Research on the heritability of intelligence quotient (IQ) inquires into the degree of variation in IQ within a population that is associated with genetic variation between individuals in that population. There has been significant controversy in the academic community about the heritability of IQ since research on the issue began in the late nineteenth century. Intelligence in the normal range is a polygenic trait, meaning that it is influenced by more than one gene, and in the case of intelligence at least 500 genes. Further, explaining the similarity in IQ of closely related persons requires careful study because environmental factors may be correlated with genetic factors. Outside the normal range, certain single gene genetic disorders, such as phenylketonuria, can negatively affect intelligence.
Estimates in the academic research of the heritability of IQ vary significantly by study and by study design. The general figure for heritability of IQ from behavioral genetic studies is about 0.5 across multiple studies in varying populations. The relationship between heritability and age is uncertain, though most researchers believe that there is an increase in heritability over the course of the lifespan and that this increase reflects the importance of gene-environment correlations. Recent genetic research has come to more equivocal results, with estimates of heritability lower than those derived from twin studies, causing what is known as the "missing heritability problem".
Although IQ differences between individuals have been shown to have a hereditary component, it does not follow that disparities in IQ between groups have a genetic basis. The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups.
Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of variation in a phenotypic trait in a population is explained by genetic variation between individuals in that population. The concept of heritability can be expressed in the form of the following question: "What is the proportion of the variation in a given trait within a population that is not explained by the environment or random chance?"
Estimates of heritability take values ranging from 0 to 1; a heritability estimate of 1 indicates that all variation in the trait in question is genetic and a heritability estimate of 0 indicates that none of the variation is genetic.
There are a number of caveats to consider when interpreting heritability:
Heritability measures the proportion of variation in a trait that can be attributed to genes, and not the proportion of a trait caused by genes. Thus, if the environment relevant to a given trait changes in a way that affects all members of the population equally, the mean value of the trait will change without any change in its heritability (because the variation or differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase. Thus, even in developed nations, a high heritability of a trait does not necessarily mean that average group differences are due to genes. Some have gone further, and used height as an example in order to argue that "even highly heritable traits can be strongly manipulated by the environment, so heritability has little if anything to do with controllability."
A common error is to assume that a heritability figure is necessarily unchangeable. The value of heritability can change if the impact of environment (or of genes) in the population is substantially altered. If the environmental variation encountered by different individuals increases, then the heritability figure would decrease. On the other hand, if everyone had the same environment, then heritability would be 100%. The population in developing nations often has more diverse environments than in developed nations. This would mean that heritability figures would be lower in developing nations. Another example is phenylketonuria which previously caused intellectual disabilities in everyone who had this genetic disorder and thus had a heritability of 100%. Today, this can be prevented by following a modified diet, resulting in a lowered heritability.
A high heritability of a trait does not mean that effects such as learning are not involved. Vocabulary size, for example, is substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions and thus heritability is high.
Contrary to popular belief, two parents of higher IQ will not necessarily produce offspring of equal or higher intelligence. Polygenic traits often appear less heritable at the extremes. A heritable trait is definitionally more likely to appear in the offspring of two parents high in that trait than in the offspring of two randomly selected parents. However, the more extreme the expression of the trait in the parents, the less likely the child is to display the same extreme as the parents. In fact, parents whose IQ is at either extreme are more likely to produce offspring with IQ closer to the mean (or average) than they are to produce offspring with high IQ. At the same time, the more extreme the expression of the trait in the parents, the more likely the child is to express the trait at all. For example, the child of two extremely tall parents is likely to be taller than the average person (displaying the trait), but unlikely to be taller than the two parents (displaying the trait at the same extreme). See also regression toward the mean.
There are multiple types of studies that are designed to estimate heritability and other variance components, stemming from the field of biometrical genetics. The variance, or more simply the differences in a trait within a population, can be partitioned into specific variance components that are based on particular decomposition models for the phenotype. First pointed out by Ronald Fisher and Sewall Wright, different sources of variance differ in their contribution to the resemblance between types of relatives.By making certain assumptions about the relationship between genes and environment, the field of quantitative genetics has developed equations that describe the expected correlation between relatives in terms of specific variance components (shared environment, additive genetic, etc.).
Correlation between relatives on a purely additive genetic model of phenotypic determination, from Coop 2020
One simple case is presented by twin studies. By making assumptions about genetic additivity and sources of environmental variation, researchers can estimate the heritability of a phenotype by comparing the resemblance of twins.Another method used to estimate heritability is adoption studies. Because adoptive parents and their adoptive children share a common environment, but not genetics, researchers in behavioral genetics have used data from adoptive families to estimate the heritability of IQ. Based on quantitative genetic theory, some researchers have taken a broader approach to analyzing familial resemblance. These researchers typically use path analysis[a] to fit models using larger sets of familiar correlations, incorporating cultural transmission, adoption, other complex living arrangements, and generalized assortative mating. However, with the transition into the postgenomic era, issues with controversial assumptions in previous models analyzing familial resemblance may be superseded by genetic analysis. Methods have developed in genetic analysis to estimate the total heritability of a trait using large datasets with millions of genetic variants.
In the case of IQ, heritability estimates based on direct observation of molecular genetics have been significantly lower (around 10%) than those employing traditional methods (40–80%). This discrepancy has been termed the "missing heritability problem." While researchers such as Robert Plomin and Sophie von Stumm contend that this gap will likely disappear with the acquisition of more genetic data, Eric Turkheimer and Lucas J. Matthews argue that it reveals a deeper set of methodological problem inherent in the concept of heritability itself, which will not be easy to overcome.
Estimates
Although individual family studies that estimate the heritability of IQ vary greatly,[b] most family studies estimate the heritability of IQ in the range from 0.4 to 0.8 and reviews of the literature typically summarize classical family design research with an estimate of 0.5. Issues with the methodology of family studies have long plagued the research field,but some researchers now believe that genome-wide association studies can provide less biased estimates of heritability. These genomic studies provide estimates of heritability that are much lower than those derived from twin studies, leading to what is denoted as the "missing heritability problem". The latest results from genomic methods are between 0.10 to 0.20, and researchers have proposed various theories to explain the gap.
Twin studies are the most common method used to estimate the heritability of most traits. Because monozygotic twins derive from one fertilized egg, they are largely genetically identical,[c] while dizygotic twins are expected to share 50% of their DNA. Twin studies compare the resemblance of monozygotic twins to the resemblance of dizygotic twins to estimate the heritability of a trait using various models.
Twin studies have shown greater resemblance between monozygotic twins than dizygotic twins. This has lead most researchers in behavioral genetics to conclude that the heritability of IQ is in a range between 0.4 to 0.8. However, it is not clear if all of the assumptions in twin studies hold true. Critics often focus on the equal environment assumption, which is the assumption that environmentally caused similarity is the same for both monozygotic and dizygotic twins. Genetic modeling has found that even in the complete absence of genetic factors for a trait, environmental similarity between monozygotic twins will cause heritability estimators to produce large estimates of genetic heritability. Researchers have produced different analyses of the equal environment assumption, some suggesting that the assumption is untenable for related traits like educational attainment.
Twins reared apart have also been the subject of significant debate. The 2006 edition of Assessing adolescent and adult intelligence by Alan S. Kaufman and Elizabeth O. Lichtenberger reports correlations of 0.86 for identical twins raised together compared to 0.76 for those raised apart and 0.47 for siblings. Theoretically, the correlation between twins reared apart is a direct estimate of heritability, as the only common factor between the twins would be the genetic variance. However, this makes an assumption of uncorrelated environments between twins, which may not be the case.For example, some researchers argue that twins experience similar education and this causes increased similarity in IQ. Thomas Bouchard argues that some previous analyses of the data on twins reared apart are an abuse of statistical theory that he calls "pseudoanalysis".
Adoption studies
Another study design researchers have used is adoption studies. In theory, "adoption creates sets of genetically related individuals who do not share a common family environment because they were adopted apart". Adoption studies have broadly found that the IQs of adoptees are more similar to their biological parents than that of their adoptive parents, in tandem with findings that adoption into better socioeconomic environments greatly increases IQ. For example, one analysis of the Texas Adoption Project estimated heritability at 0.78. However, one recent adoption study estimate heritability for cognitive ability at 0.33. Some critics point to assortative mating, range restriction and other complex family and social processes as providing issues in the interpretation of data from adoption studies. Recent studies employing polygenic scores for educational attainment in adoptees have found a more complicated interaction between genes and environments.
Other models
More broadly, researchers have analyzed the resemblance between various familial classes, such as parent-child correlations. In 1982, Bouchard and McGue reviewed such correlations reported in 111 original studies in the United States. The mean correlation of IQ scores between monozygotic twins was 0.86, between siblings 0.47, between half-siblings 0.31, and between cousins 0.15. Some more basic model fitting studies are found in the behavioral genetics literature. Loehlin fit a model to the Bouchard and McGue data which estimated broad heritability at .58 from 'direct' methods and .47 from 'indirect' methods. Chipuer et al. analyzed this data using a LISREL model and estimated broad sense heritability as .51. Other models successively add more components. In 1997, Devlin et al. also used these correlations to fit and compare different models and estimated that heritability was less than 50% in their best fitting models, finding that maternal effects were particularly important in their analysis. Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different from previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports.
Genetics researchers observed that cultural heritability and genetic heritability can produce similar patterns of observations between relatives in the 1970s. As a result, in a series of papers using models based on path analysis, a group of researchers from the University of Hawaii analyzed data collected on American families and estimated genetic heritability ranging from 0.30 to 0.34. A later study by Otto et al. applied the path analysis method to the data collected by Bouchard and McGue and estimated heritability ranging from 0.29 to 0.42. Path analysis models have been subject to criticism for further issues with modeling assumptions and statistical procedures, reflecting broader criticisms of structural equation modeling.
These findings are compatible with the view that a large number of genes, each with only a small effect, contribute to differences in intelligence.
A diagram showing the relationship between genomic relatedness and phenotypic correlation for a hypothetical trait, a method used to estimate heritability. From Kemper et al. 2021
Molecular genetic studies have been central to several major scientific advances relevant to heritability. The first is the advent of polygenic scores, which take the thousands of single-nucleotide polymorphisms (SNPs) that influence a given trait to form an estimated "genetic value" or "genetic propensity". Most genome-wide association studies (GWAS) estimate a polygenic score (PGS) and describe its overall statistical contribution to variance for the population in question, i.e. providing a R2 (coefficient of determination) for the polygenic score. This measures only the predictive power of the genetic variants in the study in question, a future study with larger sample size or more genetic variants tagged may produce a larger amount. Consequently, a given polygenic score only captures "a fraction of SNP-heritability". The second is the concept of SNP heritability, which is the total proportion of phenotypic variance explained by SNPs.
Genomic studies have found significant correlations between IQ and various polygenic scores, but the true size of this association is unclear. The largest published analysis for genetic contributions to IQ found that the polygenic score could explain 9.9% of the variance between families, while some later analyses estimated that polygenic scores can predict about 11% of the variance. However, these analyses rely on differences in genetic variants between families which introduce biases from population stratification, assortative mating, and indirect genetic effects such as dynastic effects or genetic nurture. One study found that polygenic score prediction for cognitive traits was 60% greater between families than within families.
Genomic studies have also estimated values of SNP heritability for IQ. Very early studies used a method called genome-wide complex trait analysis (GCTA) which calculates the overall genetic similarity (as indexed by the cumulative effects of all genotyped single nucleotide polymorphisms) between all pairs of individuals in a sample of unrelated individuals and then correlates this genetic similarity with phenotypic similarity across all the pairs. Studies using this method reported relatively large estimates of heritability, ranging from 40% to 51%. A later study by the same group estimated SNP heritability for different measures of cognitive ability at about 31%. However, researchers later discovered population stratification and other environmental biases were major issues affecting basic analyses. As a result, recent research has sought to estimate the SNP heritability of traits, including IQ, using within-family genome-wide association studies, though researchers have identified that these are not immune to interpretive issues. Some estimates of the SNP heritability of IQ, derived from genome-wide association studies (GWAS), put the figure much lower, at around 10%. For example, one recent published analysis estimated the within-family SNP heritability of cognitive function to be 14%. A more recent unpublished preprint estimated the SNP heritability at about 19%.
Another set of methods attempts to generalize correlations between familial resemblance and genetic resemblance to estimate heritability. These methods are free from most environmental biases and have typically found that twin estimates of heritability are inflated. When tested on similar achievement phenotypes like educational attainment, these methods find that twin methods significantly inflate the value of heritability.
The phrase 'missing heritability' is commonly used to describe both the gap between classically derived heritability estimates and the variance explained by variants and "SNP" heritability. As the heritability estimates obtained by molecular genetic studies do not match earlier research from twin and family studies, researchers have proposed many theories as to explain these discrepancies for various traits, including but not limited[d] to rare variants, untagged variants, gene-environment interactions or environmental confounding. Some researchers believe that the gap for IQ heritability will be narrowed with larger, better-powered genome-wide studies and with whole-genome sequencing,while others indicate that the gap may at least in part reflect overestimation of heritability by twin studies.
In biometrical studies that partition variance, "shared environment" refers to hypothetical factors that increase the similarity of relatives in a particular study design. Researchers draw a distinction between "objectively shared environments" (features of the environment that relatives such as siblings share) and the "shared environment" in behavioral genetics studies, which measures only the factors that increase homogeneity.
Researchers typically use the same designs to estimate shared environment effects as to estimate heritability: twin studies, adoption studies and family studies. These methods often come to disparate conclusions about the effects of shared environment.
Some adoption studies show that by adulthood adoptive siblings aren't more similar in IQ than strangers, while adult full siblings show an IQ correlation of 0.24. These findings have lead researchers such as Sandra Scarr and Robert Plomin to conclude that shared environment has minimal effect on intelligence in adulthood. Psychologists Ken Richardson and Sarah Norgate question the assumptions of adoption studies for IQ, highlighting issues with assumptions of additivity and randomization. However, adoption studies have also shown that the process of adoption has a positive effect on IQ scores, and two studies have associated this with factors such as the level of parental education and socioeconomic status of adoptive parents. In a 1991 paper, psychologist Eric Turkheimer tried to reconcile results from adoption studies that demonstrate large increases in IQ from adoption and those that show low correlations between non-biologically related relatives. He argued that early adoption studies first established that genetic effects influence behavioral phenotypes, while later studies from Schiff and colleagues demonstrate that environment has an influence. Researchers Alexandra Burt and Wendy Johnson argue that the joint study of means and variance is necessary to understand the impact of environment. In a 2024 study, Burt et al. propose that measurement error or noise might inhibit the identification of specific environmental factors that influence academic achievement.
Relationship between correlation between brothers' IQ scores and age differences in Sundet et al. 2008
Some studies of twins reared apart (e.g. Bouchard, 1990) find a significant shared environmental influence, of at least 10% going into late adulthood. Jack Kaplan suggests that the evidence from twin studies and adoption studies are inconsistent, and that twin studies suggest a role for the effect of shared environment on intelligence in adulthood. Researchers such as Briley and Tucker-Drob have suggested that low shared environmental estimates in adulthood may arise because individuals differentially respond to objectively shared environments over time, as a result of unmodelled interactions with either genetic or objectively nonshared environmental factors. One unpublished meta-analysis of twin studies finds estimates of shared environment around 20% for 15-20 year olds, but results for adults no greater than 15%.Researchers have proposed many different factors that could influence intelligence through the shared environment component. One 2018 paper by Laura Engelhardt and colleagues found that measures of socioeconomic status, school demographics and neighborhood socioeconomic status statistically account for most of shared environmental influences on several metrics of intelligence.
Another method researchers have used to estimate the shared environmental component for intelligence is studies of siblings. In a large study, Kendler and colleagues used twins, step-siblings, and half-siblings to estimate the shared environment component of many behavioral phenotypes. They found shared environment components for IQ ranging from about 10% to 20%, depending on the relationship class.Another study found an association between similarity in age between siblings and similarity in IQ scores, suggesting familial environment influences on intelligence scores.
A 2015 study by Johnathan Daw and colleagues argues that the additivity assumption of typical family studies is not correct and that measures of objectively measured shared environment could moderate the estimated nonshared environment component. They found that shared environments such as birth order, household size, sibling age dispersion, and school quality exhibit "large moderating effects on both verbal intelligence and academic achievement". They suggested that these results show how objectively shared environments can influence intelligence in contrast to a common belief that objectively shared environments do not influence children's lives.
Non-shared environment
As nonshared environmental variance is simply the leftover variance not accounted for by genetic variance and shared environmental variance, it encompasses any factors that cause differences between siblings and estimates vary based on the estimates of heritability and shared environment.Robert McCall argues nonshared environment contributes 15-25% of the variance of IQ scores. Although parents treat their children differently, known measures of differential treatment explains only a small amount of non-shared environmental influence. One suggestion is that children react differently to the same environmental factors: that objectively shared measures of the environment can have different effects on siblings. One 1994 study of the National Longitudinal Survey of Youth found that a measure of the home environment accounted for nonshared environmental variance for several measures of intelligence, while Daw et al. suggest that nonshared environment is moderated by objectively shared environment. More likely influences may be the impact of peers and other experiences outside the family. For example, siblings grown up in the same household may have different friends and teachers and even contract different illnesses. This factor may be one of the reasons why IQ score correlations between siblings decreases as they get older.
Some authors highlight measurement error as one source of variance attributed to the nonshared environment, as measurement error is captured in the nonshared environment term.Another factor that produces differences between individuals is stochastic variation, often attributed to nonlinear epigenetic processes.
Dickens and Flynn model
Dickens and Flynn (2001) argued that the "heritability" figure includes both a direct effect of the genotype on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues. This model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects.
The Flynn effect is the increase in average intelligence test scores by about 0.3% annually, resulting in the average person today scoring 15 points higher in IQ compared to the generation 50 years ago. This effect can be explained by a generally more stimulating environment for all people.
Some scientists have suggested that such enhancements are due to better nutrition, better parenting and schooling, as well as exclusion of the least intelligent people from reproduction. However, Flynn and a group of other scientists share the viewpoint that modern life implies solving many abstract problems which leads to a rise in their IQ scores.
Indirect genetic effects
Various molecular genetic studies have identified an effect of alleles not carried by an individual, but some relative, on their phenotype. These have been varyingly referred to as "dynastic effects", "non-transmitted coefficients", "interpersonal genetic effects", "genetic nurture", or more broadly as "indirect genetic effects".
Kong reports that, "Nurture has a genetic component, i.e. alleles in the parents affect the parents' phenotypes and through that influence the outcomes of the child." These results were obtained through a meta-analysis of educational attainment and polygenic scores of non-transmitted alleles. Although the study deals with educational attainment and not IQ, these two are strongly linked.
Hypotheses about the relationship between the value of heritability and socioeconomic status originated with Sandra Scarr's 1971 article Race, Social Class, and IQ. Scarr notes that existing theories of environmental disadvantage explaining the relationship between social class and IQ predict that more advantaged groups will show greater genetic variance as the advantaged groups don't experience as many detrimental environmental effects. In her study, she identified statistical interactions between socioeconomic environment and genetic variance, finding that genetic influences on IQ were stronger in higher socioeconomic groups where environmental conditions allowed genetic potential to be more fully expressed, whereas in lower socioeconomic groups, adverse environments constrained this expression and reduced the impact of genetic differences. Other than some minor criticisms and a partial replication, the topic was not broached again until the 1996 APA report "Intelligence: Knowns and Unknowns", which stated that:
"We should note, however, that low-income and non-white families are poorly represented in existing adoption studies as well as in most twin samples. Thus it is not yet clear whether these studies apply to the population as a whole. It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence."
Relationship between the genetic variance component and socioeconomic status in Turkheimer et al. 2003
A 1999 study by David Rowe and colleagues replicated the effect in the National Longitudinal Study of Adolescent Health, finding that among children from poorly-educated parents 'shared environment' was associated with most of the variance, while among children from well-educated parents, genes were associated with most of the variance.Turkheimer and colleagues (2003) argued that the proportions of IQ variance attributable to genes and environment vary with socioeconomic status. They found that in a study on seven-year-old twins, in impoverished families, 60% of the variance in early childhood IQ was accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse. The name "Scarr-Rowe effect" was proposed for this proposed interaction between heritability and socioeconomic status by Eric Turkheimer in 2009 paper presentation. Later studies found more equivocal results, some failing to replicate the finding in different locations. A 2016 meta-analysis found that the effects varied by country, with moderate interactions in studies from the United States, while the effects were zero or reversed in studies from Australia and Western Europe. A large study published the following year, however, found negligible impacts using data from twins in Florida.Recent research has focused on the relationship between socioeconomic status and heritability using genomic data. One 2020 study on educational attainment, a closely related but not identical phenotype, found that polygenic scores had greater predictive value in lower socioeconomic status quartiles. A 2021 study similarly found an increase in SNP heritability with increasing socioeconomic deprivation; lower socioeconomic status was associated with higher heritability. One 2024 study used polygenic indices and found that the Scarr-Rowe hypothesis "lacks empirical support" and suggested an alternative theory, the compensatory advantage hypothesis, that heritability was lower among populations with higher heritability.
Heritability and age
Twin correlations over development (data from Wilson 1983)
The interaction between heritability and age over the course of development has been the interest of scientists for many years. Perhaps first described by Ronald Wilson, researchers have noted changes in kinship correlations over the life course. Particularly in the case of twins, the correlation between monozygotic twins remains relatively constant, while the correlation between dizygotic twins decreases. Some early attempts at modeling this change have equivocal results, with Rao et al. finding that heritability increased in adulthood in one model, but not in another, while a later study by Devlin et al. did not find support for age moderation of heritability. Most researchers in behavioral genetics now believe that estimates of heritability from classical family studies (adoption, twin studies) increase as individuals age One review described a "linearly increasing heritability of intelligence from infancy (20%) through adulthood (60%)". This increase in heritability with age is known as the "Wilson Effect". There are two possible explanations for why measured heritability might increase over the course of the lifespan: that new genetic factors are activated (genetic innovation) or that previous genetic factors explain more variance (genetic amplification).Behavioral genetics researchers believe that genetic amplification is most supported by the evidence, where the same genetic factors that influence IQ at one age are involved later, but produce larger and larger phenotypic effects over time. One theory proposed to explain the apparent increase in heritability values is a reciprocal effects model, where environments and genes become correlated over the course of development. These models incorporate phenotype to environment associations that violate the independence assumption in traditional behavioral genetic models that may overestimate heritability. Another proposed explanation for why measured heritability can increase over the course of development is gene-environment interactions.
Recent molecular genetics research has also investigated the interaction between genetics or heritability and age over the course of development. One 2018 genome-wide association study found "comparable heritability across age". Another study found that the within-family estimate heritability was 0.12 in childhood, 0.13 in adolescence and 0.15 in adulthood.
Implications
Development
Some researchers, especially those that work in fields like developmental systems theory, have criticized the concept of heritability as misleading or meaningless. Douglas Wahlsten and Gilbert Gottlieb argue that the prevailing models of behavioral genetics are too simplistic by not accounting for gene-environment interactions. Stephen Ceci also highlights the issues with this assumption, noting that they were first raised by Jane Loevinger in 1943. They assert that the idea of partitioning variance makes no sense when environments and genes interact and argue that such interaction is ubiquitous in human development. They highlight their belief that heritability analysis requires a hidden assumption they call the "separation of causes", which isn't borne out by biological reality or experimental research. Such researchers argue that the notion of heritability gives the false impression that "genes have some direct and isolated influence on traits", rather than another developmental resource that a complex system uses over the course of ontogeny.
Although IQ differences between individuals have been shown to have a hereditary component, it does not follow that differences in average IQ test performance between racial and ethnic groups have a genetic basis. The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups. Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap.
Arguments in support of a genetic explanation of racial differences in average IQ are sometimes fallacious. For instance, some hereditarians have cited as evidence the failure of known environmental factors to account for such differences, or the high heritability of intelligence within races. In 1972, John DeFries attempted to connect "between group heritability" or h2B to "within group heritability" or h2W in an equation , where r is the intraclass genetic correlation among members of the same group, t is corresponding phenotypic intraclass correlation. However, this equation is tautological as the term r is defined in reference to variance among and between groups, meaning r, if anything, depends on h2B. Geneticists Joshua Schraiber and Michael Edge showed that even if this quantity r is known, the quantity h2B is still consistent with "infinitely many configurations of genetic differences among populations". They conclude:
Perfect knowledge of within-group heritability provides no information about between-group heritability. Crucially, even if the heritability of between-group differences is estimated correctly, it leaves the direction of the genetic and environmental components of phenotypic difference unclear.
In 2017, the editorial board of Nature issued a statement differentiating current research on the genetics of intelligence from the racist pseudoscience which it acknowledged has dogged intelligence research since its inception. It characterized the idea of genetically determined differences in intelligence between races as definitively false. Analysis of polygenic scores sampled from the 1000 Genomes Project has likewise found no evidence that intelligence was under diversifying selection in Africans and Europeans, suggesting that genetic differences cannot be a significant factor in the observed Black-White gap in average IQ test performance.
Evolutionary psychology is a theoretical approach in psychology that examines cognition and behavior from a modern evolutionary perspective. It seeks to identify human psychological adaptations with regard to the ancestral problems they evolved to solve. In this framework, psychological traits and mechanisms are either functional products of natural and sexual selection or non-adaptive by-products of other adaptive traits.
Adaptationist thinking about physiological mechanisms, such as the heart, lungs, and the liver, is common in evolutionary biology. Evolutionary psychologists apply the same line of thinking in psychology, arguing that just as the heart evolved to pump blood, the liver evolved to detoxify poisons, and the kidneys evolved to filter turbid fluids, there is modularity of mind, in that different psychological mechanisms evolved to solve distinct adaptive problems. The evolutionary psychologists argue that much of human behavior is the output of psychological adaptations that evolved to solve recurrent problems in human ancestral environments.
Some evolutionary psychologists argue that evolutionary theory can provide a foundational, metatheoretical framework that integrates the entire field of psychology, much as evolutionary biology has done for biology.
Evolutionary psychologists hold that behaviors or traits that occur universally in all cultures are good candidates for evolutionary adaptations, including the abilities to infer others' emotions, discern kin from non-kin, identify and prefer healthier mates, and cooperate with others. Findings have been made regarding human social behaviour related to infanticide, intelligence, marriage patterns, promiscuity, perception of beauty, bride price, and parental investment.
Criticism of evolutionary psychology involves questions of testability, cognitive and evolutionary assumptions (such as modular functioning of the brain, and large uncertainty about the ancestral environment), importance of non-genetic and non-adaptive explanations, and political and ethical implications of interpretations of research results.
Scope
Principles
The central assumption of evolutionary psychology is that the human brain is composed of many specialized mechanisms that were shaped by natural selection over a vast period of time to solve the recurrent information-processing problems faced by our ancestors. These problems involve food choices, social hierarchies, the distribution of resources to offspring, and selecting mates. Proponents suggest that it seeks to integrate psychology into the other natural sciences, rooting it in the organizing theory of biology (evolutionary theory), and thus understanding psychology as a branch of biology. Anthropologist John Tooby and psychologist Leda Cosmides wrote:
Evolutionary psychology is the long-forestalled scientific attempt to assemble out of the disjointed, fragmentary, and mutually contradictory human disciplines a single, logically integrated research framework for the psychological, social, and behavioral sciences – a framework that not only incorporates the evolutionary sciences on a full and equal basis, but that systematically works out all of the revisions in existing belief and research practice that such a synthesis requires.
Just as human physiology and evolutionary physiology have worked to identify physical adaptations of the body that represent "human physiological nature," the purpose of evolutionary psychology is to identify evolved emotional and cognitive adaptations that represent "human psychological nature." According to Steven Pinker, it is "not a single theory but a large set of hypotheses" and a term that "has also come to refer to a particular way of applying evolutionary theory to the mind, with an emphasis on adaptation, gene-level selection, and modularity." Evolutionary psychology adopts an understanding of the mind grounded in the computational theory of mind. It describes mental processes as computational operations, so that, for example, a fear response is described as arising from a neurological computation that takes perceptual data, e.g., a visual image of a spider, and outputs the appropriate reaction, e.g., fear of possibly dangerous animals. Under this view, any domain-general learning is impossible because of the combinatorial explosion. Evolutionary psychology specifies the domain as the problems of survival and reproduction.
While philosophers have generally considered the human mind to include broad faculties, such as reason and lust, evolutionary psychologists describe evolved psychological mechanisms as narrowly focused on specific issues, such as catching cheaters or choosing mates. The discipline sees the human brain as having evolved specialized functions, called cognitive modules, or psychological adaptations which are shaped by natural selection.
Examples include language-acquisition modules, incest-avoidance mechanisms, cheater-detection mechanisms, intelligence and sex-specific mating preferences, foraging mechanisms, alliance-tracking mechanisms, agent-detection mechanisms, and others. Some mechanisms, termed domain-specific, deal with recurrent adaptive problems over the course of human evolutionary history. Domain-general mechanisms, on the other hand, are proposed to deal with evolutionary novelty.
Nikolaas Tinbergen's four categories of questions can help clarify the distinctions among several complementary types of explanations. Evolutionary psychology focuses primarily on the "why?" questions, while traditional psychology focuses on the "how?" questions.
Table of categories
Diachronic versus synchronic perspective
Dynamic view Explanation of current form in terms of a historical sequence
Static view Explanation of the current form of species
How vs. why questions
Proximate view How an individual organism's structures function
Ontogeny (development) Developmental explanations for changes in individuals, from DNA to their current form
Mechanism (causation) Mechanistic explanations for how an organism's structures work
Ultimate (evolutionary) view Why a species evolved the structures (adaptations) it has
Phylogeny (evolution) The history of the evolution of sequential changes in a species over many generations
Function (adaptation) A species trait that solves a reproductive or survival problem in the current environment
Premises
Evolutionary psychology is founded on several core premises.
The brain is an information-processing device, and it produces behavior in response to external and internal inputs.
The brain's adaptive mechanisms were shaped by natural and sexual selection.
Different neural mechanisms are specialized for solving problems in humanity's evolutionary past.
The brain has evolved specialized neural mechanisms that were designed for solving problems that recurred over deep evolutionary time, giving modern humans stone-age minds.
Most contents and processes of the brain are unconscious, and most mental problems that seem easy to solve are actually extremely difficult problems that are solved unconsciously by complicated neural mechanisms.
Human psychology comprises many specialized mechanisms, each sensitive to different classes of information or inputs. These mechanisms combine to produce manifest behavior.
Evolutionary psychology has its historical roots in Charles Darwin's theory of natural selection. In his book On the Origin of Species, Darwin predicted that psychology would develop an evolutionary basis:
In the distant future I see open fields for far more important research. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation.
—Charles Darwin
Upon completing On the Origin of Species in 1859, Darwin shifted his focus to creating an evolution-based psychology, having viewed the development of an evolutionary explanation for higher human faculties as his most crucial undertaking since conceiving his theory of evolution in the late 1830s. He devoted more than a decade to study of the evolutionary origins of key aspects of human behaviour, in particular: the human intellect; rationality; human sexual behaviour; emotional expressions; moral behaviour; language; culture; and conscience—several of which Darwin argued had originated due to the unusual ways natural selection operates in social animals, that is, by different kinds of group selection, including kin selection and reciprocal altruism.
Initially conceived as groundwork for one book, Darwin's psychological project became so large and fruitful that he split it into two parts, the first being published as The Descent of Man, and Selection in Relation to Sex in 1871 and the second as The Expression of the Emotions in Man and Animals in 1872. Darwin also published pathbreaking evolutionary monographs on insect and invertebrate behaviour, and—believing that infant behaviour gives us privileged access to humans' evolutionary endowment—a seminal case-study of his baby son William's behaviour, containing many observations confirmed by recent research, including those on: infant-adult communication and meta-communication; infant emotional expressions; early reasoning; the experimental study of infant jealousy; and the origins of self-knowledge.
Darwin's evolutionary psychological publications inspired many of the founders of modern psychology, including: Sigmund Freud and his theories of psychoanalysis, particularly as relating to the Oedipus complex and group psychology; both the physiological and folk psychologies of Wilhelm Wundt; William James's functionalist approach to psychology; the genetic psychology of James Mark Baldwin and thus, indirectly, Jean Piaget; the social behaviourism or symbolic interactionism of George Herbert Mead which set out from Mead's appropriation of Darwin's treatment of gesture and attitude in his book on Expression; and the study of evolved 'instinctive' behaviour in animals—a major topic in On the Origin of Species—known today as ethology and pioneered by Konrad Lorenz, Karl von Frisch and Niko Tinbergen. More recently, Darwin's research on his baby son inspired Colwyn Trevarthen's observational approach to infancy and his theory of innate intersubjectivity. Overall, Darwin's theories of evolution, adaptation, and natural selection have provided and continue to provide many insights into why minds and brains function the way they do.
20th century evolutionary psychologies mostly bypass Darwin's extensive evolutionary treatment of human psychology, drawing instead from: the biological sciences of their day—especially 20th century evolutionary theory as it is imagined to relate to ancient human environments; the study of paleoanthropology; ethology; and various human sciences, most obviously cognitive psychology. For example, writing from a gene-centred perspective on evolution, W.D. Hamilton's (1964) papers on inclusive fitness and Robert Trivers's (1972) theories on reciprocity and parental investment helped to re-establish evolutionary thinking in psychology and the other social sciences, though without reference to the place of parental investment, reciprocal altruism and kin selection in Darwin's psychology. Likewise, in 1975, Edward O. Wilson combined evolutionary theory with studies of animal and social behavior, principally referencing the works of Lorenz and Tinbergen, in his book Sociobiology: The New Synthesis.
In the 1970s, two major branches of ethology developed. Firstly, the study of animal social behavior (including humans) generated sociobiology, defined by its pre-eminent proponent Edward O. Wilson in 1975 as "the systematic study of the biological basis of all social behavior." and in 1978 as "the extension of population biology and evolutionary theory to social organization." Secondly, there was behavioral ecology, which placed less emphasis on social behavior; it focused on the ecological and evolutionary basis of animal and human behavior.
In the 1970s and 1980s, university departments began to include the term evolutionary biology in their titles. The modern era of evolutionary psychology was ushered in, in particular, by Donald Symons' 1979 book The Evolution of Human Sexuality and Leda Cosmides and John Tooby's 1992 book The Adapted Mind. David Buller observed that the term "evolutionary psychology" is sometimes seen as denoting research based on the specific methodological and theoretical commitments of certain researchers from the Santa Barbara school (University of California), thus some evolutionary psychologists prefer to term their work "human ecology", "human behavioural ecology" or "evolutionary anthropology" instead.
In psychology, there are the primary streams of developmental, social, and cognitive psychology. Establishing some measure of the relative influence of genetics and environment on behavior has been at the core of behavioral genetics and its variants, notably studies at the molecular level that examine the relationship between genes, neurotransmitters, and behavior. Dual inheritance theory (DIT), developed in the late 1970s and early 1980s, has a slightly different perspective by trying to explain how human behavior is a product of two different and interacting evolutionary processes: genetic evolution and cultural evolution. DIT is seen by some as a "middle-ground" between views that emphasize human universals versus those that emphasize cultural variation.
The theories on which evolutionary psychology is based originated with Charles Darwin's work, including his speculations about the evolutionary origins of social instincts in humans. Modern evolutionary psychology, however, is possible only because of advances in evolutionary theory in the 20th century. Evolutionary psychologists say that natural selection has provided humans with many psychological adaptations, much the same way it has generated humans' anatomical and physiological adaptations. As with adaptations in general, psychological adaptations are said to be specialized for the environment in which an organism evolved, the environment of evolutionary adaptedness. Sexual selection provides organisms with adaptations related to mating. For male mammals, which have a relatively high maximal potential reproduction rate, sexual selection leads to adaptations that help them compete for females. For female mammals, with a relatively low maximal potential reproduction rate, sexual selection leads to choosiness, which helps females select higher-quality mates. Charles Darwin described both natural selection and sexual selection, and he relied on group selection to explain the evolution of altruistic (self-sacrificing) behavior. But group selection was argued to be a weak explanation by those who adopted the 20th century's individualistic gene-centred view of evolution, because it was thought that in any loosely-bonded group which lacked strong in-group dynamics (of the kind assumed by Darwin), less altruistic individuals will be more likely to survive, and the group will become less self-sacrificing as a whole.
In 1964, the evolutionary biologist William D. Hamilton proposed inclusive fitness theory, emphasizing a gene-centered view of evolution. Hamilton noted that genes can increase the replication of copies of themselves into the next generation by influencing the organism's social traits in such a way that (statistically) results in helping the survival and reproduction of other copies of the same genes (most simply, identical copies in the organism's close relatives). According to Hamilton's rule, self-sacrificing behaviors (and the genes influencing them) can evolve if they typically help the organism's close relatives so much that it more than compensates for the individual animal's sacrifice. Inclusive fitness theory resolved the issue of how altruism can evolve. Other theories also help explain the evolution of altruistic behavior, including evolutionary game theory, tit-for-tat reciprocity, and generalized reciprocity. These theories help to explain the development of altruistic behavior, and account for hostility toward cheaters (individuals that take advantage of others' altruism).
Several mid-level evolutionary theories inform evolutionary psychology. The r/K selection theory proposes that some species prosper by having many offspring, while others follow the strategy of having fewer offspring but investing much more in each one. Humans follow the second strategy. Parental investment theory explains how parents invest more or less in individual offspring based on how successful those offspring are likely to be, and thus how much they might improve the parents' inclusive fitness. According to the Trivers–Willard hypothesis, parents in good conditions tend to invest more in sons (who are best able to take advantage of good conditions), while parents in poor conditions tend to invest more in daughters (who are best able to have successful offspring even in poor conditions). According to life history theory, animals evolve life histories to match their environments, determining details such as age at first reproduction and number of offspring. Dual inheritance theory posits that genes and human culture have interacted, with genes affecting the development of culture and culture, in turn, affecting human evolution on a genetic level, in a similar way to the Baldwin effect.
Evolutionary psychology posits that psychological traits, like biological organs, have a genetic foundation and have developed through natural selection. Therefore, cognition, having a genetic foundation, is functionally structured and has evolved through natural selection. Similar to other organs and tissues, this functional structure ought to be common to all individuals of a species, tackling essential challenges for survival and reproduction. Evolutionary psychologists seek to understand psychological mechanisms by understanding the survival and reproductive functions they might have served over the course of evolutionary history. These might include abilities to infer others' emotions, discern kin from non-kin, identify and prefer healthier mates, cooperate with others, and follow leaders. Consistent with the theory of natural selection, evolutionary psychology sees humans as often in conflict with others, including mates and relatives. For instance, a mother may wish to wean her offspring from breastfeeding earlier than her infant does, freeing her to invest in additional offspring. Evolutionary psychology also recognizes the roles of kin selection and reciprocity in the evolution of prosocial traits such as altruism. Like chimpanzees and bonobos, humans have subtle and flexible social instincts, allowing them to form extended families, lifelong friendships, and political alliances. In studies testing theoretical predictions, evolutionary psychologists have made modest findings on topics such as infanticide, intelligence, marriage patterns, promiscuity, perception of beauty, bride price, and parental investment.
Historical topics
Proponents of evolutionary psychology in the 1990s explored historical events, but the response from historical experts was highly negative, and there has been little effort to continue that line of research. Historian Lynn Hunt says that the historians complained that the researchers:
have read the wrong studies, misinterpreted the results of experiments, or worse yet, turned to neuroscience looking for a universalizing, anti-representational, and anti-intentional ontology to bolster their claims.
Hunt states that "the few attempts to build up a subfield of psychohistory collapsed under the weight of its presuppositions." She concludes that, as of 2014, the "'iron curtain' between historians and psychology...remains standing."
Products of evolution: adaptations, exaptations, byproducts, and random variation
Not all traits of organisms are evolutionary adaptations. As noted in the table below, traits may also be exaptations, byproducts of adaptations (sometimes called "spandrels"), or random variation between individuals.
Psychological adaptations are hypothesized to be innate or relatively easy to learn and to manifest in cultures worldwide. For example, the toddler's ability to learn a language with virtually no training is likely to be a psychological adaptation. On the other hand, ancestral humans did not read or write; thus, today, learning to read and write requires extensive training, and presumably involves the repurposing of cognitive capacities that evolved under selection pressures unrelated to written language. However, variations in manifest behavior can result from universal mechanisms interacting with different local environments. For example, light-skinned Europeans who move from a northern climate to the equator will develop darker skin. The mechanisms regulating their pigmentation do not change; rather, the input to those mechanisms changes, resulting in different outputs.
Adaptation
Exaptation
Byproduct
Random variation
Definition
Organismic trait designed to solve an ancestral problem(s). Shows complexity, special "design", functionality
Adaptation that has been "re-purposed" to solve a different adaptive problem.
Byproduct of an adaptive mechanism with no current or ancestral function
Random variations in an adaptation or byproduct
Physiological example
Bones / Umbilical cord
Small bones of the inner ear
White color of bones / Belly button
Bumps on the skull, convex or concave belly button shape
Psychological example
Toddlers' ability to learn to talk with minimal instruction
Voluntary attention
Ability to learn to read and write
Variations in verbal intelligence
One of the tasks of evolutionary psychology is to identify which psychological traits are likely to be adaptations, byproducts, or random variation. George C. Williams suggested that an"adaptation is a special and onerous concept that should only be used where it is really necessary." As noted by Williams and others, adaptations can be identified by their improbable complexity, species universality, and adaptive functionality.
Obligate and facultative adaptations
A question that may be asked about an adaptation is whether it is generally obligate (relatively robust in the face of typical environmental variation) or facultative (sensitive to typical environmental variation). The sweet taste of sugar and the pain of hitting one's knee against concrete are the result of fairly obligate psychological adaptations; typical environmental variability during development does not much affect their operation. By contrast, facultative adaptations are somewhat like "if-then" statements. For example, adult attachment style seems particularly sensitive to early childhood experiences. As adults, our propensity to develop close, trusting bonds with others is dependent on whether early childhood caregivers can be trusted to provide reliable assistance and attention. The adaptation for skin to tan is conditional on exposure to sunlight; this is an example of another facultative adaptation. When a psychological adaptation is facultative, evolutionary psychologists concern themselves with how developmental and environmental inputs influence its expression.
Evolutionary psychologists hold that behaviors or traits that occur universally in all cultures are good candidates for evolutionary adaptations. Cultural universals include behaviors related to language, cognition, social roles, gender roles, and technology. Evolved psychological adaptations (such as the ability to learn a language) interact with cultural inputs to produce specific behaviors (e.g., the specific language learned).
Basic gender differences, such as greater eagerness for sex among men and greater coyness among women, are explained as sexually dimorphic psychological adaptations that reflect the different reproductive strategies of males and females. It has been found that both male and female personality traits vary across a wide spectrum. Males had a higher rate of traits relating to dominance, tension, and directness. Females had higher rates of organizational behavior and more emotionally-oriented characteristics.
Evolutionary psychologists contrast their approach to what they term the "standard social science model," according to which the mind is a general-purpose cognition device shaped almost entirely by culture.
Evolutionary psychology argues that to properly understand the functions of the brain, one must understand the properties of the environment in which the brain evolved. That environment is often referred to as the "environment of evolutionary adaptiveness."
The idea of an environment of evolutionary adaptedness was first explored as a part of attachment theory by John Bowlby. This is the environment to which a particular evolved mechanism is adapted. More specifically, the environment of evolutionary adaptedness is defined as the set of historically recurring selection pressures that formed a given adaptation, as well as those aspects of the environment that were necessary for the proper development and functioning of the adaptation.
Humans, the genus Homo, appeared between 1.5 and 2.5 million years ago, a time that roughly coincides with the start of the Pleistocene 2.6 million years ago. Because the Pleistocene ended a mere 12,000 years ago, most human adaptations either evolved during the Pleistocene or were maintained by stabilizing selection during the Pleistocene. Evolutionary psychology, therefore, proposes that most of human psychological mechanisms are adapted to reproductive problems frequently encountered in Pleistocene environments. In broad terms, these problems include those of growth, development, differentiation, maintenance, mating, parenting, and social relationships.
The environment of evolutionary adaptation is significantly different from modern society. The ancestors of modern humans lived in smaller groups, had more cohesive cultures, and had more stable and rich contexts for identity and meaning.
Researchers look to existing hunter-gatherer societies for clues as to how hunter-gatherers lived in the environment of evolutionary adaptation. Unfortunately, the few surviving hunter-gatherer societies differ from each other, and they have been pushed out of the best land and into harsh environments, so it is not clear how closely they reflect ancestral culture. However, across the world, small-band hunter-gatherers offer a similar developmental system for the young. The characteristics of the niche are largely the same as for social mammals, who evolved over 30 million years ago: soothing perinatal experience, several years of on-demand breastfeeding, nearly constant affection or physical proximity, responsiveness to need (mitigating offspring distress), self-directed play, and, for humans, multiple responsive caregivers. Initial studies show the importance of these components in early life for positive child outcomes.
Evolutionary psychologists sometimes look to chimpanzees, bonobos, and other great apes for insights into human ancestral behavior.
When an organism's traits are suited for its old environment, a change in surroundings can result in a mismatch. Our minds, evolved for Pleistocene environments, may possess traits incongruent with the modern world. For instance, it is noteworthy that more than 20,000 people in the US die from gun violence each year. However, spiders and snakes are responsible for very few fatalities. Even so, individuals acquire a fear of spiders and snakes as easily as they do a pointed gun, and more easily than an unpointed gun, rabbits, or flowers. One possible reason is that spiders and snakes posed a danger to early humans during the Pleistocene, unlike guns, rabbits, and flowers. There is a mismatch between our evolved fear responses and the modern world.
This mismatch also shows up in the phenomenon of the supernormal stimulus, a stimulus that elicits a response more strongly than the stimulus for which the response evolved. The term was coined by Niko Tinbergen to refer to non-human animal behavior, but psychologist Deirdre Barrett said that supernormal stimulation governs human behavior as powerfully as that of other animals. She explained junk food as an exaggerated stimulus for cravings for salt, sugar, and fat, and she says that television exaggerates social cues of laughter, smiling faces, and attention-grabbing action.
Magazine centerfolds and double cheeseburgers pull instincts intended for an environment of evolutionary adaptedness where breast development was a sign of health, youth, and fertility in a prospective mate, and fat was a rare and vital nutrient.
The psychologist Mark van Vugt recently argued that modern organizational leadership is a mismatch. His argument is that humans are not adapted to work in large, anonymous bureaucratic structures with formal hierarchies. The human mind still responds to personalized, charismatic leadership primarily in informal, egalitarian settings. Hence, the dissatisfaction and alienation that many employees experience. Salaries, bonuses, and other privileges exploit instincts for relative status, which attract men in particular to senior executive positions.
Research methods
Evolutionary theory is heuristic in that it may generate hypotheses that might not be developed from other theoretical approaches. One of the main goals of adaptationist research is to identify which organismic traits are likely to be adaptations, and which are byproducts or random variations. As noted earlier, adaptations are expected to show evidence of complexity, functionality, and species universality, while byproducts or random variation will not. In addition, adaptations are expected to be presented as proximate mechanisms that interact with the environment in either a generally obligate or facultative fashion (see above). Evolutionary psychologists are also interested in identifying these proximate mechanisms (sometimes termed "mental mechanisms" or "psychological adaptations") and what type of information they take as input, how they process that information, and their outputs. Evolutionary developmental psychology, or "evo-devo," focuses on how adaptations may be activated at certain developmental times (e.g., losing baby teeth, adolescence, etc.) or how events during the development of an individual may alter life-history trajectories.
Evolutionary psychologists use several strategies to develop and test hypotheses about whether a psychological trait is likely to be an evolved adaptation. David Buss notes that these methods include:
Cross-cultural Consistency. Characteristics that have been demonstrated to be cross-cultural human universals, such as smiling, crying, and facial expressions, are presumed to be evolved psychological adaptations. Several evolutionary psychologists have collected massive datasets from cultures around the world to assess cross-cultural universality.
Function to Form (or "problem to solution"). The fact that males, but not females, risk potential misidentification of genetic offspring (referred to as "paternity uncertainty") led evolutionary psychologists to hypothesize that, compared to females, male jealousy would be more focused on sexual than on emotional infidelity.
Form to Function (reverse-engineering – or "solution to problem").Morning sickness and associated food aversions during pregnancy seemed to have the characteristics of an evolved adaptation (complexity and universality). Margie Profet hypothesized that the function was to avoid the ingestion of toxins during early pregnancy that could damage the fetus (but which are otherwise likely to be harmless to healthy non-pregnant women).
Corresponding Neurological Modules. Evolutionary psychology and cognitive neuropsychology are mutually compatible – evolutionary psychology helps to identify psychological adaptations and their ultimate, evolutionary functions, while neuropsychology helps to identify the proximate manifestations of these adaptations.
Current Evolutionary Adaptiveness. In addition to evolutionary models that suggest evolution occurs over long timescales, recent research has demonstrated that some evolutionary shifts can be fast and dramatic. Consequently, some evolutionary psychologists have focused on the impact of psychological traits in the current environment. Such research can inform estimates of trait prevalence over time. Such work has been informative in the study of evolutionary psychopathology.
Evolutionary psychologists also use various sources of data for testing, including experiments, archaeological records, data from hunter-gatherer societies, observational studies, neuroscience data, self-reports and surveys, public records, and human products. Recently, additional methods and tools have been introduced based on fictional scenarios, mathematical models, and multi-agent computer simulations.
Main areas of research
Foundational areas of research in evolutionary psychology can be divided into broad categories of adaptive problems that arise from evolutionary theory itself: survival, mating, parenting, family and kinship, interactions with non-kin, and cultural evolution.
Survival and individual-level psychological adaptations
Problems of survival are clear targets for the evolution of physical and psychological adaptations. Major problems the ancestors of present-day humans faced included food selection and acquisition; territory selection and physical shelter; and avoiding predators and other environmental threats.
Consciousness meets George Williams' criteria of species universality, complexity, and functionality, and it is a trait that apparently increases fitness.
In his paper "Evolution of consciousness," John Eccles argues that special anatomical and physical adaptations of the mammalian cerebral cortex gave rise to consciousness. In contrast, others have argued that the recursive circuitry underwriting consciousness is much more primitive, having evolved initially in pre-mammalian species because it improves the capacity for interaction with both social and natural environments by providing an energy-saving "neutral" gear in an otherwise energy-expensive motor output machine. Once in place, this recursive circuitry may well have provided a basis for the subsequent development of many of the functions that consciousness facilitates in higher organisms, as outlined by Bernard J. Baars. Richard Dawkins suggested that humans evolved consciousness in order to make themselves the subjects of thought. Daniel Povinelli suggests that large, tree-climbing apes evolved consciousness to account for their own mass when moving safely among tree branches. Consistent with this hypothesis, Gordon Gallup found that chimpanzees and orangutans, but not little monkeys or terrestrial gorillas, demonstrated self-awareness in mirror tests.
The concept of consciousness can refer to voluntary action, awareness, or wakefulness. However, even voluntary behavior involves unconscious mechanisms. Many cognitive processes take place in the cognitive unconscious, unavailable to conscious awareness. Some behaviors are conscious when learned but then become unconscious, seemingly automatic. Learning, especially implicitly learning a skill, can take place seemingly outside of consciousness. For example, plenty of people know how to turn right when they ride a bike, but very few can accurately explain how they actually do so.
Evolutionary psychology views self-deception as an adaptation that can improve one's results in social exchanges.
Sleep may have evolved to conserve energy when activity would be less fruitful or more dangerous, such as at night, and especially during the winter season.
Many experts, such as Jerry Fodor, write that the purpose of perception is knowledge, but evolutionary psychologists hold that its primary purpose is to guide action. For example, they say, depth perception seems to have evolved not to help us know the distances to other objects but rather to help us move around in space. Evolutionary psychologists say that animals from fiddler crabs to humans use eyesight for collision avoidance, suggesting that vision is basically for directing action, not providing knowledge.
Building and maintaining sense organs is metabolically expensive, so these organs evolve only when they improve an organism's fitness. More than half the brain is devoted to processing sensory information, and the brain itself consumes roughly one-fourth of one's metabolic resources, so the senses must provide exceptional benefits to fitness. Perception accurately mirrors the world; animals get useful, accurate information through their senses.
Scientists who study perception and sensation have long understood the human senses as adaptations to their surrounding worlds. Depth perception involves of processing over half a dozen visual cues, each of which is based on a regularity of the physical world. Vision evolved to respond to the narrow range of electromagnetic energy that is plentiful and passes through objects. Sound waves go around corners and interact with obstacles, creating a complex pattern that includes useful information about the sources of and distances to objects. Larger animals naturally make lower-pitched sounds as a consequence of their size. The range over which an animal hears, on the other hand, is determined by adaptation. Homing pigeons, for example, can hear the very low-pitched sound (infrasound) that carries great distances, even though most smaller animals detect higher-pitched sounds. Taste and smell respond to chemicals in the environment that are thought to have been significant for fitness in the environment of evolutionary adaptation. For example, salt and sugar were apparently both valuable to the human or pre-human inhabitants of the environment of evolutionary adaptations, so present-day humans have an intrinsic hunger for salty and sweet tastes. The sense of touch is actually many senses, including pressure, heat, cold, tickle, and pain. Pain, while unpleasant, is adaptive. An important adaptation for senses is range shifting, by which the organism becomes temporarily more or less sensitive to sensation. For example, one's eyes automatically adjust to dim or bright ambient light. Sensory abilities of different organisms often coevolve, as is the case with the hearing of echolocating bats and that of the moths that have evolved to respond to the sounds that the bats make.
Evolutionary psychologists contend that perception demonstrates the principle of modularity, with specialized mechanisms handling particular perception tasks. For example, people with damage to a particular part of the brain have the specific defect of not being able to recognize faces (prosopagnosia). Evolutionary psychology suggests that this indicates a so-called face-reading module.
Learning and facultative adaptations
In evolutionary psychology, learning is said to be accomplished through evolved capacities, specifically facultative adaptations. Facultative adaptations manifest differently in response to environmental input. Sometimes input comes during development and helps shape it. For example, migrating birds learn to orient themselves by the stars during a critical period in their maturation. Evolutionary psychologists believe that humans also learn language along an evolved program, with critical periods. The input can also come during daily tasks, helping the organism cope with changing environmental conditions. For example, animals evolved Pavlovian conditioning to solve problems involving causal relationships. Animals learn most easily when those tasks resemble problems that they faced in their evolutionary past, such as a rat learning where to find food or water. Learning capacities sometimes demonstrate differences between the sexes. In many animal species, for example, males can solve spatial problems faster and more accurately than females, due to the effects of male hormones during development. The same might be true of humans.
Motivations direct and energize behavior, while emotions provide the affective component to motivation, positive or negative. In the early 1970s, Paul Ekman and colleagues began a line of research that suggests that many emotions are universal. He found evidence that humans share at least five basic emotions: fear, sadness, happiness, anger, and disgust. Social emotions evidently evolved to motivate social behaviors that were adaptive in the environment of evolutionary adaptation. For example, spite seems to work against the individual, but it can establish an individual's reputation as someone to be feared. Shame and pride can motivate behaviors that help one maintain one's standing in a community, and self-esteem is one's estimate of one's status. Motivation has a neurobiological basis in the brain's reward system. Recently, it has been suggested that reward systems may evolve in such a way that there may be an inherent or unavoidable trade-off in the motivational system for activities of short versus long duration.
Cognition
Cognition refers to internal representations of the world and internal information processing. From an evolutionary psychology perspective, cognition is not "general purpose". Cognition uses heuristics, or strategies, that generally increase the likelihood of solving problems that the ancestors of present-day humans routinely faced. For example, present-day humans are far more likely to solve a logic problem that involves detecting cheating (a common problem given humans' social nature) than the same problem presented in purely abstract terms. Since the ancestors of present-day humans did not encounter truly random events and lived under simpler life conditions, present-day humans may be cognitively predisposed to misperceive patterns in random sequences. Gambler's fallacy is one example of this. Gamblers may falsely believe that they have hit a lucky streak even when each outcome is actually random and independent of previous trials. Many people assume that a coin flip sequence of nine Heads makes Tails more probable on the tenth try. Humans find it far easier to make diagnoses or predictions using frequency data than when the same information is presented as probabilities or percentages. Perhaps these evaluations are connected to ancient human groups residing in small tribes (typically fewer than 150 individuals), where frequency data was more prevalent and unexpected events were less common.
Personality
Evolutionary psychology is primarily interested in identifying commonalities between people, or the basic human psychological nature. From an evolutionary perspective, the fact that people have fundamental differences in personality traits initially presents something of a puzzle.
Personality traits are conceptualized by evolutionary psychologists as arising from normal variation around an optimum, due to frequency-dependent selection (behavioral polymorphisms), or as facultative adaptations. Like variability in height, some personality traits may simply reflect inter-individual variability around a general optimum. Or, personality traits may represent different genetically predisposed "behavioral morphs" – alternate behavioral strategies that depend on the frequency of competing behavioral strategies in the population. For example, if most of the population is generally trusting and gullible, the behavioral morph of being a "cheater" (or, in the extreme case, a sociopath) may be advantageous. Finally, like many other psychological adaptations, personality traits may be facultative – sensitive to typical variations in the social environment, especially during early development. For example, later-born children are more likely than firstborns to be rebellious, less conscientious, and more open to new experiences, which may be advantageous to them given their particular niche in family structure.
Shared environmental influences do play a role in personality and are not always of less importance than genetic factors. However, shared environmental influences often decrease to near zero after adolescence but do not completely disappear.
According to Steven Pinker, who builds on the work by Noam Chomsky, the universal human ability to learn to talk between the ages of 1–4, basically without training, suggests that language acquisition is a distinctly human psychological adaptation. Pinker and Bloom argue that language as a mental faculty shares many likenesses with the complex organs of the body, which suggests that, like these organs, language has evolved as an adaptation, since this is the only known mechanism by which such complex organs can develop.
Pinker follows Chomsky in arguing that the fact that children can learn any human language with no explicit instruction suggests that language, including most of grammar, is basically innate and that it only needs to be activated by interaction. Chomsky himself does not believe language to have evolved as an adaptation, but suggests that it likely evolved as a byproduct of some other adaptation, a so-called spandrel. But Pinker and Bloom argue that the organic nature of language strongly suggests an adaptive origin.
Evolutionary psychologists hold that the FOXP2 gene may well be associated with the evolution of human language. In the 1980s, psycholinguist Myrna Gopnik identified a dominant gene that causes language impairment in the KE family of Britain. This gene turned out to be a mutation of the FOXP2 gene. Humans have a unique allele of this gene, which has otherwise been closely conserved through most of mammalian evolutionary history. This unique allele appears to have first arisen between 100 and 200 thousand years ago, and it is now all but universal in humans. However, the once-popular idea that FOXP2 is a 'grammar gene' or that it triggered the emergence of language in Homo sapiens is now widely discredited.
Currently, several competing theories about the evolutionary origin of language coexist, none of them having achieved a general consensus. Researchers of language acquisition in primates and humans, such as Michael Tomasello and Talmy Givón, argue that the innatist framework has understated the role of imitation in learning and that it is not at all necessary to posit the existence of an innate grammar module to explain human language acquisition. Tomasello argues that studies of how children and primates actually acquire communicative skills suggest that humans learn complex behavior through experience, so that instead of a module specifically dedicated to language acquisition, language is acquired by the same cognitive mechanisms used to acquire all other kinds of socially transmitted behavior.
On the issue of whether language is best seen as having evolved as an adaptation or as a spandrel, evolutionary biologist W. Tecumseh Fitch, following Stephen J. Gould, argues that it is unwarranted to assume that every aspect of language is an adaptation, or that language as a whole is an adaptation. He criticizes some strands of evolutionary psychology for suggesting a pan-adaptationist view of evolution, and dismisses Pinker and Bloom's question of whether "Language has evolved as an adaptation" as being misleading. He argues instead that from a biological viewpoint, the evolutionary origins of language are best conceptualized as being the probable result of a convergence of many separate adaptations into a complex system. A similar argument is made by Terrence Deacon who in The Symbolic Species argues that the different features of language have co-evolved with the evolution of the mind and that the ability to use symbolic communication is integrated in all other cognitive processes.
If the theory that language could have evolved as a single adaptation is accepted, the question becomes which of its many functions has been the basis of adaptation. Several evolutionary hypotheses have been posited: that language evolved for social grooming, to signal mating potential, or to form social contracts. Evolutionary psychologists recognize that these theories are all speculative and that much more evidence is required to understand how language might have been selectively adapted.
Given that sexual reproduction is the means by which genes are propagated into future generations, sexual selection plays a large role in human evolution. Human mating, then, is of interest to evolutionary psychologists who aim to investigate evolved mechanisms for attracting and securing mates. Several lines of research have stemmed from this interest, such as studies of mate selection, mate poaching, mate retention, mating preferences, and conflict between the sexes.
In 1972, Robert Trivers published an influential paper on sex differences, which is now referred to as parental investment theory. The size differences of gametes (anisogamy) are the fundamental, defining difference between males (small gametes – sperm) and females (large gametes – ova). Trivers noted that anisogamy typically results in different levels of parental investment between the sexes, with females initially investing more. Trivers proposed that this difference in parental investment leads to the sexual selection of different reproductive strategies between the sexes and to sexual conflict. For example, he suggested that the sex that invests less in offspring will generally compete for access to the higher-investing sex to increase their inclusive fitness. Trivers posited that differential parental investment led to the evolution of sexual dimorphisms in mate choice, intra- and inter- sexual reproductive competition, and courtship displays. In mammals, including humans, females make a much larger parental investment than males (i.e., gestation followed by childbirth and lactation). Parental investment theory is a branch of life history theory.
Buss and Schmitt's 1993 sexual strategies theory proposed that, due to differential parental investment, humans have evolved sexually dimorphic adaptations related to "sexual accessibility, fertility assessment, commitment seeking and avoidance, immediate and enduring resource procurement, paternity certainty, assessment of mate value, and parental investment." Their strategic interference theory suggested that conflict between the sexes occurs when the preferred reproductive strategies of one sex interfere with those of the other sex, resulting in the activation of emotional responses such as anger or jealousy
Women are generally more selective when choosing mates, especially under long-term mating conditions. However, under some circumstances, engaging in multiple sexual relationships can provide benefits to women as well, such as fertility insurance, trading up to better genes, reducing the risk of inbreeding, and insurance protection of her offspring.
Due to male paternity uncertainty, sex differences have been found in the domains of sexual jealousy. Females generally react more adversely to emotional infidelity, and males will react more to sexual infidelity. This pattern is predicted because mating costs differ between the sexes. Women, on average, should prefer a mate who can offer resources (e.g., financial, commitment); thus, a woman risks losing such resources when a mate commits emotional infidelity. Men, on the other hand, are never certain of the genetic paternity of their children because they do not bear the offspring themselves. This suggests that for men, sexual infidelity would generally be more aversive than emotional infidelity because investing resources in another man's offspring does not lead to the propagation of their own genes.
Another interesting line of research examines women's mate preferences across the ovulatory cycle. The theoretical underpinning of this research is that ancestral women would have evolved mechanisms to select mates with certain traits depending on their hormonal status. Known as the ovulatory shift hypothesis, the theory posits that, during the ovulatory phase of a woman's cycle (approximately days 10–15 of a woman's cycle), a woman who mated with a male with high genetic quality would have been more likely, on average, to produce and bear a healthy offspring than a woman who mated with a male with low genetic quality. These putative preferences are expected to be especially apparent in short-term mating domains because a potential male mate would offer only genes to a potential offspring. This hypothesis allows researchers to examine whether women select mates with characteristics indicative of high genetic quality during the high fertility phase of their ovulatory cycles. Indeed, studies have shown that women's preferences vary across the ovulatory cycle. In particular, Haselton and Miller (2006) showed that highly fertile women prefer creative but poor men as short-term mates. Creativity may be a proxy for good genes. Research by Steven Gangestad in 2004 indicates that highly fertile women prefer men who display social presence and intrasexual competition; these traits may act as cues that help women predict which men may have, or be able to acquire, resources.
Reproduction inherently involves a cost for women, and men may also bear this cost. Individuals have limited time and resources to dedicate to their children, and this commitment can harm their own future health, survival, and reproductive potential. Parental investment is any parental expenditure (time, energy, etc.) that benefits one offspring at a cost to parents' ability to invest in other components of fitness. Components of fitness (Beatty 1992Components of fitness (Beatty 1992) include the well-being of existing offspring, parents' future reproduction, and inclusive fitness through aid to kin (Hamilton, 1964). Parental investment theory is a branch of life history theory.
The benefits of parental investment to the offspring are large and are associated with the effects on condition, growth, survival, and ultimately, on the reproductive success of the offspring. However, these benefits can come at the cost of the parent's future reproductive ability, e.g., increased risk of injury when defending offspring against predators, the loss of mating opportunities whilst rearing offspring, and an increase in the time to the next reproduction. Overall, parents are selected to maximize the difference between the benefits and the costs, and parental care will likely evolve when the benefits exceed the costs.
The Cinderella effect is an alleged high incidence of stepchildren being physically, emotionally, or sexually abused, neglected, murdered, or otherwise mistreated at the hands of their stepparents at significantly higher rates than their genetic counterparts. It takes its name from the fairy tale character Cinderella, who in the story was cruelly mistreated by her stepmother and stepsisters. Martin Daly and Margo Wilson noted in their 1999 book:
Evolutionary thinking led to the discovery of the most important risk factor for child homicide – the presence of a stepparent. Parental efforts and investments are valuable resources, and selection favors those parental psyches that allocate effort effectively to promote fitness. The adaptive problems that challenge parental decision-making include accurately identifying one's offspring, allocating resources among them with sensitivity to their needs and abilities, and converting parental investment into fitness increments.... Stepchildren were seldom or never so valuable to one's expected fitness as one's own offspring would be, and those parental psyches that were easily parasitized by just any appealing youngster must always have incurred a selective disadvantage.
—Martin Daly and Margo Wilson, The Truth about Cinderella: A Darwinian View of Parental Love,
However, they note that not all stepparents will "want" to abuse their partner's children, or that genetic parenthood is any insurance against abuse. They see step-parental care as primarily "mating effort" towards the genetic parent.
Inclusive fitness is the sum of an organism's classical fitness (how many of its own offspring it produces and supports) and the number of equivalents of its own offspring it can add to the population by supporting others. The first component is called classical fitness by Hamilton (1964).
From the gene's point of view, evolutionary success ultimately depends on leaving behind the maximum number of copies of itself in the population. Until 1964, it was generally believed that genes only achieved this by causing the individual to leave the maximum number of viable offspring. However, in 1964, W. D. Hamilton proved mathematically that, because close relatives of an organism share some identical genes, a gene can also increase its evolutionary success by promoting the reproduction and survival of these related or otherwise similar individuals. Hamilton concluded that this leads natural selection to favor organisms that would behave in ways that maximize their inclusive fitness. It is also true that natural selection favors behavior that maximizes personal fitness.
Hamilton's rule describes mathematically whether or not a gene for altruistic behavior will spread in a population:
rb > c
where
c is the reproductive cost to the altruist,
b is the reproductive benefit to the recipient of the altruistic behavior, and
r is the probability, above the population average, of the individuals sharing an altruistic gene – commonly viewed as "degree of relatedness".
The concept explains how natural selection can perpetuate altruism. If there is an "altruism gene" (or complex of genes) that influences an organism's behavior to be helpful and protective of relatives and their offspring, this behavior also increases the proportion of the altruism gene in the population, because relatives are likely to share genes with the altruist due to common descent. Altruists may also recognize altruistic behavior in unrelated individuals and be inclined to support them. As Dawkins points out in The Selfish Gene (Chapter 6) and The Extended Phenotype, this must be distinguished from the green-beard effect.
Although it is generally true that humans tend to be more altruistic toward their kin than toward non-kin, the relevant proximate mechanisms that mediate this cooperation have been debated (see kin recognition), with some arguing that kin status is determined primarily via social and cultural factors (such as co-residence, maternal association of sibs, etc.), while others have argued that kin recognition can also be mediated by biological factors such as facial resemblance and immunogenetic similarity of the major histocompatibility complex (MHC).
The interaction between these social and biological kin-recognition factors was discussed in the 2007 Nature article – The architecture of human kin detection.
Whatever the proximate mechanisms of kin recognition, there is substantial evidence that humans are generally more altruistic towards close genetic kin compared to genetic non-kin.
Interactions with non-kin / reciprocity
Although interactions with non-kin are generally less altruistic compared to those with kin, cooperation can be maintained with non-kin via mutually beneficial reciprocity as was proposed by Robert Trivers. If there are repeated encounters between the same two players in an evolutionary game in which each of them can choose either to "cooperate" or "defect", then a strategy of mutual cooperation may be favored even if it pays each player, in the short term, to defect when the other cooperates. Direct reciprocity can lead to the evolution of cooperation only if the probability, w, of another encounter between the same two individuals exceeds the cost-to-benefit ratio of the altruistic act:
w > c/b
Reciprocity can also be indirect if information about previous interactions is shared. Reputation enables the evolution of cooperation by indirect reciprocity. Natural selection favors strategies that base the decision to help on the recipient's reputation: studies show that people who are more helpful are more likely to receive help. The calculations of indirect reciprocity are complicated, and only a tiny fraction of this universe has been uncovered, but again, a simple rule has emerged. Indirect reciprocity can only promote cooperation if the probability, q, of knowing someone's reputation exceeds the cost-to-benefit ratio of the altruistic act:
q > c/b
One important problem with this explanation is that individuals may be able to evolve the capacity to obscure their reputations, reducing the probability, q, that it will be known.
Trivers argues that friendship and various social emotions evolved in order to manage reciprocity. Liking and disliking, he says, evolved to help present-day humans' ancestors form coalitions with others who reciprocated and to exclude those who did not reciprocate. Moral indignation may have evolved to prevent one's altruism from being exploited by cheaters, and gratitude may have motivated present-day humans' ancestors to reciprocate appropriately after benefiting from others' altruism. Likewise, present-day humans feel guilty when they fail to reciprocate. These social motivations match what evolutionary psychologists expect to see in adaptations that evolved to maximize the benefits and minimize the drawbacks of reciprocity.
According to evolutionary psychologists, humans possess psychological adaptations that developed to help us spot cheaters, also known as non-reciprocators.
In 1993, Robert Frank and his associates found that participants in a prisoner's dilemma scenario were often able to predict whether their partners would "cheat", based on a half-hour of unstructured social interaction. In a 1996 experiment, for example, Linda Mealey and her colleagues found that people were better at remembering faces when those faces were associated with stories about those individuals cheating (such as embezzling money from a church).
Humans may have an evolved set of psychological adaptations that predispose them to be more cooperative than otherwise would be expected with members of their tribal in-group, and nastier to members of tribal out-groups. These adaptations may have been a consequence of tribal warfare.
Humans may also have predispositions for "altruistic punishment" – to punish in-group members who violate in-group rules, even when this altruistic behavior cannot be justified in terms of helping those you are related to (kin selection), cooperating with those who you will interact with again (direct reciprocity), or cooperating to better your reputation with others (indirect reciprocity).
Though evolutionary psychology has traditionally focused on individual-level behaviors, determined by species-typical psychological adaptations, considerable work has examined how these adaptations shape and, ultimately, govern culture. In 1989, Tooby and Cosmides argued that the mind consists of many domain-specific psychological adaptations, some of which may constrain what cultural material is learned or taught. As opposed to a domain-general cultural acquisition program, where an individual passively receives culturally-transmitted material from the group,
Tooby and Cosmides, among others, argue that:
The psyche evolved to generate adaptive rather than repetitive behavior, and hence critically analyzes the behavior of those surrounding it in highly structured and patterned ways, to be used as a rich (but by no means the only) source of information out of which to construct a 'private culture' or individually tailored adaptive system; in consequence, this system may or may not mirror the behavior of others in any given respect. Tooby and Cosmides 1989.
Biological explanations of human culture also brought criticism to evolutionary psychology: Evolutionary psychologists see the human psyche and physiology as a genetic product and assume that genes contain the information for the development and control of the organism and that this information is transmitted from one generation to the next via genes. Evolutionary psychologists, therefore, view the physical and psychological characteristics of humans as genetically programmed. Even then, when evolutionary psychologists acknowledge the influence of the environment on human development, they understand the environment only as an activator or trigger for the programmed developmental instructions encoded in genes. Evolutionary psychologists, for example, believe that the human brain is composed of innate modules, each specialised for a specific task, e.g., an anxiety module. According to evolutionary psychologists, these modules are present before the organism actually develops and are then activated by some environmental event. Critics object that this view is reductionist and that cognitive specialisation arises only through the interaction of humans with their real environment, rather than the environment of distant ancestors. Interdisciplinary approaches are increasingly striving to mediate between these opposing points of view and to highlight that biological and cultural causes need not be antithetical to explain human behaviour, including complex cultural achievements.
According to Paul Baltes, the benefits granted by evolutionary selection decrease with age. Natural selection has not eliminated many harmful conditions and nonadaptive characteristics that appear among older adults, such as Alzheimer's disease. If it were a disease that killed 20-year-olds instead of 70-year-olds, this might have been a disease that natural selection could have eliminated ages ago. Thus, unaided by evolutionary pressures against non-adaptive conditions, modern humans suffer the aches, pains, and infirmities of aging, and as the benefits of evolutionary selection decrease with age, the need for modern technological mediums against non-adaptive conditions increases.
Social psychology
As humans are a highly social species, there are many adaptive problems associated with navigating the social world (e.g., maintaining allies, managing status hierarchies, interacting with outgroup members, coordinating social activities, collective decision-making). Researchers in the emerging field of evolutionary social psychology have made many discoveries pertaining to topics traditionally studied by social psychologists, including person perception, social cognition, attitudes, altruism, emotions, group dynamics, leadership, motivation, prejudice, intergroup relations, and cross-cultural differences.
When endeavouring to solve a problem, humans, at an early age, show determination, whereas chimpanzees do not exhibit a comparable facial expression. Researchers suspect the human-determined expression evolved because when a human is determinedly working on a problem, other people will frequently help.
Adaptationist hypotheses regarding the etiology of psychological disorders are often based on analogies between physiological and psychological dysfunctions, as noted in the table below. Prominent theorists and evolutionary psychiatrists include Michael T. McGuire, Anthony Stevens, and Randolph M. Nesse. They, and others, suggest that mental disorders are due to the interactive effects of both nature and nurture, and often have multiple contributing causes.
Possible causes of psychological abnormalities from an adaptationist perspective
Summary based on information contained in these textbooks
Causal mechanism of failure or malfunction of adaptation
Physiological Example
Hypothesized Psychological Example
Functioning adaptation (adaptive defense)
Fever / Vomiting (functional responses to infection or ingestion of toxins)
Mild depression or anxiety (functional responses to mild loss or stress/ reduction of social interactions to prevent infection by contagious pathogens)
By-product of an adaptation(s)
Intestinal gas (byproduct of digestion of fiber)
Sexual fetishes (?) (possible byproduct of normal sexual arousal adaptations that have 'imprinted' on unusual objects or situations)
Adaptations with multiple effects
Sickle cell disease (Gene that imparts malaria resistance, in homozygous form, causes sickle cell anemia)
Schizophrenia or bipolar disorder (May be side-effects of adaptations for high levels of creativity, perhaps dependent on alternate developmental trajectories)
Evolutionary psychologists have suggested that schizophrenia and bipolar disorder may reflect a side-effect of genes with fitness benefits, such as increased creativity. A 1994 report by the American Psychiatry Association found that the incidence of schizophrenia occurs at roughly the same rate in Western and non-Western cultures, and in industrialized and pastoral societies, suggesting that schizophrenia is not a disease of civilization nor an arbitrary social invention. Sociopathy may represent an evolutionarily stable strategy, by which a small number of people who cheat on social contracts benefit in a society consisting mostly of non-sociopaths Mild depression may be an adaptive response to withdraw from, and re-evaluate, situations that have led to disadvantageous outcomes (the "analytical rumination hypothesis") (see Evolutionary approaches to depression).
Trofimova reviewed the most consistent psychological and behavioural sex differences in psychological abilities and disabilities and linked them to Geodakyan's Evolutionary Theory of Sex (ETS). She pointed out that a pattern of consistent sex differences in physical, verbal, and social disabilities corresponds to the idea of the ETS, considering sex dimorphism as a functional specialization of a species.
Sex differentiation, according to the ETS, creates two partitions within a species:
conservational (females)
variational (males).
In females, superiority in verbal abilities, higher rule obedience, socialisation, empathy, and agreeableness can be seen as reflecting the systemic conservation function of the female sex. Men often excel in areas such as exploration, risk-taking, spatial abilities, physical strength, and aggression. This pattern, along with elevated birth and accidental death rates, could indicate the male sex's systematic variational function, which explores the limits of advantageous traits. This suggests that psychological sex differences are affected by a species' general drive to expand its adaptive flexibility while retaining valuable characteristics.
Moreover, Trofimova suggested a "redundancy pruning" hypothesis as an upgrade of the ETS theory. She pointed out higher rates of psychopathy, dyslexia, autism, and schizophrenia in males, in comparison to females. She suggested that the variational function of the "male partition" might also provide irrelevance/redundancy pruning of excesses in a bank of beneficial characteristics of a species, while continuing resistance to any changes from the norm-driven conservational partition of species. This might explain a contradictory allocation of a high drive for social status/power in the male sex, with their lower (among the two sexes) abilities for social interaction. The high rates of communicative disorders and psychopathy in males might facilitate their higher rates of disengagement from normative expectations and their insensitivity to social disapproval when they deliberately do not follow social norms.
Some of these speculations have yet to be developed into fully testable hypotheses, and much research is required to confirm their validity.
Evolutionary psychology has been applied to explain criminal or otherwise immoral behavior as being adaptive or related to adaptive behaviors. Males are generally more aggressive than females, who are more selective of their partners because of the far greater effort they have to contribute to pregnancy and child-rearing. Males' greater aggressiveness is hypothesized to stem from the more intense reproductive competition they faced. Males of low status may be especially vulnerable to being childless. It may have been evolutionarily advantageous to engage in highly risky, violent behavior to increase their status and therefore reproductive success. This may explain why males are generally involved in more crimes, and why low status and being unmarried are associated with criminality. Furthermore, competition over females is argued to have been particularly intensive in late adolescence and young adulthood, which is theorized to explain why crime rates are particularly high during this period. Some sociologists have underlined differential exposure to androgens as the cause of these behaviors, notably Lee Ellis in his evolutionary neuroandrogenic (ENA) theory.
Many conflicts that result in harm and death involve status, reputation, and seemingly trivial insults. Steven Pinker in his book The Better Angels of Our Nature argues that in non-state societies without a police it was very important to have a credible deterrence against aggression. Therefore, it was important to be perceived as having a credible reputation for retaliation, leading humans in developing instincts for revenge as well as for protecting reputation ("honor"). Pinker argues that the development of the state and the police has dramatically reduced the level of violence compared to the ancestral environment. Whenever the state breaks down, which can be very local, such as in poor areas of a city, humans again organize in groups for protection and aggression, and concepts such as violent revenge and protecting honor again become extremely important.
Rape is theorized to be a reproductive strategy that facilitates the propagation of the rapist's progeny. Such a strategy may be adopted by men who otherwise are unlikely to be appealing to women and therefore cannot form legitimate relationships, or by high-status men on socially vulnerable women who are unlikely to retaliate to increase their reproductive success even further. The sociobiological theories of rape are highly controversial, as traditional theories typically do not consider rape to be a behavioral adaptation, and objections to this theory are made on ethical, religious, political, as well as scientific grounds.
Adaptationist perspectives on religious belief suggest that, like all behavior, religious beliefs are a product of the human brain. As with all other organ functions, cognition's functional structure has been argued to have a genetic foundation and is therefore subject to the effects of natural selection and sexual selection. Like other organs and tissues, this functional structure should be universally shared amongst humans and should have solved important problems of survival and reproduction in ancestral environments. However, evolutionary psychologists remain divided on whether religious belief is more likely a consequence of evolved psychological adaptations or a byproduct of other cognitive adaptations.
Coalitional psychology
Coalitional psychology is an approach to explaining political behaviors among different coalitions and the conditionality of these behaviors in an evolutionary psychological perspective. This approach assumes that since human beings appeared on the earth, they have evolved to live in groups instead of living as individuals to achieve benefits such as more mating opportunities and increased status. Human beings thus naturally think and act in a way that manages and negotiates group dynamics.
Coalitional psychology offers falsifiable ex ante prediction by positing five hypotheses on how these psychological adaptations operate:
Humans represent groups as a special category of individual, unstable, and with a short shadow of the future
Political entrepreneurs strategically manipulate the coalitional environment, often appealing to emotional devices such as "outrage" to inspire collective action.
Critics of evolutionary psychology accuse it of promoting genetic determinism, pan-adaptationism (the idea that all behaviors and anatomical features are adaptations), unfalsifiable hypotheses, distal or ultimate explanations of behavior when proximate explanations are superior, and malevolent political or moral ideas.
Ethical implications
Critics have argued that evolutionary psychology might be used to justify existing social hierarchies and reactionary policies. It has also been suggested by critics that evolutionary psychologists' theories and interpretations of empirical data rely heavily on ideological assumptions about race and gender.
In response to such criticism, evolutionary psychologists often caution against committing the naturalistic fallacy – the assumption that "what is natural" is necessarily a moral good. However, their caution against committing the naturalistic fallacy has been criticized as a means to stifle legitimate ethical discussions.
Contradictions in models
Some criticisms of evolutionary psychology point to contradictions between different aspects of adaptive scenarios posited by evolutionary psychology. One example is evolutionary psychology's claim that extended social groups select for modern human brains. However, the synaptic function of modern human brains requires high amounts of many specific essential nutrients. Sharing this requirement among all individuals in a population would decrease the possibility of forming large groups, due to bottleneck foods with rare essential nutrients capping group sizes. Furthermore, it is noted that some insects have societies with distinct ranks for individuals, despite having smaller connectomes relative to humans. Also, monkeys remain socially functioning after the removal of most of their brains, which doesn't support the notion that big brains promote social networking.
The model of males as both providers and protectors is criticized for the impossibility of being in two places at once; the male cannot both protect his family at home and be out hunting at the same time. In the case of the claim that a provider male could buy protection service for his family from other males by bartering food that he had hunted, critics point at the fact that the most valuable food (the food that contained the rarest essential nutrients) would be different in different ecologies. As such, valuable foods may be primarily vegetable in some geographical areas and animal in others, making it impossible for hunting styles relying on physical strength or risk-taking to be universally of similar value.
A contradiction also exists between evolutionary psychology's claim that men need to assess a mate's sexual viability more than women. Further, the claim that male sexual jealousy guards against infidelity is also criticized, as it would be pointless for a male to be fast to assess female fertility if he needed to assess the risk of there being a jealous male nearby. In that case, his chances of defeating him before mating would have to be assessed.
Evolutionary psychology has been entangled in the broader philosophical and social-science controversies surrounding nature versus nurture debate. Evolutionary psychologists typically contrast evolutionary psychology with what they call the standard social science model (SSSM). They characterize the SSSM as the "blank slate", "relativist", "social constructionist", and "cultural determinist" perspective that dominated the social sciences throughout the 20th century and assumed that the mind was shaped almost entirely by culture. Critics have argued that evolutionary psychologists created a false dichotomy between their own view and the caricature of the SSSM.
Other critics regard the SSSM as a rhetorical device or a straw man and suggest that the scientists whom evolutionary psychologists associate with the SSSM did not believe that the mind was a blank slate devoid of any natural predispositions.
Reductionism and determinism
Some critics view evolutionary psychology as a form of genetic reductionism and genetic determinism, a common critique being that evolutionary psychology does not address the complexity of individual development and experience and fails to explain the influence of genes on behavior in individual cases. Evolutionary psychologists respond that they work within a nature-nurture interactionist framework that acknowledges that many psychological adaptations are facultative (sensitive to environmental variations during individual development). The discipline is generally not focused on proximate analyses of behavior; rather, it focuses on the study of distal/ultimate causality (the evolution of psychological adaptations). The field of behavioral genetics focuses on the proximate influence of genes on behavior.
Evolutionary psychology's scientific credibility suffers from common criticisms that its hypotheses are untestable and arbitrary. For example, many current traits likely evolved for purposes other than their current ones. Therefore, since infinite explanations could exist for a trait's evolution, critics argue that pinpointing the precise reason is impossible.
Although evolutionary psychology hypotheses are hard to verify, evolutionary psychologists maintain that verification is possible.
Testing ideas about the evolutionary origins of psychological phenomena is indeed a challenging task, but not an impossible one.
Part of the critique of the scientific base of evolutionary psychology includes a critique of the concept of the Environment of Evolutionary Adaptation (EEA). Some critics have argued that researchers know so little about the environment in which Homo sapiens evolved that explaining specific traits as an adaptation to that environment becomes highly speculative. Evolutionary psychologists respond that they do know many things about this environment, including that present-day humans' ancestors were hunter-gatherers, and that they generally lived in small tribes.
Edward Hagen argues that the human past environments were not radically different in the same sense as the Carboniferous or Jurassic periods and that the animal and plant taxa of the era were similar to those of the modern world, as was the geology and ecology. Hagen argues that few would deny that other organs evolved in the EEA (for example, lungs evolving in an oxygen-rich atmosphere), yet critics question whether the brain's EEA is truly knowable, which he argues constitutes selective scepticism. Hagen also argues that most evolutionary psychology research rests on the fact that females can get pregnant and males cannot, which Hagen observes was also true in the EEA.
John Alcock describes this as the "No Time Machine Argument": critics argue that since it is not possible to travel back in time to the EEA, then it cannot be determined what was going on there, and thus what was adaptive. Alcock argues that present-day evidence allows researchers to be reasonably confident about the conditions of the EEA and that the fact that so many human behaviours are adaptive in the current environment is evidence that the ancestral environment of humans had much in common with the present one, as these behaviours would have evolved in the ancestral environment. Thus, Alcock concludes that researchers can make predictions on the adaptive value of traits. Similarly, Dominic Murphy argues that alternative explanations cannot merely just be forwarded but instead need their own evidence and predictions – if one explanation makes predictions that the others cannot, it is reasonable to have confidence in that explanation. In addition, Murphy argues that other historical sciences also make predictions about future phenomena to develop explanations about past phenomena. For example, cosmologists look for evidence of what we would expect to see in modern times if the Big Bang were true, while geologists make predictions about modern phenomena to determine if an asteroid wiped out the dinosaurs. Murphy argues that if other historical disciplines can conduct tests without a time machine, then the onus is on the critics to show why evolutionary psychology is untestable if other historical disciplines are not, as "methods should be judged across the board, not singled out for ridicule in one context."
Evolutionary psychologists generally presume that, like the body, the mind is composed of many evolved modular adaptations, although there is some disagreement within the discipline regarding the degree of general plasticity, or "generality," of some modules. One suggestion for the evolution of modularity is that it offered greater fitness and reduced connection costs than non-modular networks.
In contrast, some academics argue that it is unnecessary to posit highly domain-specific modules and suggest that the brain's neural anatomy supports a model based on more domain-general faculties and processes. Moreover, empirical support for the domain-specific theory stems almost entirely from performance on variations of the Wason selection task, which is extremely limited in scope as it tests only one subtype of deductive reasoning.
Cultural rather than genetic development of cognitive tools
Psychologist Cecilia Heyes has argued that the picture presented by some evolutionary psychology of the human mind as a collection of cognitive instincts–organs of thought shaped by genetic evolution over very long time periods
–does not fit the research results. She posits instead that humans have cognitive gadgets–"special-purpose organs of thought" built in the course of development through social interaction. Similar criticisms are articulated by Subrena E. Smith of the University of New Hampshire.
Response by evolutionary psychologists
Evolutionary psychologists have addressed many of their critics (e.g., in books by Segerstråle, Barlow, and Alcock.). Among their rebuttals are that some criticisms are straw men, or are based on an incorrect nature versus nurture dichotomy, or on basic misunderstandings of the discipline.
Robert Kurzban suggested that "...critics of the field, when they err, are not slightly missing the mark. Their confusion is profound. It's not like they are marksmen who can't quite hit the center of the target; they're holding the gun backwards." Many have written specifically to correct basic misconceptions.
