Nature versus nurture

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The nature versus nurture debate involves whether human behaviour is determined by the environment, either prenatal or during a person's life, or by a person's genes. The alliterative expression "nature and nurture" in English has been in use since at least the Elizabethan period^[1] and goes back to medieval French.^[2] The combination of the two concepts as complementary is ancient (Greek: ἁπό φύσεως καὶ εὐτροφίας^[3]). Nature is what we think of as pre-wiring and is influenced by genetic inheritance and other biological factors. Nurture is generally taken as the influence of external factors after conception e.g. the product of exposure, experience and learning on an individual.^[4]

The phrase in its modern sense was popularized by the English Victorian polymath Francis Galton, the modern founder of eugenics and behavioral genetics, discussing the influence of heredity and environment on social advancement.^[5][6][7] Galton was influenced by the book On the Origin of Species written by his half-cousin, Charles Darwin.

The view that humans acquire all or almost all their behavioral traits from "nurture" was termed tabula rasa ("blank slate") by John Locke in 1690. A "blank slate view" in human developmental psychology assuming that human behavioral traits develop almost exclusively from environmental influences, was widely held during much of the 20th century (sometimes termed "blank-slatism"). The debate between "blank-slate" denial of the influence of heritability, and the view admitting both environmental and heritable traits, has often been cast in terms of nature versus nurture. These two conflicting approaches to human development were at the core of an ideological dispute over research agendas throughout the second half of the 20th century. As both "nature" and "nurture" factors were found to contribute substantially, often in an extricable manner, such views were seen as naive or outdated by most scholars of human development by the 2000s.^{[8][9][10][11][12][13]}

The strong dichotomy of nature versus nurture has thus been claimed to have limited relevance in some fields of research. Close feedback loops have been found in which "nature" and "nurture" influence one another constantly, as seen in self-domestication. In ecology and behavioral genetics, researchers think nurture has an essential influence on nature.^[14][15] Similarly in other fields, the dividing line between an inherited and an acquired trait becomes unclear, as in epigenetics^[16] or fetal development.^[17][18]

History of the debate

John Locke's An Essay Concerning Human Understanding (1690) is often cited as the foundational document of the "blank slate" view. Locke was criticizing René Descartes' claim of an innate idea of God universal to humanity. Locke's view was harshly criticized in his own time. Anthony Ashley-Cooper, 3rd Earl of Shaftesbury, complained that by denying the possibility of any innate ideas, Locke "threw all order and virtue out of the world", leading to total moral relativism. Locke's was not the predominant view in the 19th century, which on the contrary tended to focus on "instinct". Leda Cosmides and John Tooby noted that William James (1842–1910) argued that humans have more instincts than animals, and that greater freedom of action is the result of having more psychological instincts, not fewer.^[19]

The question of "innate ideas" or "instincts" were of some importance in the discussion of free will in moral philosophy. In 18th-century philosophy, this was cast in terms of "innate ideas" establishing the presence of a universal virtue, prerequisite for objective morals. In the 20th century, this argument was in a way inverted, as some philosophers now argued that the evolutionary origins of human behavioral traits forces us to concede that there is no foundation for ethics (J. L. Mackie), while others treat ethics as a field in complete isolation from evolutionary considerations (Thomas Nagel).^[20]

In the early 20th century, there was an increased interest in the role of the environment, as a reaction to the strong focus on pure heredity in the wake of the triumphal success of Darwin's theory of evolution.^[21]

During this time, the social sciences developed as the project of studying the influence of culture in clean isolation from questions related to "biology". Franz Boas's The Mind of Primitive Man (1911) established a program that would dominate American anthropology for the next fifteen years. In this study he established that in any given population, biology, language, material and symbolic culture, are autonomous; that each is an equally important dimension of human nature, but that no one of these dimensions is reducible to another.

The tool of twin studies was developed as an research design intended to exclude all confounders based on inherited behavioral traits.^[22] Such studies are designed to decompose the variability of a given trait in a given population into a genetic and an environmental component.

John B. Watson in the 1920s and 1930s established the school of purist behaviorism that would become dominant over the following decades. Watson was convinced of the complete dominance of cultural influence over anything that heredity might contribute, to the point of claiming

"Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select – doctor, lawyer, artist, merchant-chief and, yes, even beggar-man and thief, regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors" (Behaviorism, 1930, p. 82).

During the 1940s to 1960s, Ashley Montagu was a notable proponent of this purist form of behaviorism which allowed no contribution from heredity whatsoever:

"Man is man because he has no instincts, because everything he is and has become he has learned, acquired, from his culture ... with the exception of the instinctoid reactions in infants to sudden withdrawals of support and to sudden loud noises, the human being is entirely instinctless."^[23]

In 1951, Calvin Hall^[24] suggested that the dichotomy opposing nature to nurture is ultimately fruitless.

Robert Ardrey in the 1960s argued for innate attributes of human nature, especially concerning territoriality, in the widely read African Genesis (1961) and The Territorial Imperative. Desmond Morris in The Naked Ape (1967) expressed similar views. Organised opposition to Montagu's kind of purist "blank-slatism" began to pick up in the 1970s, notably led by E. O. Wilson (On Human Nature 1979). Twin studies established that there was, in many cases, a significant heritable component. These results did not in any way point to overwhelming contribution of heritable factors, with heritability typically ranging around 40% to 50%, so that the controversy may not be cast in terms of purist behaviorism vs. purist nativism. Rather, it was purist behaviorism which was gradually replaced by the now-predominant view that both kinds of factors usually contribute to a given trait, anecdotally phrased by Donald Hebb as an answer to the question "which, nature or nurture, contributes more to personality?" by asking in response, "Which contributes more to the area of a rectangle, its length or its width?"^[25] In a comparable avenue of research, anthropologist Donald Brown in the 1980s surveyed hundreds of anthropological studies from around the world and collected a set of cultural universals. He identified approximately 150 such features, coming to the conclusion there is indeed a "universal human nature", and that these features point to what that universal human nature is.^[26]

At the height of the controversy, during the 1970s to 1980s, the debate was highly ideologised. In Not in Our Genes: Biology, Ideology and Human Nature (1984), Richard Lewontin, Steven Rose and Leon Kamin criticise "genetic determinism" from a Marxist framework, arguing that "Science is the ultimate legitimator of bourgeois ideology ... If biological determinism is a weapon in the struggle between classes, then the universities are weapons factories, and their teaching and research faculties are the engineers, designers, and production workers." The debate thus shifted away from whether heritable traits exist to whether it was politically or ethically permissible to admit their existence. The authors deny this, requesting that that evolutionary inclinations could be discarded in ethical and political discussions regardless of whether they exist or not.^[27]

Heritability studies became much easier to perform, and hence much more numerous, with the advances of genetic studies during the 1990s. By the late 1990s, an overwhelming amount of evidence had accumulated that amounts to a refutation of the extreme forms of "blank-slatism" advocated by Watson or Montagu.

This revised state of affairs was summarized in books aimed at a popular audience from the late 1990s. In The Nurture Assumption: Why Children Turn Out the Way They Do (1998), Judith Rich Harris was heralded by Steven Pinker as a book that "will come to be seen as a turning point in the history of psychology".^[28] but Harris was criticized for exaggerating the point of "parental upbringing seems to matter less than previously thought" to the implication that "parents do not matter".^[29]

The situation as it presented itself by the end of the 20th century was summarized in The Blank Slate: The Modern Denial of Human Nature (2002) by Steven Pinker. The book became a best-seller, and was instrumental in bringing to the attention of a wider public the paradigm shift away from the behaviourist purism of the 1940s to 1970s that had taken place over the preceding decades. Pinker portrays the adherence to pure blank-slatism as an ideological dogma linked to two other dogmas found in the dominant view of human nature in the 20th century, which he termed "noble savage" (in the sense that people are born good and corrupted by bad influence) and "ghost in the machine" (in the sense that there is a human soul capable of moral choices completely detached from biology). Pinker argues that all three dogmas were held onto for an extended period even in the face of evidence because they were seen as desirable in the sense that if any human trait is purely conditioned by culture, any undesired trait (such as crime or aggression) may be engineered away by purely cultural (political means). Pinker focuses on reasons he assumes were responsible for unduly repressing evidence to the contrary, notably the fear of (imagined or projected) political or ideological consequences.^[30]

Heritability estimates

This chart illustrates three patterns one might see when studying the influence of genes and environment on traits in individuals. Trait A shows a high sibling correlation, but little heritability (i.e. high shared environmental variance c²; low heritability h²). Trait B shows a high heritability since correlation of trait rises sharply with degree of genetic similarity. Trait C shows low heritability, but also low correlations generally; this means Trait C has a high nonshared environmental variance e². In other words, the degree to which individuals display Trait C has little to do with either genes or broadly predictable environmental factors—roughly, the outcome approaches random for an individual. Notice also that even identical twins raised in a common family rarely show 100% trait correlation.

It is important to note that the term heritability refers only to the degree of genetic variation between people on a trait. It does not refer to the degree to which a trait of a particular individual is due to environmental or genetic factors. The traits of an individual are always a complex interweaving of both.^[31] For an individual, even strongly genetically influenced, or "obligate" traits, such as eye color, assume the inputs of a typical environment during ontogenetic development (e.g., certain ranges of temperatures, oxygen levels, etc.).

In contrast, the "heritability index" statistically quantifies the extent to which variation between individuals on a trait is due to variation in the genes those individuals carry. In animals where breeding and environments can be controlled experimentally, heritability can be determined relatively easily. Such experiments would be unethical for human research. This problem can be overcome by finding existing populations of humans that reflect the experimental setting the researcher wishes to create.

One way to determine the contribution of genes and environment to a trait is to study twins. In one kind of study, identical twins reared apart are compared to randomly selected pairs of people. The twins share identical genes, but different family environments. In another kind of twin study, identical twins reared together (who share family environment and genes) are compared to fraternal twins reared together (who also share family environment but only share half their genes). Another condition that permits the disassociation of genes and environment is adoption. In one kind of adoption study, biological siblings reared together (who share the same family environment and half their genes) are compared to adoptive siblings (who share their family environment but none of their genes).

In many cases, it has been found that genes make a substantial contribution, including psychological traits such as intelligence and personality.^[32] Yet heritability may differ in other circumstances, for instance environmental deprivation. Examples of low, medium, and high heritability traits include:

Low heritability	Medium heritability	High heritability
Specific language	Weight	Blood type
Specific religion	Religiosity	Eye color

Twin and adoption studies have their methodological limits. For example, both are limited to the range of environments and genes which they sample. Almost all of these studies are conducted in Western, first-world countries, and therefore cannot be extrapolated globally to include poorer, non-western populations. Additionally, both types of studies depend on particular assumptions, such as the equal environments assumption in the case of twin studies, and the lack of pre-adoptive effects in the case of adoption studies.

Since the definition of "nature" in this context is tied to "heritability", the definition of "nurture" has necessarily become very wide, including any type of causality that is not heritable. The term has thus moved away from its original connotation of "cultural influences" to include all effects of the environment, including; indeed, a substantial source of environmental input to human nature may arise from stochastic variations in prenatal development and is thus in no sense of the term "cultural".^[33][34]

Interaction of genes and environment

Heritability refers to the origins of differences between people. Individual development, even of highly heritable traits, such as eye color, depends on a range of environmental factors, from the other genes in the organism, to physical variables such as temperature, oxygen levels etc. during its development or ontogenesis.

The variability of trait can be meaningfully spoken of as being due in certain proportions to genetic differences ("nature"), or environments ("nurture"). For highly penetrant Mendelian genetic disorders such as Huntington's disease virtually all the incidence of the disease is due to genetic differences. Huntington's animal models live much longer or shorter lives depending on how they are cared for^{[citation needed]}.

At the other extreme, traits such as native language are environmentally determined: linguists have found that any child (if capable of learning a language at all) can learn any human language with equal facility.^[35] With virtually all biological and psychological traits, however, genes and environment work in concert, communicating back and forth to create the individual.

At a molecular level, genes interact with signals from other genes and from the environment. While there are many thousands of single-gene-locus traits, so-called complex traits are due to the additive effects of many (often hundreds) of small gene effects. A good example of this is height, where variance appears to be spread across many hundreds of loci.^[36]

Extreme genetic or environmental conditions can predominate in rare circumstances—if a child is born mute due to a genetic mutation, it will not learn to speak any language regardless of the environment; similarly, someone who is practically certain to eventually develop Huntington's disease according to their genotype may die in an unrelated accident (an environmental event) long before the disease will manifest itself.

The "two buckets" view of heritability.

More realistic "homogenous mudpie" view of heritability.

Steven Pinker likewise described several examples:^{[37] [38]}

concrete behavioral traits that patently depend on content provided by the home or culture—which language one speaks, which religion one practices, which political party one supports—are not heritable at all. But traits that reflect the underlying talents and temperaments—how proficient with language a person is, how religious, how liberal or conservative—are partially heritable.

When traits are determined by a complex interaction of genotype and environment it is possible to measure the heritability of a trait within a population. However, many non-scientists who encounter a report of a trait having a certain percentage heritability imagine non-interactional, additive contributions of genes and environment to the trait. As an analogy, some laypeople may think of the degree of a trait being made up of two "buckets," genes and environment, each able to hold a certain capacity of the trait. But even for intermediate heritabilities, a trait is always shaped by both genetic dispositions and the environments in which people develop, merely with greater and lesser plasticities associated with these heritability measures.

Heritability measures always refer to the degree of variation between individuals in a population. That is, as these statistics cannot be applied at the level of the individual, it would be incorrect to say that while the heritability index of personality is about 0.6, 60% of one's personality is obtained from one's parents and 40% from the environment. To help to understand this, imagine that all humans were genetic clones. The heritability index for all traits would be zero (all variability between clonal individuals must be due to environmental factors). And, contrary to erroneous interpretations of the heritability index, as societies become more egalitarian (everyone has more similar experiences) the heritability index goes up (as environments become more similar, variability between individuals is due more to genetic factors).

One should also take into account the fact that the variables of heritability and environmentality are not precise and vary within a chosen population and across cultures. It would be more accurate to state that the degree of heritability and environmentality is measured in its reference to a particular phenotype in a chosen group of a population in a given period of time. The accuracy of the calculations is further hindered by the number of coefficients taken into consideration, age being one such variable. The display of the influence of heritability and environmentality differs drastically across age groups: the older the studied age is, the more noticeable the heritability factor becomes, the younger the test subjects are, the more likely it is to show signs of strong influence of the environmental factors.

Some have pointed out that environmental inputs affect the expression of genes^[16] (see the article on epigenetics). This is one explanation of how environment can influence the extent to which a genetic disposition will actually manifest.^{[citation needed]} The interactions of genes with environment, called gene–environment interactions, are another component of the nature–nurture debate. A classic example of gene–environment interaction is the ability of a diet low in the amino acid phenylalanine to partially suppress the genetic disease phenylketonuria. Yet another complication to the nature–nurture debate is the existence of gene-environment correlations. These correlations indicate that individuals with certain genotypes are more likely to find themselves in certain environments. Thus, it appears that genes can shape (the selection or creation of) environments. Even using experiments like those described above, it can be very difficult to determine convincingly the relative contribution of genes and environment.

A study conducted by T.J. Bouchard, Jr. showed data that has been evidence for the importance of genes when testing middle-aged twins reared together and reared apart. The results shown have been important evidence against the importance of environment when determining, happiness, for example. In the Minnesota study of twins reared apart, it was actually found that there was higher correlation for monozygotic twins reared apart (0.52)than monozygotic twins reared together (0.44). Also, highlighting the importance of genes, these correlations found much higher correlation among monozygotic than dizygotic twins that had a correlation of 0.08 when reared together and −0.02 when reared apart.^[39]

Social pre-wiring

The social pre-wiring hypothesis refers to the ontogeny of social interaction. Also informally referred to as, "wired to be social." The theory questions whether there is a propensity to socially oriented action already present before birth. Research in the theory concludes that newborns are born into the world with a unique genetic wiring to be social^[40].

Circumstantial evidence supporting the social pre-wiring hypothesis can be revealed when examining newborns' behavior. Newborns, not even hours after birth, have been found to display a preparedness for social interaction. This preparedness is expressed in ways such as their imitation of facial gestures. This observed behavior cannot be contributed to any current form of socialization or social construction. Rather, newborns most likely inherit to some extent social behavior and identity through genetics^[40].

Principal evidence of this theory is uncovered by examining Twin pregnancies. The main argument is, if there are social behaviors that are inherited and developed before birth, then one should expect twin foetuses to engage in some form of social interaction before they are born. Thus, ten foetuses were analyzed over a period of time using ultrasound techniques. Using kinematic analysis, the results of the experiment were that the twin foetuses would interact with each other for longer periods and more often as the pregnancies went on. Researchers were able to conclude that the performance of movements between the co-twins were not accidental but specifically aimed^[40].

The social pre-wiring hypothesis was proved correct, "The central advance of this study is the demonstration that 'social actions' are already performed in the second trimester of gestation. Starting from the 14th week of gestation twin foetuses plan and execute movements specifically aimed at the co-twin. These findings force us to predate the emergence of social behavior: when the context enables it, as in the case of twin foetuses, other-directed actions are not only possible but predominant over self-directed actions."^[40].

Obligate vs. facultative adaptations

Traits may be considered to be adaptations (such as the umbilical cord), byproducts of adaptations (the belly button) or due to random variation (convex or concave belly button shape).^[41] An alternative to contrasting nature and nurture focuses on "obligate vs. facultative" adaptations.^[41] Adaptations may be generally more obligate (robust in the face of typical environmental variation) or more facultative (sensitive to typical environmental variation). For example, the rewarding sweet taste of sugar and the pain of bodily injury are obligate psychological adaptations—typical environmental variability during development does not much affect their operation.^[42] On the other hand, facultative adaptations are somewhat like "if-then" statements.^[43] An example of a facultative psychological adaptation may be adult attachment style. The attachment style of adults, (for example, a "secure attachment style," the propensity to develop close, trusting bonds with others) is proposed to be conditional on whether an individual's early childhood caregivers could be trusted to provide reliable assistance and attention. An example of a facultative physiological adaptation is tanning of skin on exposure to sunlight (to prevent skin damage).

Advanced techniques

Quantitative studies of heritable traits throw light on the question.

Developmental genetic analysis examines the effects of genes over the course of a human lifespan. Early studies of intelligence, which mostly examined young children, found that heritability measured 40–50%. Subsequent developmental genetic analyses found that variance attributable to additive environmental effects is less apparent in older individuals,^[44][45][46] with estimated heritability of IQ increasing in adulthood.

Multivariate genetic analysis examines the genetic contribution to several traits that vary together. For example, multivariate genetic analysis has demonstrated that the genetic determinants of all specific cognitive abilities (e.g., memory, spatial reasoning, processing speed) overlap greatly, such that the genes associated with any specific cognitive ability will affect all others. Similarly, multivariate genetic analysis has found that genes that affect scholastic achievement completely overlap with the genes that affect cognitive ability.

Extremes analysis examines the link between normal and pathological traits. For example, it is hypothesized that a given behavioral disorder may represent an extreme of a continuous distribution of a normal behavior and hence an extreme of a continuous distribution of genetic and environmental variation. Depression, phobias, and reading disabilities have been examined in this context.

For a few highly heritable traits, studies have identified loci associated with variance in that trait, for instance in some individuals with schizophrenia.^[47]

Entrepreneurship

Through studies of identical twins separated at birth, one-third of their creative thinking abilities come from genetics and two-thirds come from learning.^[48] Research suggests that between 37 and 42 percent of the explained variance can be attributed to genetic factors.^[49] The learning primarily comes in the form of human capital transfers of entrepreneurial skills through parental role modeling.^[50] Other findings agree that the key to innovative entrepreneurial success comes from environmental factors and working “10,000 hours” to gain mastery in entrepreneurial skills.^[51]

Heritability of intelligence

Evidence from behavioral genetic research suggests that family environmental factors may have an effect upon childhood IQ, accounting for up to a quarter of the variance. The American Psychological Association's report "Intelligence: Knowns and Unknowns" (1995) states that there is no doubt that normal child development requires a certain minimum level of responsible care. Here, environment is playing a role in what is believed to be fully genetic (intelligence) but it was found that severely deprived, neglectful, or abusive environments have highly negative effects on many aspects of children's intellect development. Beyond that minimum, however, the role of family experience is in serious dispute. On the other hand, by late adolescence this correlation disappears, such that adoptive siblings no longer have similar IQ scores.^[52]

Moreover, adoption studies indicate that, by adulthood, adoptive siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: monozygotic (identical) twins raised separately are highly similar in IQ (0.74), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adoptive siblings (~0.0).^[53] Recent adoption studies also found that supportive parents can have a positive effect on the development of their children.^[54]

Personality traits

Personality is a frequently cited example of a heritable trait that has been studied in twins and adoptees using behavioral genetic study designs. The most famous categorical organization of heritable personality traits were created by Goldberg (1990) in which he had college students rate their personalities on 1400 dimensions to begin, and then narrowed these down into "The Big Five" factors of personality—Openness, conscientiousness, extraversion, agreeableness, and neuroticism. The close genetic relationship between positive personality traits and, for example, our happiness traits are the mirror images of comorbidity in psychopathology. These personality factors were consistent across cultures, and many studies have also tested the heritability of these traits.

Identical twins reared apart are far more similar in personality than randomly selected pairs of people. Likewise, identical twins are more similar than fraternal twins. Also, biological siblings are more similar in personality than adoptive siblings. Each observation suggests that personality is heritable to a certain extent. A supporting article had focused on the heritability of personality (which is estimated to be around 50% for subjective well-being) in which a study was conducted using a representative sample of 973 twin pairs to test the heritable differences in subjective well-being which were found to be fully accounted for by the genetic model of the Five-Factor Model’s personality domains.^[55] However, these same study designs allow for the examination of environment as well as genes.

Adoption studies also directly measure the strength of shared family effects. Adopted siblings share only family environment. Most adoption studies indicate that by adulthood the personalities of adopted siblings are little or no more similar than random pairs of strangers. This would mean that shared family effects on personality are zero by adulthood.

In the case of personality traits, non-shared environmental effects are often found to out-weigh shared environmental effects. That is, environmental effects that are typically thought to be life-shaping (such as family life) may have less of an impact than non-shared effects, which are harder to identify. One possible source of non-shared effects is the environment of pre-natal development. Random variations in the genetic program of development may be a substantial source of non-shared environment. These results suggest that "nurture" may not be the predominant factor in "environment". Environment and our situations, do in fact impact our lives, but not the way in which we would typically react to these environmental factors. We are preset with personality traits that are the basis for how we would react to situations. An example would be how extraverted prisoners become less happy than introverted prisoners and would react to their incarceration more negatively due to their preset extraverted personality.^{[31]:Ch 19} Behavioral genes are somewhat proven to exist when we take a look at fraternal twins. When fraternal twins are reared apart, they show the same similarities in behavior and response as if they have been reared together.^[56]

Genetics

Genomics

The relationship between personality and people's own well-being is influenced and mediated by genes (Weiss, Bates, & Luciano, 2008). There has been found to be a stable set point for happiness that is characteristic of the individual (largely determined by the individual's genes). Happiness fluctuates around that setpoint (again, genetically determined) based on whether good things or bad things are happening to us ("nurture"), but only fluctuates in small magnitude in a normal human. The midpoint of these fluctuations is determined by the "great genetic lottery" that people are born with, which leads them to conclude that how happy they may feel at the moment or over time is simply due to the luck of the draw, or gene. This fluctuation was also not due to educational attainment, which only accounted for less than 2% of the variance in well-being for women, and less than 1% of the variance for men.^[39]

They consider that the individualities measured together with personality tests remain steady throughout an individual’s lifespan. They further believe that human beings may refine their forms or personality but can never change them entirely. Darwin's Theory of Evolution steered naturalists such as George Williams and William Hamilton to the concept of personality evolution. They suggested that physical organs and also personality is a product of natural selection.^[57]

With the advent of genomic sequencing, it has become possible to search for and identify specific gene polymorphisms that affect traits such as IQ and personality. These techniques work by tracking the association of differences in a trait of interest with differences in specific molecular markers or functional variants. An example of a visible human trait for which the precise genetic basis of differences are relatively well known is eye color. For traits with many genes affecting the outcome, a smaller portion of the variance is currently understood: For instance for height known gene variants account for around 5–10% of height variance at present.^{[citation needed]} When discussing the significant role of genetic heritability in relation to one's level of happiness, it has been found that from 44% to 52% of the variance in one's well-being is associated with genetic variation. Based on the retest of smaller samples of twins studies after 4,5, and 10 years, it is estimated that the heritability of the genetic stable component of subjective well-being approaches 80%.^[39] Other studies that have found that genes are a large influence in the variance found in happiness measures, exactly around 35–50%.^{[58][59][60][61]}

In contrast to views developed in 60's that gender identity is primarily learned (which led to policy-based surgical sex changed in children such as David Reimer), genomics has provided solid evidence that both sex and gender identities are primarily influenced by genes:

It is now clear that genes are vastly more influential than virtually any other force in shaping sex identity and gender identity…[T]he growing consensus in medicine is that…children should be assigned to their chromosomal (i.e., genetic) sex regardless of anatomical variations and differences—with the option of switching, if desired, later in life.

— Siddhartha Mukherjee, The Gene: An Intimate History

Linkage and association studies

In their attempts to locate the genes responsible for configuring certain phenotypes, researches resort to two different techniques. Linkage study facilitates the process of determining a specific location in which a gene of interested is located. This methodology is applied only among individuals that are related and does not serve to pinpoint specific genes. It does, however, narrow down the area of search, making it easier to locate one or several genes in the genome which constitute a specific trait.

Association studies, on the other hand, are more hypothetic and seek to verify whether a particular genetic variable really influences the phenotype of interest. In association studies it is more common to use case-control approach, comparing the subject with relatively higher or lower hereditary determinants with the control subject.

Behavioural genetics

From Wikipedia, the free encyclopedia

Behavioural genetics also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" connotes a focus on genetic influences, the field broadly investigates genetic and environmental influences, using research designs that allow removal of the confounding of genes and environment. Behavioural genetics was founded as a scientific discipline by Francis Galton in the late 19th century, only to be discredited through association with eugenics movements before and during World War II. In the latter half of the 20th century, the field saw renewed prominence with research on inheritance of behaviour and mental illness in humans (typically using twin and family studies), as well as research on genetically informative model organisms through selective breeding and crosses. In the late 20th and early 21st centuries, technological advances in molecular genetics made it possible to measure and modify the genome directly. This led to major advances in model organism research (e.g., knockout mice) and in human studies (e.g., genome-wide association studies), leading to new scientific discoveries.

Findings from behavioural genetic research have broadly impacted modern understanding of the role of genetic and environmental influences on behaviour. These include evidence that nearly all researched behaviors are under a significant degree of genetic influence, and that influence tends to increase as individuals develop into adulthood. Further, most researched human behaviours are influenced by a very large number of genes and the individual effects of these genes are very small. Environmental influences also play a strong role, but they tend to make family members more different from one another, not more similar.

History

Farmers with wheat and cattle - Ancient Egyptian art 1,422 BCE displaying domesticated animals.

Selective breeding and the domestication of animals is perhaps the earliest evidence that humans considered the idea that individual differences in behaviour could be due to natural causes.^[1] Plato and Aristotle each speculated on the basis and mechanisms of inheritance of behavioural characteristics.^[2] Plato, for example, argued in The Republic that selective breeding among the citizenry to encourage the development of some traits and discourage others, what today might be called eugenics, was to be encouraged in the pursuit of an ideal society.^[2][3] Behavioural genetic concepts also existed during the English renaissance, where William Shakespeare perhaps first coined the terms "nature" versus "nurture" in The Tempest, where he wrote in Act IV, Scene I, that Caliban was "A devil, a born devil, on whose nature Nurture can never stick".^[3][4]

Modern-day behavioural genetics began with Sir Francis Galton, a nineteenth-century intellectual and cousin of Charles Darwin.^[3] Galton was a polymath who studied many subjects, including the heritability of human abilities and mental characteristics. One of Galton's investigations involved a large pedigree study of social and intellectual achievement in the English upper class. In 1869, 10 years after Darwin's Origin of the species, Galton published his results in Hereditary Genius.^[5] In this work, Galton found that the rate of "eminence" was highest among close relatives of eminent individuals, and decreased as the degree of relationship to eminent individuals decreased. While Galton could not rule out the role of environmental influences on eminence, a fact which he acknowledged, the study served to initiate an important debate about the relative roles of genes and environment on behavioural characteristics. Through his work, Galton also "introduced multivariate analysis and paved the way towards modern Bayesian statistics" that are used throughout the sciences—launching what has been dubbed the "Statistical Enlightenment".^[6]

Galton in his later years

The field of behavioural genetics, as founded by Galton, was ultimately undermined by another of Galton's intellectual contributions, the founding of the eugenics movement in 20th century society.^[3] The primary idea behind eugenics was to use selective breeding combined with knowledge about the inheritance of behaviour to improve the human species.^[3] The eugenics movement was subsequently discredited by scientific corruption and genocidal actions in Nazi Germany. Behavioural genetics was thereby discredited through its association to eugenics.^[3] The field once again gained status as a distinct scientific discipline through the publication of early texts on behavioural genetics, such as Calvin S. Hall's 1951 book chapter on behavioural genetics, in which he introduced the term "psychogenetics",^[7] which enjoyed some limited popularity in the 1960s and 1970s.^[8][9] However, it eventually disappeared from usage in favour of "behaviour genetics".

The start of behavior genetics as a well-identified field was marked by the publication in 1960 of the book Behavior Genetics by John L. Fuller and William Robert (Bob) Thompson.^[1][10] It is widely accepted now that many if not most behaviours in animals and humans are under significant genetic influence, although the extent of genetic influence for any particular trait can differ widely.^[11][12] A decade later, in February 1970, the first issue of the journal Behavior Genetics was published and in 1972 the Behavior Genetics Association was formed with Theodosius Dobzhansky elected as the association's first president. The field has since grown and diversified, touching many scientific disciplines.^[3][13]

Methods

The primary goal of behavioural genetics is to investigate the nature and origins of individual differences in behaviour.^[3] A wide variety of different methodological approaches are used in behavioral genetic research,^[14] only a few of which are outlined below.

Animal studies

In animal research selection experiments have often been employed. For example, laboratory house mice have been bred for open-field behaviour,^[15] thermoregulatory nesting,^[16] and voluntary wheel-running behaviour.^[17] A range of methods in these designs are covered on those pages.

Behavioural geneticists using model organisms employ a range of molecular techniques to alter, insert, or delete genes. These techniques include knockouts, floxing, gene knockdown, or genome editing using methods like CRISPR-Cas9.^[18] These techniques allow behavioural geneticists different levels of control in the model organism's genome, to evaluate the molecular, physiological, or behavioural outcome of genetic changes.^[19]

Twin and family studies

Pedigree chart showing an inheritance pattern consistent with autosomal dominant transmission. Behavioural geneticists have used pedigree studies to investigate the genetic and environmental basis of behaviour.

One research design used in behavioural genetic research are variations on family designs (also known as pedigree designs), including twin studies and adoption studies.^[14] Quantitative genetic modelling of individuals with known genetic relationships (e.g., parent-child, sibling, dizygotic and monozygotic twins) allows one to estimate to what extent genes and environment contribute to phenotypic differences among individuals.^[20] The basic intuition of the twin study is that monozygotic twins share 100% of their genome and dizygotic twins share, on average, 50% of their segregating genome. Thus, differences between the two members of a monozygotic twin pair can only be due to differences in their environment, whereas dizygotic twins will differ from one another due to environment as well as genes. Under this simplistic model, if dizygotic twins differ more than monozygotic twins it can only be attributable to genetic influences. An important assumption of the twin model is the equal environment assumption^[21] that monozygotic twins have the same shared environmental experiences as dizygotic twins. If, for example, monozygotic twins tend to have more similar experiences than dizygotic twins—and these experiences themselves are not genetically mediated through gene-environment correlation mechanisms—then monozygotic twins will tend to be more similar to one another than dizygotic twins for reasons that have nothing to do with genes.^[22]

Twin studies of monozygotic and dizygotic twins use a biometrical formulation to describe the influences on twin similarity and to infer heritability.^[20][23] The formulation rests on the basic observation that the variance in a phenotype is due to two sources, genes and environment. More formally, ${\displaystyle Var(P)=g+(g\times \epsilon )+\epsilon }$, where ${\displaystyle P}$ is the phenotype, ${\displaystyle g}$ is the effect of genes, ${\displaystyle \epsilon }$ is the effect of the environment, and ${\displaystyle (g\times \epsilon )}$ is a gene by environment interaction. The ${\displaystyle g}$ term can be expanded to include additive (${\displaystyle a^{2}}$), dominance (${\displaystyle d^{2}}$), and epistatic (${\displaystyle i^{2}}$) genetic effects. Similarly, the environmental term ${\displaystyle \epsilon }$ can be expanded to include shared environment (${\displaystyle c^{2}}$) and non-shared environment (${\displaystyle e^{2}}$), which includes any measurement error. Dropping the gene by environment interaction for simplicity (typical in twin studies) and fully decomposing the ${\displaystyle g}$ and ${\displaystyle \epsilon }$ terms, we now have ${\displaystyle Var(P)=(a^{2}+d^{2}+i^{2})+(c^{2}+e^{2})}$. Twin research then models the similarity in monozygotic twins and dizogotic twins using simplified forms of this decomposition, shown in the table.^[20]

Decomposing the genetic and environmental contributions to twin similarity.^[20]

Type of relationship	Full decomposition	Falconer's decomposition
Perfect similarity between siblings	${\displaystyle 1.0=a^{2}+d^{2}+i^{2}+c^{2}+e^{2}}$	${\displaystyle 1.0=a^{2}+c^{2}+e^{2}}$
Monozygotic twin correlation(${\displaystyle r_{MZ}}$)	${\displaystyle r_{MZ}=a^{2}+d^{2}+i^{2}+c^{2}}$	${\displaystyle r_{MZ}=a^{2}+c^{2}}$
Dizygotic twin correlation (${\displaystyle r_{DZ}}$)	${\displaystyle r_{DZ}={\frac {1}{2}}a^{2}+{\frac {1}{4}}d^{2}+(k)i^{2}+c^{2}}$	${\displaystyle r_{DZ}={\frac {1}{2}}a^{2}+c^{2}}$
Where ${\displaystyle k}$ is an unknown (probably very small) quantity.

The simplified Falconer formulation can then be used to derive estimates of ${\displaystyle a^{2}}$, ${\displaystyle c^{2}}$, and ${\displaystyle e^{2}}$. Rearranging and substituting the ${\displaystyle r_{MZ}}$ and ${\displaystyle r_{DZ}}$ equations one can obtain an estimate of the additive genetic variance, or heritability, ${\displaystyle a^{2}=2(r_{MZ}-r_{DZ})}$, the non-shared environmental effect ${\displaystyle e^{2}=1-r_{MZ}}$ and, finally, the shared environmental effect ${\displaystyle c^{2}=r_{MZ}-a^{2}}$.^[20] The Falconer formulation is presented here to illustrate how the twin model works. Modern approaches use maximum likelihood to estimate the genetic and environmental variance components.^[24]

Measured genetic variants

The Human Genome Project has allowed scientists to directly genotype the sequence of human DNA nucleotides.^[25] Once genotyped, genetic variants can be tested for association with a behavioural phenotype, such as mental disorder, cognitive ability, personality, and so on.^[26]

• Candidate Genes. One popular approach has been to test for association candidate genes with behavioural phenotypes, where the candidate gene is selected based on some a priori theory about biological mechanisms involved in the manifestation of a behavioural trait or phenotype.^[27] In general, such studies have proven difficult to broadly replicate^[28][29] and there has been concern raised that the false positive rate in this type of research is high.^[27][30]
• Genome-wide association studies. In genome-wide association studies, researchers test the relationship of millions of genetic polymorphisms with behavioural phenotypes across the genome.^[26] This approach to genetic association studies is largely atheoretical, and typically not guided by a particular biological hypothesis regarding the phenotype.^[26] Genetic association findings for behavioural traits and psychiatric disorders have been found to be highly polygenic (involving many small genetic effects).^{[31][32][33][34][35]}
• SNP heritability and co-heritability. Recently, researchers have begun to use similarity between classically unrelated people at their measured single nucleotide polymorphisms (SNPs) to estimate genetic variation or covariation that is tagged by SNPs, using mixed effects models implemented in software such as Genome-wide complex trait analysis (GCTA).^[36][37] To do this, researchers find the average genetic relatedness over all SNPs between all individuals in a (typically large) sample, and use Haseman-Elston regression or restricted maximum likelihood to estimate the genetic variation that is "tagged" by, or predicted by, the SNPs. The proportion of phenotypic variation that is accounted for by the genetic relatedness has been called "SNP heritability".^[38] Intuitively, SNP heritability increases to the degree that phenotypic similarity is predicted by genetic similarity at measured SNPs, and is expected to be lower than the true narrow-sense heritability to the degree that measured SNPs fail to tag (typically rare) causal variants.^[39] The value of this method is that it is an independent way to estimate heritability that does not require the same assumptions as those in twin and family studies, and that it gives insight into the allelic frequency spectrum of the causal variants underlying trait variation.^[40]

Quasi-experimental designs

Some behavioural genetic designs are useful not to understand genetic influences on behaviour, but to control for genetic influences to test environmentally-mediated influences on behaviour.^[41] Such behavioural genetic designs may be considered a subset of natural experiments,^[42] quasi-experiments that attempt to take advantage of naturally occurring situations that mimic true experiments by providing some control over an independent variable. Natural experiments can be particularly useful when experiments are infeasible, due to practical or ethical limitations.^[42]

A general limitation of observational studies is that the relative influences of genes and environment are confounded. A simple demonstration of this fact is that measures of 'environmental' influence are heritable.^[43] Thus, observing a correlation between an environmental risk factor and a health outcome is not necessarily evidence for environmental influence on the health outcome. Similarly, in observational studies of parent-child behavioural transmission, for example, it is impossible to know if the transmission is due to genetic or environmental influences, due to the problem of passive gene-environment correlation.^[42] The simple observation that the children of parents who use drugs are more likely to use drugs as adults does not indicate why the children are more likely to use drugs when they grow up. It could be because the children are modelling their parents' behaviour. Equally plausible, it could be that the children inherited drug-use-predisposing genes from their parent, which put them at increased risk for drug use as adults regardless of their parents' behaviour. Adoption studies, which parse the relative effects of rearing environment and genetic inheritance, find a small to negligible effect of rearing environment on smoking, alcohol, and marijuana use in adopted children,^[44] but a larger effect of rearing environment on harder drug use.^[45]

Other behavioural genetic designs include discordant twin studies,^[41] children of twins designs,^[46] and Mendelian randomization.^[47]

General findings

There are many broad conclusions to be drawn from behavioural genetic research about the nature and origins of behaviour.^[3][48] Three major conclusions include: 1) all behavioural traits and disorders are influenced by genes; 2) environmental influences tend to make members of the same family more different, rather than more similar; and 3) the influence of genes tends to increase in relative importance as individuals age.^[3]

Genetic influences on behaviour are pervasive

It is clear from multiple lines of evidence that all researched behavioural traits and disorders are influenced by genes; that is, they are heritable. The single largest source of evidence comes from twin studies, where it is routinely observed that monozygotic (identical) twins are more similar to one another than are same-sex dizygotic (fraternal) twins.^[11][12]

The conclusion that genetic influences are pervasive has also been observed in research designs that do not depend on the assumptions of the twin method. Adoption studies show that adoptees are routinely more similar to their biological relatives than their adoptive relatives for a wide variety of traits and disorders.^[3] In the Minnesota Study of Twins Reared Apart, monozygotic twins separated shortly after birth were reunited in adulthood.^[49] These adopted, reared-apart twins were as similar to one another as were twins reared together on a wide range of measures including general cognitive ability, personality, religious attitudes, and vocational interests, among others.^[49] Approaches using genome-wide genotyping have allowed researchers to measure genetic relatedness between individuals and estimate heritability based on millions of genetic variants. Methods exist to test whether the extent of genetic similarity (aka, relatedness) between nominally unrelated individuals (individuals who are not close or even distant relatives) is associated with phenotypic similarity.^[37] Such methods do not rely on the same assumptions as twin or adoption studies, and routinely find evidence for heritability of behavioural traits and disorders.^[33][35][50]

Nature of environmental influence

Just as all researched human behavioural phenotypes are influenced by genes (i.e., are heritable), all such phenotypes are also influenced by the environment.^[11][48] The basic fact that monozygotic twins are genetically identical but are never perfectly concordant for psychiatric disorder or perfectly correlated for behavioural traits, indicates that the environment shapes human behaviour.^[48]

The nature of this environmental influence, however, is such that it tends to make individuals in the same family more different from one another, not more similar to one another.^[3] That is, estimates of shared environmental effects (${\displaystyle c^{2}}$) in human studies are small, negligible, or zero for the vast majority of behavioural traits and psychiatric disorders, whereas estimates of non-shared environmental effects (${\displaystyle e^{2}}$) are moderate to large.^[11] From twin studies ${\displaystyle c^{2}}$ is typically estimated at 0 because the correlation (${\displaystyle r_{MZ}}$) between monozygotic twins is at least twice the correlation (${\displaystyle r_{DZ}}$) for dizygotic twins. When using the Falconer variance decomposition (${\displaystyle 1.0=a^{2}+c^{2}+e^{2}}$) this difference between monozygotic and dizygotic twin similarity results in an estimated ${\displaystyle c^{2}=0}$. It is important to note that the Falconer decomposition is simplistic.^[20] It removes the possible influence of dominance and epistatic effects which, if present, will tend to make monozygotic twins more similar than dizygotic twins and mask the influence of shared environmental effects.^[20] This is a limitation of the twin design for estimating ${\displaystyle c^{2}}$. However, the general conclusion that shared environmental effects are negligible does not rest on twin studies alone. Adoption research also fails to find large (${\displaystyle c^{2}}$) components; that is, adoptive parents and their adopted children tend to show much less resemblance to one another than the adopted child and his or her non-rearing biological parent.^[3] In studies of adoptive families with at least one biological child and one adopted child, the sibling resemblance also tends be nearly zero for most traits that have been studied.^[11][51]

Similarity in twins and adoptees indicates a small role for shared environment in personality.

The figure provides an example from personality research, where twin and adoption studies converge on the conclusion of zero to small influences of shared environment on broad personality traits measured by the Multidimensional Personality Questionnaire including positive emotionality, negative emotionality, and constraint.^[52]

Given the conclusion that all researched behavioural traits and psychiatric disorders are heritable, biological siblings will always tend to be more similar to one another than will adopted siblings. However, for some traits, especially when measured during adolescence, adopted siblings do show some significant similarity (e.g., correlations of .20) to one another. Traits that have been demonstrated to have significant shared environmental influences include internalizing and externalizing psychopathology,^[53] substance use^[54] and dependence,^[45] and intelligence.^[54]

Nature of genetic influence

Genetic effects on human behavioural outcomes can be described in multiple ways.^[20] One way to describe the effect is in terms of how much variance in the behaviour can be accounted for by alleles in the genetic variant, otherwise known as the coefficient of determination or ${\displaystyle R^{2}}$. An intuitive way to think about ${\displaystyle R^{2}}$ is that it describes the extent to which the genetic variant makes individuals, who harbour different alleles, different from one another on the behavioural outcome. A complementary way to describe effects of individual genetic variants is in how much change one expects on the behavioural outcome given a change in the number of risk alleles an individual harbours, often denoted by the Greek letter ${\displaystyle \beta }$ (denoting the slope in a regression equation), or, in the case of binary disease outcomes by the odds ratio ${\displaystyle OR}$ of disease given allele status. Note the difference: ${\displaystyle R^{2}}$ describes the population-level effect of alleles within a genetic variant; ${\displaystyle \beta }$ or ${\displaystyle OR}$ describe the effect of having a risk allele on the individual who harbours it, relative to an individual who does not harbour a risk allele.^[55]

When described on the ${\displaystyle R^{2}}$ metric, the effects of individual genetic variants on complex human behavioural traits and disorders are vanishingly small, with each variant accounting for ${\displaystyle R^{2}<0 .3="" annotation="">$ of variation in the phenotype.^[3] This fact has been discovered primarily through genome-wide association studies of complex behavioural phenotypes, including results on substance use,^[56][57] personality,^[58] fertility,^[59] schizophrenia,^[32] depression,^[58][60] and endophenotypes including brain structure^[61] and function.^[62] There are a small handful of replicated and robustly studied exceptions to this rule, including the effect of APOE on Alzheimer's disease,^[63] and CHRNA5 on smoking behaviour,^[56] and ALDH2 (in individuals of East Asian ancestry) on alcohol use.^[64]

On the other hand, when assessing effects according to the ${\displaystyle \beta }$ metric, there are a large number of genetic variants that have very large effects on complex behavioural phenotypes. The risk alleles within such variants are exceedingly rare, such that their large behavioural effects impact only a small number of individuals. Thus, when assessed at a population level using the ${\displaystyle R^{2}}$ metric, they account for only a small amount of the differences in risk between individuals in the population. Examples include variants within APP that result in familial forms of severe early onset Alzheimer's disease but affect only relatively few individuals. Compare this to risk alleles within APOE, which pose much smaller risk compared to APP, but are far more common and therefore affect a much greater proportion of the population.^[65]

Finally, there are classical behavioural disorders that are genetically simple in their etiology, such as Huntington's disease. Huntington's is caused by a single autosomal dominant variant in the HTT gene, which is the only variant that accounts for any differences among individuals in their risk for developing the disease, assuming they live long enough.^[66] In the case of genetically simple and rare diseases such as Huntington's, the variant ${\displaystyle R^{2}}$ and the ${\displaystyle OR}$ are simultaneously large.^[55]

Criticisms and Controversies

Behavioural genetic research and findings have at times been controversial. Some of this controversy has arisen because behavioural genetic findings can challenge societal beliefs about the nature of human behaviour and abilities, other controversies have arisen due to misunderstandings of behavioural genetic research, whether by the lay public or the researchers themselves.^[3] Major areas of controversy have included genetic research on topics such as racial differences, intelligence, violence, and human sexuality.^[67]

Perhaps the most controversial subject has been on race and genetics,^[67] where fringe research groups have claimed that observed racial differences on a behavioral trait are a product of racial differences in allele frequencies. Such claims are made most frequently to differences between White and Black racial groups. These are complicated issues that are extremely difficult to resolve due to the confounding of the racial group and environmental experience, such as discrimination and oppression. Indeed, race is a social construct that is not very useful for genetic research. Instead, geneticists use concepts such as ancestry, which is more rigorously defined.^[68] For example, a so-called "Black" race may include all individuals of relatively recent African descent ("recent" because all humans are descended from African ancestors). However, there is more genetic diversity in Africa than the rest of the world combined,^[69] so speaking of a "Black" race is without a precise genetic meaning.^[68]