Sex differences in humans

From Wikipedia, the free encyclopedia

Sex differences in humans have been studied in a variety of fields. In humans, biological sex is determined by five factors present at birth: the presence or absence of a Y chromosome, the type of gonads, the sex hormones, the internal reproductive anatomy (such as the uterus), and the external genitalia. Genetic sex is determined solely by the presence or absence of a Y chromosome. Phenotypic sex refers to an individual's sex as determined by their internal and external genitalia, expression of secondary sex characteristics, and behavior.

The sex of the individual can be defined in different ways, giving rise to different conceptual frameworks about what determines sex. Sex differences generally refer to traits that are sexually dimorphic. A subset of such differences is hypothesized to be the product of the evolutionary process of sexual selection.

Medicine

Sex differences in medicine include sex-specific diseases, which are diseases that occur only in people of one sex; and sex-related diseases, which are diseases that are more usual to one sex, or which manifest differently in each sex. For example, certain autoimmune diseases may occur predominantly in one sex, for unknown reasons. 90% of primary biliary cirrhosis cases are women, whereas primary sclerosing cholangitis is more common in men. Gender-based medicine, also called "gender medicine", is the field of medicine that studies the biological and physiological differences between the human sexes and how that affects differences in disease. Traditionally, medical research has mostly been conducted using the male body as the basis for clinical studies. Similar findings are also reported in the sport medicine literature where males typically account for >60% of the individuals studied. The findings of these studies have often been applied across the sexes and healthcare providers have assumed a uniform approach in treating both male and female patients. More recently, medical research has started to understand the importance of taking the sex into account as the symptoms and responses to medical treatment may be very different between sexes.

Neither concept should be confused with sexually transmitted diseases, which are diseases that have a significant probability of transmission through sexual contact.

Sex-related illnesses have various causes:

• Sex-linked genetic illnesses
• Parts of the reproductive system that are specific to one sex
• Social causes that relate to the gender role expected of that sex in a particular society.
• Different levels of prevention, reporting, diagnosis or treatment in each gender.

Research has found that female doctors may be providing better care than male doctors in the U.S. and also in Canada.

Physiology

Sex differences in human physiology are distinctions of physiological characteristics associated with either male or female humans. These can be of several types, including direct and indirect, direct being the direct result of differences prescribed by the Y-chromosome, and indirect being characteristics influenced indirectly (e.g., hormonally) by the Y-chromosome. Sexual dimorphism is a term for the phenotypic difference between males and females of the same species.

Direct sex differences follow a bimodal distribution. Through the process of meiosis and fertilization (with rare exceptions), each individual is created with zero or one Y-chromosome. The complementary result for the X-chromosome follows, either a double or a single X. Therefore, direct sex differences are usually binary in expression (although the deviations in complex biological processes produce a menagerie of exceptions). These include, most conspicuously, male (vs female) gonads.

Indirect sex differences are general differences as quantified by empirical data and statistical analysis. Most differing characteristics will conform to a bell-curve (i.e., normal) distribution which can be broadly described by the mean (peak distribution) and standard deviation (indicator of size of range). Often only the mean or mean difference between sexes is given. This may or may not preclude overlap in distributions. For example, most males are taller and stronger than females, but an individual female could be taller and stronger than an individual male. The extents of these differences vary across societies.

The most obvious differences between males and females include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physiological and behavioral effects, including gonadal differentiation, internal and external genital and breast differentiation, and differentiation of muscle mass, height, and hair distribution. There are also differences in the structure of specific areas of the brain. For example, on average, the SDN (INAH3 in humans) has been repeatedly found to be considerably larger in males than in females.

Psychology

Research on biological sex differences in human psychology investigates cognitive and behavioral differences between men and women. This research employs experimental tests of cognition, which take a variety of forms. Tests focus on possible differences in areas such as IQ, spatial reasoning, aggression, emotion, and brain structure and function. 

Chromosomal make up is important in human psychology. Women have two X chromosomes while males have an X and Y chromosomal structure. The X chromosome is much more active than Y and it affects behavior. Genetic researchers theorize that the X chromosome may contain a gene responsible for sociability.

Most IQ tests are constructed so that there are no overall score differences between females and males. Areas where differences have been found include verbal and mathematical ability. IQ tests that measure fluid g and have not been constructed to eliminate sex differences also tend to show that sex differences are either non-existent or negligible. 2008 research found that, for grades 2 to 11, there were no significant gender differences in math skills among the general population. Differences in variability of IQ scores have been observed in studies, with more men falling at the extremes of the spectrum.

Because social and environmental factors affect brain activity and behavior, where differences are found, it can be difficult for researchers to assess whether or not the differences are innate. Studies on this topic explore the possibility of social influences on how both sexes perform in cognitive and behavioral tests. Stereotypes about differences between men and women have been shown to affect a person's behavior (this is called stereotype threat).

In his book titled Gender, Nature, and Nurture, psychologist Richard Lippa found that there were large differences in women's and men's preferences for realistic occupations (for example, mechanic or carpenters) and moderate differences in their preferences for social and artistic occupations. His results also found that women tend to be more people-oriented and men more thing-oriented.

Hartung & Widiger (1998) found that many kinds of mental illnesses and behavioral problems show gender differences in prevalence and incidence. "Of the 80 disorders diagnosed in adulthood for which sex ratios are provided, 35 are said to be more common in men than in women (17 of which are substance related or a paraphilia), 31 are said to be more common in women than men, and 14 are said to be equally common in both sexes."

Differences in male and female jealousy can also be observed. While female jealousy is more likely to be inspired by emotional infidelity, male jealousy is most likely to be brought on by sexual infidelity. A clear majority of approximately 62% to 86% of women reported that they would be more bothered by emotional infidelity and 47% to 60% of men reported that they would be more bothered by sexual infidelity.

In 2005, Janet Shibley Hyde from the University of Wisconsin-Madison introduced the gender similarities hypothesis, which suggests that males and females are similar on most, but not all, psychological variables. The research focused on cognitive variables (for example, reading comprehension, mathematics), communication (for example, talkativeness, facial expressions), social and personality (for example, aggression, sexuality), psychological well-being, and motor behaviors. Using results from a review of 46 meta-analyses, she found that 78% of gender differences were small or close to zero. A few exceptions were some motor behaviors (such as throwing distance) and some aspects of sexuality (such as attitudes about casual sex), which show the largest gender differences. She concludes her article by stating: "It is time to consider the costs of overinflated claims of gender differences. Arguably, they cause harm in numerous realms, including women’s opportunities in the workplace, couple conflict and communication, and analyses of self-esteem problems among adolescents." Hyde also stated elsewhere that "variations within genders are greater than variations between genders."

In 2011 Irina Trofimova found a significant female advantage in time on the lexical task and on the temperament scale of social-verbal tempo, and a male advantage on the temperament scale of physical endurance which were more pronounced in young age groups and faded in older groups. She suggested that there is a "middle age - middle sex" effect: sex differences in these two types of abilities observed in younger groups might be entangled with age and hormonal changes. The study concluded that a one-dimensional approach to sex differences (common in meta-analytic studies) therefore overlooks a possible interaction of sex differences with age. This hormones-based "middle age-middle sex effect", and also specifics of the few psychological sex differences (verbal and physical) were analysed in terms of the systemic evolutional tendencies driving sex dimorphism. 

Sociology

Crime

Statistics have been consistent in reporting that men commit more criminal acts than women. Self-reported delinquent acts are also higher for men than women across many different actions. Many professionals have offered explanations for this sex difference. Some differing explanations include men's evolutionary tendency toward risk and violent behavior, sex differences in activity, social support, and gender inequality. 

Despite the difficulty of interpreting them, crime statistics may provide a way to investigate such a relationship from a gender differences perspective. An observable difference in crime rates between men and women might be due to social and cultural factors, crimes going unreported, or to biological factors (for example, testosterone or sociobiological theories). Taking the nature of the crime itself into consideration may also be a factor. Crime can be measured by such data as arrest records, imprisonment rates, and surveys. However, not all crimes are reported or investigated. Moreover, some studies show that men can have an overwhelming bias against reporting themselves to be the victims of a crime (particularly when victimized by a woman), and some studies have argued that men reporting intimate partner violence find disadvantageous biases in law enforcement. Burton et al. (1998) found that low levels of self control are associated with criminal activity.

Education

A world map showing countries by gender education disparity, 2010.

 

Sometimes and in some places, there are sex differences in educational achievement. This may be caused by sex discrimination in law or culture, or may reflect natural differences in the interests of the sexes.

Leadership

Research has been undertaken to examine whether or not there are sex differences in leadership. Until recently, leadership positions have predominantly been held by men. Women were rarely seen in senior leadership positions leading to a lack of data on how they behave in such positions. The two main lines of research contradict one another, the first being that there are significant sex differences in leadership and the second being that gender does not have an effect on leadership.

Women and men have been surveyed by Gallup each year concerning workplace topics, and when questioned about preferences of a female boss or a male boss, women chose a preference for a male boss 39% of the time, compared to 26% of men displaying preference for a male boss. Only 27% of females would prefer a boss of the same gender. This preference, among both sexes, for male leadership in the workplace has continued unabated for sixty years, according to the survey results.

Religion

Sex differences in religion can be classified as either "internal" or "external." Internal religious issues are studied from the perspective of a given religion, and might include religious beliefs and practices about the roles and rights of men and women in government, education and worship; beliefs about the sex or gender of deities and religious figures; and beliefs about the origin and meaning of human gender. External religious issues can be broadly defined as an examination of a given religion from an outsider's perspective, including possible clashes between religious leaders and laity; and the influence of, and differences between, religious perspectives on social issues. For example, various religious perspectives have either endorsed or condemned alternative family structures, homosexual relationships, and abortion. External religious issues can also be examined from the "lens of gender" perspective embraced by some in feminism or critical theory and its offshoots.

Social capital

Sex differences in social capital are differences between men and women in their ability to coordinate actions and achieve their aims through trust, norms and networks. Social capital is often seen as the missing link in development; as social networks facilitate access to resources and protect the commons, while cooperation makes markets work more efficiently. Social capital has been thought of as women's capital as whereas there are gendered barriers to accessing economic capital, women's role in family, and community ensures that they have strong networks. There is potential that the concept can help to bring women's unpaid 'community and household labor', vital to survival and development, to the attention of economists. However, research analyzing social capital from a gendered perspective is rare, and the notable exceptions are very critical.

Suicide

Sex differences in suicide have been shown to be significant; there are highly asymmetric rates of attempted and completed suicide between males and females. The gap, also called the gender paradox of suicidal behavior, can vary significantly between different countries. Statistics indicate that males die much more often by means of suicide than do females, however reported suicide attempts are 3 times more common among females than males. This paradox is partially explained by the methodology, with females more often choosing medication-induced overdosage, and males more often turning to weapons such as firearms or knives.

Financial risk-taking

Sex differences in financial decision making are relevant and significant. Numerous studies have found that women tend to be financially more risk-averse than men and hold safer portfolios. A May 3, 2015 article in the Wall Street Journal by Georgette Jasen reported that "when it comes to investing, men sometimes have their way of doing things, and women have different ways." Scholarly research has documented systematic differences in financial decisions such as buying investments versus insurance, donating to ingroups versus outgroups (such as terrorism victims in Iraq versus USA), spending in stores, and the endowment effect-or asking price for goods people have. The majority of these studies are based on the theory of agency-communion developed by David Bakan in 1966; according to this theory, due to factors such as socialization, males are typically more agentic (focus on self, upside potential, aggressiveness) and females typically more communal (focus on others, downside potential, and nurturing). This framework robustly explains many financial decision making outcomes.

Sex differences in intelligence

From Wikipedia, the free encyclopedia

 

Differences in human intelligence have long been a topic of debate among researchers and scholars. With the advent of the concept of g or general intelligence, many researchers have argued for no significant sex differences in g factor or general intelligence, while others have argued for greater intelligence for males, and others for females. These results depend on the methodology, tests researchers used for their claims, and the personal performances of the participants.

 

Some research indicates male advantages on some cognitive tests are minimized when controlling for socioeconomic factors. Other research has concluded that there is larger variability in male scores compared to female scores, which results in more males than females in the top and bottom of the IQ distribution. Additionally, there are differences in the capacity of males and females in performing certain tasks, such as rotation of objects in space, often categorized as spatial ability.

Historical perspectives

Prior to the 20th century, it was a commonly held view that men were intellectually superior to women. In 1801, Thomas Gisborne said that women were naturally suited to domestic work and not spheres suited to men such as politics, science, or business. He stated that this was because women did not possess the same level of rational thinking that men did and had naturally superior abilities in skills related to family support. 

In 1875, Herbert Spencer said that women were incapable of abstract thought and could not understand issues of justice and had only the ability to understand issues of care. In 1925, Sigmund Freud also stated that women were less morally developed in the concept of justice and, unlike men, were more influenced by feeling than rational thought. Early brain studies comparing mass and volumes between the sexes concluded that women were intellectually inferior because they have smaller and lighter brains. Many believed that the size difference caused women to be excitable, emotional, sensitive, and therefore not suited for political participation.

In the nineteenth century, whether men and women had equal intelligence was seen by many as a prerequisite for the granting of suffrage. Leta Hollingworth argued that women were not permitted to realize their full potential, as they were confined to the roles of child-rearing and housekeeping. 

During the early twentieth century, the scientific consensus shifted to the view that gender plays no role in intelligence, but men consistently overestimated their IQs and women consistently underestimated their IQs.

In his 1916 study of children's IQs, psychologist Lewis Terman concluded that "the intelligence of girls, at least up to 14 years, does not differ materially from that of boys". He did, however, find "rather marked" differences on a minority of tests. For example, he found boys were "decidedly better" in arithmetical reasoning, while girls were "superior" at answering comprehension questions. He also proposed that discrimination, lack of opportunity, women's responsibilities in motherhood, or emotional factors may have accounted for the fact that few women had careers in intellectual fields.

Research on general intelligence

Background

Chamorro-Premuzic et al. stated, "The g factor, which is often used synonymously with general intelligence, is a latent variable which emerges in a factor analysis of various cognitive ('IQ') tests. They are not exactly the same thing. g is an indicator or measure of general intelligence; it's not general intelligence itself."

All or most of the major tests commonly used to measure intelligence have been constructed so that there are no overall score differences between males and females. Thus, there is little difference between the average IQ scores of men and women. Differences have been reported, however, in specific areas such as mathematics and verbal measures. Also, studies have found the variability of male scores is greater than that of female scores, resulting in more males than females in the top and bottom of the IQ distribution.

In favor of males or females in g factor

Research, using the Wechsler Adult Intelligence Scale (WAIS III and WAIS-R), that finds general intelligence in favor of males indicates a very small difference. This is consistent across countries. In the United States and Canada, the IQ points range from two to three points in favor of males, while the points rise to four points in favor of males in China and Japan. By contast, some research finds greater advantage for adult females. For children in the United States and the Netherlands, there are one to two IQ point differences in favor of boys. Other research has found a slight advantage for girls on the residual verbal factor.

A 2004 meta-analysis by Richard Lynn and Paul Irwing published in 2005 found that the mean IQ of men exceeded that of women by up to 5 points on the Raven's Progressive Matrices test. Lynn's findings were debated in a series of articles for Nature. He argued that there is a greater male advantage than most tests indicate, stating that because girls mature faster than boys, and that cognitive competence increases with physiological age, rather than with calender age, the male-female difference is small or negative prior to puberty, but males have an advantage after adolescence and this advantage continues into adulthood.

In favor of no sex differences or inconclusive consensus

Most studies find either a very small difference in favor of males or no sex difference with regard to general intelligence. In 2000, researchers Roberto Colom and Francisco J. Abad conducted a large study of 10,475 adults on five IQ tests taken from the Primary Mental Abilities and found negligible or no significant sex differences. The tests conducted were on vocabulary, spatial rotation, verbal fluency and inductive reasoning.

The literature on sex differences in intelligence has produced inconsistent results due to the type of testing used, and this has resulted in debate among researchers. The issue remains unresolved if one uses standardized tests as Jensen (1998) and Colom did. Garcia (2002) agrees that there might be a small insignificant sex difference in intelligence in general (IQ) but this may not necessarily reflect a sex difference in general intelligence or g factor. The difference between the two concepts is that IQ is a psychometric scoring system measured with standardized testing, while g factor is a latent scientific construct that correlates with all cognitive tests and achievements in life. Although most researchers distinguish between g and IQ, those that argue for greater male intelligence assert that IQ and g are synonymous (Lynn & Irwing 2004) and so the real division comes from defining IQ in relation to g factor. However, in 2008 Lynn and Irwing proposed that since working memory ability correlates highest with g factor, researchers would have no choice but to accept greater male intelligence if differences on working memory tasks are found. As a result, a neuroimaging study published by Schmidt (2009) conducted an investigation into this proposal by measuring sex differences on an n-back working memory task. The results found no sex difference in working memory capacity, thus contradicting the position put forward by Lynn and Irwing (2008) and more in line with those arguing for no sex differences in intelligence.

A 2012 review by researchers Richard E. Nisbett, Joshua Aronson, Clancy Blair, William Dickens, James Flynn, Diane F. Halpern and Eric Turkheimer discussed Arthur Jensen's 1998 studies on sex differences in intelligence. Jensen's tests were significantly g-loaded but were not set up to get rid of any sex differences. They summarized his conclusions as he quoted, "No evidence was found for sex differences in the mean level of g or in the variability of g. Males, on average, excel on some factors; females on others." Jensen's conclusion that no overall sex differences existed for g has been reinforced by researchers who analyzed this issue with a battery of 42 mental ability tests and found no overall sex difference.

Although most of the tests showed no difference, there were some that did. For example, they found female subjects performed better on verbal abilities while males performed better on visuospatial abilities. For verbal fluency, females have been specifically found to perform slightly better in vocabulary and reading comprehension but significantly higher in speech production and essay writing. Males have been specifically found to perform better on spatial visualization, spatial perception, and mental rotation. Researchers had then recommended that general models such as fluid and crystallized intelligence be divided into verbal, perceptual and visuospatial domains of g; this is because, as this model is applied, females excel at verbal and perceptual tasks while males on visuospatial tasks, thus evening out the sex differences on IQ tests.

Variability

Some studies have identified the degree of IQ variance as a difference between males and females. Males tend to show greater variability on many traits; for example having both highest and lowest scores on tests of cognitive abilities.

Feingold (1992b) and Hedges and Nowell (1995) have reported that, despite average sex differences being small and relatively stable over time, test score variances of males were generally larger than those of females." Feingold "found that males were more variable than females on tests of quantitative reasoning, spatial visualisation, spelling, and general knowledge. ... Hedges and Nowell go one step further and demonstrate that, with the exception of performance on tests of reading comprehension, perceptual speed, and associative memory, more males than females were observed among high-scoring individuals."

Brain and intelligence

Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Although men have larger brains, men and women typically achieve similar IQ results. For men, the gray matter volume in the frontal and parietal lobes correlates with IQ; for women, the gray matter volume in the frontal lobe and Broca's area (which is used in language processing) correlates with IQ.

Women have greater cortical thickness, cortical complexity and cortical surface area (controlling for body size) which compensates for smaller brain size. Meta-analysis and studies have found that brain size explains 6–12% of variance among individual intelligence and cortical thickness explains 5%.

Mathematics performance

Girl scouts compete in the USS California Science Experience at Naval Surface Warfare. In 2008, the National Science Foundation reported that, on average, girls perform as well as boys on standardized math tests, while boys are overrepresented on both ends of the spectrum.

 

A performance difference in mathematics on the SAT and international PISA exists in favor of males, though differences in mathematics course performance measures favor females. In 1983, Benbow concluded that the study showed a large sex difference by age 13 and that it was especially pronounced at the high end of the distribution. However, Gallagher and Kaufman criticized Benbow's and others' reports, which found that males were over-represented in the highest percentages, on the grounds that they had not ensured representative sampling.

In nearly every study on the subject, males have out-performed females on mathematics in high school, but the size of the male-female difference, across countries, is related to gender inequality in social roles. In a 2008 study paid for by the National Science Foundation in the United States, however, researchers stated that "girls perform as well as boys on standardized math tests. Although 20 years ago, high school boys performed better than girls in math, the researchers found that is no longer the case. The reason, they said, is simple: Girls used to take fewer advanced math courses than boys, but now they are taking just as many." However, the study indicated that, while boys and girls performed similarly on average, boys were over-represented among the very best performers as well as among the very worst.

A 2011 meta-analysis with 242 studies from 1990 to 2007 involving 1,286,350 people found no overall sex difference of performance in mathematics. The meta-analysis also found that although there were no overall differences, a small sex difference that favored males in complex problem solving is still present in high school.

With regard to gender inequality, some psychologists believe that many historical and current sex differences in mathematics performance may be related to boys' higher likelihood of receiving math encouragement than girls. Parents were, and sometimes still are, more likely to consider a son's mathematical achievement as being a natural skill while a daughter's mathematical achievement is more likely to be seen as something she studied hard for. This difference in attitude may contribute to girls and women being discouraged from further involvement in mathematics-related subjects and careers.

Stereotype threat has been shown to affect performance and confidence in mathematics of both males and females.

Spatial ability

Examples of figures from mental rotation tests.

 

A man playing a video game at the Japan Media Arts Festival. Spatial abilities can be affected by experiences such as playing video games, complicating research on sex differences in spatial abilities.

 

Metastudies show a male advantage in mental rotation and assessing horizontality and verticality and a female advantage in spatial memory. A proposed hypothesis is that men and women evolved different mental abilities to adapt to their different roles in society. This explanation suggests that men may have evolved greater spatial abilities as a result of certain behaviors, such as navigating during a hunt.

A number of studies have shown that women tend to rely more on visual information than men in a number of spatial tasks related to perceived orientation.

Results from studies conducted in the physical environment are not conclusive about sex differences, with various studies on the same task showing no differences. For example, there are studies that show no difference in finding one's way between two places.

Performance in mental rotation and similar spatial tasks is affected by gender expectations. For example, studies show that being told before the test that men typically perform better, or that the task is linked with jobs like aviation engineering typically associated with men versus jobs like fashion design typically associated with women, will negatively affect female performance on spatial rotation and positively influence it when subjects are told the opposite. Experiences such as playing video games also increase a person's mental rotation ability.

The possibility of testosterone and other androgens as a cause of sex differences in psychology has been a subject of study. Adult women who were exposed to unusually high levels of androgens in the womb due to congenital adrenal hyperplasia score significantly higher on tests of spatial ability. Some research has found positive correlations between testosterone levels in healthy males and measures of spatial ability. However, the relationship is complex.

Sex differences in academics

A 2014 meta-analysis of sex differences in scholastic achievement published in the journal of Psychological Bulletin found females outperformed males in teacher-assigned school marks throughout elementary, junior/middle, high school and at both undergraduate and graduate university level. The meta-analysis, done by researchers Daniel Voyer and Susan D. Voyer from the University of New Brunswick, drew from 97 years of 502 effect sizes and 369 samples stemming from the year 1914 to 2011.

Beyond sex differences in academic ability, recent research has also been focusing on women's underrepresentation in higher education, especially in the fields of natural science, technology, engineering and mathematics (STEM).

Human intelligence

From Wikipedia, the free encyclopedia

 

Human intelligence is the intellectual prowess of humans, which is marked by complex cognitive feats and high levels of motivation and self-awareness. Through their intelligence, humans possess the cognitive abilities to learn, form concepts, understand, apply logic, and reason, including the capacities to recognize patterns, comprehend ideas, plan, solve problems, make decisions, retain information, and use language to communicate.

Correlates

As a construct and measured by intelligence tests, intelligence is considered to be one of the most useful concepts used in psychology, because it correlates with lots of relevant variables, for instance the probability of suffering an accident, earning a higher salary, and more.

Education

According to a 2018 metastudy of educational effects on intelligence, education appears to be the "most consistent, robust, and durable method" known for raising intelligence.

Myopia

A number of studies have shown a correlation between IQ and myopia. Some suggest that the reason for the correlation is environmental, whereby intelligent people are more likely to damage their eyesight with prolonged reading, while others contend that a genetic link exists.

Aging

There is evidence that aging causes decline in cognitive functions. In one cross-sectional study, various cognitive functions measured declines by about 0.8 in z-score from age 20 to age 50, the cognitive functions included speed of processing, working memory and long term memory.

Theories

Relevance of IQ tests

In psychology, human intelligence is commonly assessed by IQ scores, determined by IQ tests. However, there are critics of IQ who do not dispute the stability of IQ test scores, or the fact that they predict certain forms of achievement rather effectively. They do argue, however, that to base a concept of intelligence on IQ test scores alone is to ignore many important aspects of mental ability.

On the other hand, Linda S. Gottfredson (2006) has argued that the results of thousands of studies support the importance of IQ for school and job performance (see also the work of Schmidt & Hunter, 2004). She says that IQ also predicts or correlates with numerous other life outcomes. In contrast, empirical support for non-g intelligences is lacking or very poor.

Theory of multiple intelligences

Howard Gardner's theory of multiple intelligences is based on studies not only of normal children and adults, but also of gifted individuals (including so-called "savants"), of persons who have suffered brain damage, of experts and virtuosos, and of individuals from diverse cultures. Gardner breaks intelligence down into at least a number of different components. In the first edition of his book Frames of Mind (1983), he described seven distinct types of intelligence—logical-mathematical, linguistic, spatial, musical, kinesthetic, interpersonal, and intrapersonal. In a second edition of this book, he added two more types of intelligence—naturalist and existential intelligences. He argues that psychometric (IQ) tests address only linguistic and logical plus some aspects of spatial intelligence. A major criticism of Gardner's theory is that it has never been tested, or subjected to peer review, by Gardner or anyone else, and indeed that it is unfalsifiable. Others (e.g. Locke, 2005) have suggested that recognizing many specific forms of intelligence (specific aptitude theory) implies a political—rather than scientific—agenda, intended to appreciate the uniqueness in all individuals, rather than recognizing potentially true and meaningful differences in individual capacities. Schmidt and Hunter (2004) suggest that the predictive validity of specific aptitudes over and above that of general mental ability, or "g", has not received empirical support. On the other hand, Jerome Bruner agreed with Gardner that the intelligences were "useful fictions," and went on to state that "his approach is so far beyond the data-crunching of mental testers that it deserves to be cheered."

Howard Gardner describes his first seven intelligences as follows:

• Linguistic intelligence: People high in linguistic intelligence have an affinity for words, both spoken and written.
• Logical-mathematical intelligence: It implies logical and mathematical abilities.
• Spatial intelligence: The ability to form a mental model of a spatial world and to be able to maneuver and operate using that model.
• Musical intelligence: Those with musical intelligence have excellent pitch, and may even be absolute pitch.
• Bodily-kinesthetic intelligence: The ability to solve problems or to fashion products using one's whole body, or parts of the body. Gifted people in this intelligence may be good dancers, athletes, surgeons, craftspeople, and others.
• Interpersonal intelligence: The ability to see things from the perspective of others, or to understand people in the sense of empathy. Strong interpersonal intelligence would be an asset in those who are teachers, politicians, clinicians, religious leaders, etc.
• Intrapersonal intelligence: It is a capacity to form an accurate, veridical model of oneself and to be able to use that model to operate effectively in life.

Triarchic theory of intelligence

Robert Sternberg proposed the triarchic theory of intelligence to provide a more comprehensive description of intellectual competence than traditional differential or cognitive theories of human ability. The triarchic theory describes three fundamental aspects of intelligence. Analytic intelligence comprises the mental processes through which intelligence is expressed. Creative intelligence is necessary when an individual is confronted with a challenge that is nearly, but not entirely, novel or when an individual is engaged in automatizing the performance of a task. Practical intelligence is bound in a sociocultural milieu and involves adaptation to, selection of, and shaping of the environment to maximize fit in the context. The triarchic theory does not argue against the validity of a general intelligence factor; instead, the theory posits that general intelligence is part of analytic intelligence, and only by considering all three aspects of intelligence can the full range of intellectual functioning be fully understood. 

More recently, the triarchic theory has been updated and renamed as the Theory of Successful Intelligence by Sternberg. Intelligence is now defined as an individual's assessment of success in life by the individual's own (idiographic) standards and within the individual's sociocultural context. Success is achieved by using combinations of analytical, creative, and practical intelligence. The three aspects of intelligence are referred to as processing skills. The processing skills are applied to the pursuit of success through what were the three elements of practical intelligence: adapting to, shaping of, and selecting of one's environments. The mechanisms that employ the processing skills to achieve success include utilizing one's strengths and compensating or correcting for one's weaknesses. 

Sternberg's theories and research on intelligence remain contentious within the scientific community.

PASS theory of intelligence

Based on A. R. Luria's (1966) seminal work on the modularization of brain function, and supported by decades of neuroimaging research, the PASS Theory of Intelligence proposes that cognition is organized in three systems and four processes. The first process is the Planning, which involves executive functions responsible for controlling and organizing behavior, selecting and constructing strategies, and monitoring performance. The second is the Attention process, which is responsible for maintaining arousal levels and alertness, and ensuring focus on relevant stimuli. The next two are called Simultaneous and Successive processing and they involve encoding, transforming, and retaining information. Simultaneous processing is engaged when the relationship between items and their integration into whole units of information is required. Examples of this include recognizing figures, such as a triangle within a circle vs. a circle within a triangle, or the difference between 'he had a shower before breakfast' and 'he had breakfast before a shower.' Successive processing is required for organizing separate items in a sequence such as remembering a sequence of words or actions exactly in the order in which they had just been presented. These four processes are functions of four areas of the brain. Planning is broadly located in the front part of our brains, the frontal lobe. Attention and arousal are combined functions of the frontal lobe and the lower parts of the cortex, although the parietal lobes are also involved in attention as well. Simultaneous processing and Successive processing occur in the posterior region or the back of the brain. Simultaneous processing is broadly associated with the occipital and the parietal lobes while Successive processing is broadly associated with the frontal-temporal lobes. The PASS (Planning/Attention/Simultaneous/Successive) theory is heavily indebted to both Luria (1966, 1973), and studies in cognitive psychology involved in promoting a better look at intelligence.

Piaget's theory and Neo-Piagetian theories

In Piaget's theory of cognitive development the focus is not on mental abilities but rather on a child's mental models of the world. As a child develops, increasingly more accurate models of the world are developed which enable the child to interact with the world better. One example being object permanence where the child develops a model where objects continue to exist even when they cannot be seen, heard, or touched.

Piaget's theory described four main stages and many sub-stages in the development. These four main stages are:

• sensory motor stage (birth-2yrs);
• pre-operational stage (2yrs-7rs);
• concrete operational stage (7rs-11yrs); and
• formal operations stage (11yrs-16yrs)

Degree of progress through these stages are correlated, but not identical with psychometric IQ. Piaget conceptualizes intelligence as an activity more than a capacity. 

One of Piaget's most famous studies focused purely on the discriminative abilities of children between the ages of two and a half years old, and four and a half years old. He began the study by taking children of different ages and placing two lines of sweets, one with the sweets in a line spread further apart, and one with the same number of sweets in a line placed more closely together. He found that, "Children between 2 years, 6 months old and 3 years, 2 months old correctly discriminate the relative number of objects in two rows; between 3 years, 2 months and 4 years, 6 months they indicate a longer row with fewer objects to have "more"; after 4 years, 6 months they again discriminate correctly". Initially younger children were not studied, because if at the age of four years a child could not conserve quantity, then a younger child presumably could not either. The results show however that children that are younger than three years and two months have quantity conservation, but as they get older they lose this quality, and do not recover it until four and a half years old. This attribute may be lost temporarily because of an overdependence on perceptual strategies, which correlates more candy with a longer line of candy, or because of the inability for a four-year-old to reverse situations. By the end of this experiment several results were found. First, younger children have a discriminative ability that shows the logical capacity for cognitive operations exists earlier than acknowledged. This study also reveals that young children can be equipped with certain qualities for cognitive operations, depending on how logical the structure of the task is. Research also shows that children develop explicit understanding at age 5 and as a result, the child will count the sweets to decide which has more. Finally the study found that overall quantity conservation is not a basic characteristic of humans' native inheritance.

Piaget's theory has been criticized for the age of appearance of a new model of the world, such as object permanence, being dependent on how the testing is done (see the article on object permanence). More generally, the theory may be very difficult to test empirically because of the difficulty of proving or disproving that a mental model is the explanation for the results of the testing.

Neo-Piagetian theories of cognitive development expand Piaget's theory in various ways such as also considering psychometric-like factors such as processing speed and working memory, "hypercognitive" factors like self-monitoring, more stages, and more consideration on how progress may vary in different domains such as spatial or social.

Parieto-frontal integration theory of intelligence

Based on a review of 37 neuroimaging studies, Jung and Haier (2007) proposed that the biological basis of intelligence stems from how well the frontal and parietal regions of the brain communicate and exchange information with each other. Subsequent neuroimaging and lesion studies report general consensus with the theory. A review of the neuroscience and intelligence literature concludes that the parieto-frontal integration theory is the best available explanation for human intelligence differences.

Investment theory

Based on the Cattell–Horn–Carroll theory, the tests of intelligence most often used in the relevant studies include measures of fluid ability (Gf) and crystallized ability (Gc); that differ in their trajectory of development in individuals. The 'investment theory' by Cattell states that the individual differences observed in the procurement of skills and knowledge (Gc) are partially attributed to the 'investment' of Gf, thus suggesting the involvement of fluid intelligence in every aspect of the learning process. It is essential to highlight that the investment theory suggests that personality traits affect 'actual' ability, and not scores on an IQ test. In association, Hebb's theory of intelligence suggested a bifurcation as well, Intelligence A (physiological), that could be seen as a semblance of fluid intelligence and Intelligence B (experiential), similar to crystallized intelligence.

Intelligence compensation theory (ICT)

The intelligence compensation theory (a term first coined by Wood and Englert, 2009) states that individuals who are comparatively less intelligent work harder, more methodically, become more resolute and thorough (more conscientious) in order to achieve goals, to compensate for their 'lack of intelligence' whereas more intelligent individuals do not require traits/behaviours associated with the personality factor conscientiousness to progress as they can rely on the strength of their cognitive abilities as opposed to structure or effort. The theory suggests the existence of a causal relationship between intelligence and conscientiousness, such that the development of the personality trait conscientiousness is influenced by intelligence. This assumption is deemed plausible as it is unlikely that the reverse causal relationship could occur; implying that the negative correlation would be higher between fluid intelligence (Gf) and conscientiousness. The justification being the timeline of development of Gf, Gc and personality, as crystallized intelligence would not have developed completely when personality traits develop. Subsequently, during school-going ages, more conscientious children would be expected to gain more crystallized intelligence (knowledge) through education, as they would be more efficient, thorough, hard-working and dutiful.

This theory has recently been contradicted by evidence, that identifies compensatory sample selection. Thus, attributing the previous findings to the bias in selecting samples with individuals above a certain threshold of achievement.

Bandura's theory of self-efficacy and cognition

The view of cognitive ability has evolved over the years, and it is no longer viewed as a fixed property held by an individual. Instead, the current perspective describes it as a general capacity, comprising not only cognitive, but motivational, social and behavioural aspects as well. These facets work together to perform numerous tasks. An essential skill often overlooked is that of managing emotions, and aversive experiences that can compromise one's quality of thought and activity. The link between intelligence and success has been bridged by crediting individual differences in self-efficacy. Bandura's theory identifies the difference between possessing skills and being able to apply them in challenging situations. Thus, the theory suggests that individuals with the same level of knowledge and skill may perform badly, averagely or excellently based on differences in self-efficacy.

A key role of cognition is to allow for one to predict events and in turn devise methods to deal with these events effectively. These skills are dependent on processing of stimuli that is unclear and ambiguous. To learn the relevant concepts, individuals must be able to rely on the reserve of knowledge to identify, develop and execute options. They must be able to apply the learning acquired from previous experiences. Thus, a stable sense of self-efficacy is essential to stay focused on tasks in the face of challenging situations.

To summarize, Bandura's theory of self-efficacy and intelligence suggests that individuals with a relatively low sense of self-efficacy in any field will avoid challenges. This effect is heightened when they perceive the situations as personal threats. When failure occurs, they recover from it more slowly than others, and credit it to an insufficient aptitude. On the other hand, persons with high levels of self-efficacy hold a task-diagnostic aim that leads to effective performance.

Process, personality, intelligence and knowledge theory (PPIK)

Predicted growth curves for Intelligence as process, crystallized intelligence, occupational knowledge and avocational knowledge based on Ackerman's PPIK Theory.

 

Developed by Ackerman, the PPIK (process, personality, intelligence and knowledge) theory further develops the approach on intelligence as proposed by Cattell, the Investment theory and Hebb, suggesting a distinction between intelligence as knowledge and intelligence as process (two concepts that are comparable and related to Gc and Gf respectively, but broader and closer to Hebb's notions of "Intelligence A" and "Intelligence B") and integrating these factors with elements such as personality, motivation and interests.

Ackerman describes the difficulty of distinguishing process from knowledge, as content cannot be entirely eliminated from any ability test. Personality traits have not shown to be significantly correlated with the intelligence as process aspect except in the context of psychopathology. One exception to this generalization has been the finding of sex differences in cognitive abilities, specifically abilities in mathematical and spatial form. On the other hand, the intelligence as knowledge factor has been associated with personality traits of Openness and Typical Intellectual Engagement, which also strongly correlate with verbal abilities (associated with crystallized intelligence).

Latent inhibition

It appears that Latent inhibition can influence one's creativity.

Improving

Because intelligence appears to be at least partly dependent on brain structure and the genes shaping brain development, it has been proposed that genetic engineering could be used to enhance the intelligence, a process sometimes called biological uplift in science fiction. Experiments on mice have demonstrated superior ability in learning and memory in various behavioral tasks.

IQ leads to greater success in education, but independently education raises IQ scores. A 2017 meta-analysis suggests education increases IQ by 1-5 points per year of education, or at least increases IQ test taking ability.

Attempts to raise IQ with brain training have led to increases on aspects related with the training tasks – for instance working memory – but it is yet unclear if these increases generalise to increased intelligence per se.

A 2008 research paper claimed that practicing a dual n-back task can increase fluid intelligence (Gf), as measured in several different standard tests. This finding received some attention from popular media, including an article in Wired. However, a subsequent criticism of the paper's methodology questioned the experiment's validity and took issue with the lack of uniformity in the tests used to evaluate the control and test groups. For example, the progressive nature of Raven's Advanced Progressive Matrices (APM) test may have been compromised by modifications of time restrictions (i.e., 10 minutes were allowed to complete a normally 45-minute test). 

Substances which actually or purportedly improve intelligence or other mental functions are called nootropics. A meta analysis shows omega 3 fatty acids improves cognitive performance among those with cognitive deficits, but not among healthy subjects. A meta-regression shows omega 3 fatty acids improve the moods of patients with major depression (major depression is associated with mental deficits). However, exercise, not just performance-enhancing drugs, enhances cognition for healthy and non healthy subjects as well.

On the philosophical front, conscious efforts to influence intelligence raise ethical issues. Neuroethics considers the ethical, legal and social implications of neuroscience, and deals with issues such as the difference between treating a human neurological disease and enhancing the human brain, and how wealth impacts access to neurotechnology. Neuroethical issues interact with the ethics of human genetic engineering. 

Transhumanist theorists study the possibilities and consequences of developing and using techniques to enhance human abilities and aptitudes. 

Eugenics is a social philosophy which advocates the improvement of human hereditary traits through various forms of intervention. Eugenics has variously been regarded as meritorious or deplorable in different periods of history, falling greatly into disrepute after the defeat of Nazi Germany in World War II.

Measuring

Score distribution chart for sample of 905 children tested on 1916 Stanford-Binet Test

 

The approach to understanding intelligence with the most supporters and published research over the longest period of time is based on psychometric testing. It is also by far the most widely used in practical settings. Intelligence quotient (IQ) tests include the Stanford-Binet, Raven's Progressive Matrices, the Wechsler Adult Intelligence Scale and the Kaufman Assessment Battery for Children. There are also psychometric tests that are not intended to measure intelligence itself but some closely related construct such as scholastic aptitude. In the United States examples include the SSAT, the SAT, the ACT, the GRE, the MCAT, the LSAT, and the GMAT. Regardless of the method used, almost any test that requires examinees to reason and has a wide range of question difficulty will produce intelligence scores that are approximately normally distributed in the general population.

Intelligence tests are widely used in educational, business, and military settings because of their efficacy in predicting behavior. IQ and g (discussed in the next section) are correlated with many important social outcomes—individuals with low IQs are more likely to be divorced, have a child out of marriage, be incarcerated, and need long-term welfare support, while individuals with high IQs are associated with more years of education, higher status jobs and higher income. Intelligence is significantly correlated with successful training and performance outcomes, and IQ/g is the single best predictor of successful job performance.

General intelligence factor or g

There are many different kinds of IQ tests using a wide variety of test tasks. Some tests consist of a single type of task, others rely on a broad collection of tasks with different contents (visual-spatial, verbal, numerical) and asking for different cognitive processes (e.g., reasoning, memory, rapid decisions, visual comparisons, spatial imagery, reading, and retrieval of general knowledge). The psychologist Charles Spearman early in the 20th century carried out the first formal factor analysis of correlations between various test tasks. He found a trend for all such tests to correlate positively with each other, which is called a positive manifold. Spearman found that a single common factor explained the positive correlations among tests. Spearman named it g for "general intelligence factor". He interpreted it as the core of human intelligence that, to a larger or smaller degree, influences success in all cognitive tasks and thereby creates the positive manifold. This interpretation of g as a common cause of test performance is still dominant in psychometrics. (Although, an alternative interpretation was recently advanced by van der Maas and colleagues. Their mutualism model assumes that intelligence depends on several independent mechanisms, none of which influences performance on all cognitive tests. These mechanisms support each other so that efficient operation of one of them makes efficient operation of the others more likely, thereby creating the positive manifold.) 

IQ tasks and tests can be ranked by how highly they load on the g factor. Tests with high g-loadings are those that correlate highly with most other tests. One comprehensive study investigating the correlations between a large collection of tests and tasks has found that the Raven's Progressive Matrices have a particularly high correlation with most other tests and tasks. The Raven's is a test of inductive reasoning with abstract visual material. It consists of a series of problems, sorted approximately by increasing difficulty. Each problem presents a 3 x 3 matrix of abstract designs with one empty cell; the matrix is constructed according to a rule, and the person must find out the rule to determine which of 8 alternatives fits into the empty cell. Because of its high correlation with other tests, the Raven's Progressive Matrices are generally acknowledged as a good indicator of general intelligence. This is problematic, however, because there are substantial gender differences on the Raven's, which are not found when g is measured directly by computing the general factor from a broad collection of tests.

General collective intelligence factor or c

A recent scientific understanding of collective intelligence, defined as a group's general ability to perform a wide range of tasks, expands the areas of human intelligence research applying similar methods and concepts to groups. Definition, operationalization and methods are similar to the psychometric approach of general individual intelligence where an individual's performance on a given set of cognitive tasks is used to measure intelligence indicated by the general intelligence factor g extracted via factor analysis. In the same vein, collective intelligence research aims to discover a ‘c factor’ explaining between-group differences in performance as well as structural and group compositional causes for it.

Historical psychometric theories

Several different theories of intelligence have historically been important for psychometrics. Often they emphasized more factors than a single one like in g factor.

Cattell–Horn–Carroll theory

Many of the broad, recent IQ tests have been greatly influenced by the Cattell–Horn–Carroll theory. It is argued to reflect much of what is known about intelligence from research. A hierarchy of factors for human intelligence is used. g is at the top. Under it there are 10 broad abilities that in turn are subdivided into 70 narrow abilities. The broad abilities are:

• Fluid intelligence (Gf): includes the broad ability to reason, form concepts, and solve problems using unfamiliar information or novel procedures.
• Crystallized intelligence (Gc): includes the breadth and depth of a person's acquired knowledge, the ability to communicate one's knowledge, and the ability to reason using previously learned experiences or procedures.
• Quantitative reasoning (Gq): the ability to comprehend quantitative concepts and relationships and to manipulate numerical symbols.
• Reading & writing ability (Grw): includes basic reading and writing skills.
• Short-term memory (Gsm): is the ability to apprehend and hold information in immediate awareness and then use it within a few seconds.
• Long-term storage and retrieval (Glr): is the ability to store information and fluently retrieve it later in the process of thinking.
• Visual processing (Gv): is the ability to perceive, analyze, synthesize, and think with visual patterns, including the ability to store and recall visual representations.
• Auditory processing (Ga): is the ability to analyze, synthesize, and discriminate auditory stimuli, including the ability to process and discriminate speech sounds that may be presented under distorted conditions.
• Processing speed (Gs): is the ability to perform automatic cognitive tasks, particularly when measured under pressure to maintain focused attention.
• Decision/reaction time/speed (Gt): reflect the immediacy with which an individual can react to stimuli or a task (typically measured in seconds or fractions of seconds; not to be confused with Gs, which typically is measured in intervals of 2–3 minutes).

Modern tests do not necessarily measure of all of these broad abilities. For example, Gq and Grw may be seen as measures of school achievement and not IQ. Gt may be difficult to measure without special equipment. 

g was earlier often subdivided into only Gf and Gc which were thought to correspond to the nonverbal or performance subtests and verbal subtests in earlier versions of the popular Wechsler IQ test. More recent research has shown the situation to be more complex.

Controversies

While not necessarily a dispute about the psychometric approach itself, there are several controversies regarding the results from psychometric research.

One criticism has been against the early research such as craniometry. A reply has been that drawing conclusions from early intelligence research is like condemning the auto industry by criticizing the performance of the Model T.

Several critics, such as Stephen Jay Gould, have been critical of g, seeing it as a statistical artifact, and that IQ tests instead measure a number of unrelated abilities. The American Psychological Association's report "Intelligence: Knowns and Unknowns" stated that IQ tests do correlate and that the view that g is a statistical artifact is a minority one.

Intelligence across cultures

Psychologists have shown that the definition of human intelligence is unique to the culture that one is studying. Robert Sternberg is among the researchers who have discussed how one's culture affects the person's interpretation of intelligence, and he further believes that to define intelligence in only one way without considering different meanings in cultural contexts may cast an investigative and unintentionally egocentric view on the world. To negate this, psychologists offer the following definitions of intelligence:

1. Successful intelligence is the skills and knowledge needed for success in life, according to one's own definition of success, within one's sociocultural context.
2. Analytical intelligence is the result of intelligence's components applied to fairly abstract but familiar kinds of problems.
3. Creative intelligence is the result of intelligence's components applied to relatively novel tasks and situations.
4. Practical intelligence is the result of intelligence's components applied to experience for purposes of adaption, shaping and selection.

Although typically identified by its western definition, multiple studies support the idea that human intelligence carries different meanings across cultures around the world. In many Eastern cultures, intelligence is mainly related with one's social roles and responsibilities. A Chinese conception of intelligence would define it as the ability to empathize with and understand others — although this is by no means the only way that intelligence is defined in China. In several African communities, intelligence is shown similarly through a social lens. However, rather than through social roles, as in many Eastern cultures, it is exemplified through social responsibilities. For example, in the language of Chi-Chewa, which is spoken by some ten million people across central Africa, the equivalent term for intelligence implies not only cleverness but also the ability to take on responsibility. Furthermore, within American culture there are a variety of interpretations of intelligence present as well. One of the most common views on intelligence within American societies defines it as a combination of problem-solving skills, deductive reasoning skills, and Intelligence quotient (IQ), while other American societies point out that intelligent people should have a social conscience, accept others for who they are, and be able to give advice or wisdom.