Sex and intelligence

From Wikipedia, the free encyclopedia

Sex and intelligence research investigates differences in the distributions of cognitive skills between men and women. This research employs experimental tests of cognitive ability, which take a variety of forms, including written tests like the SAT. Research focuses on differences in individual skills as well as overall differences in general cognitive ability, which is often called g. IQ tests, specially designed to measure cognitive ability, usually test a variety of skills, and IQ scores are often used as a measure of g.

The populations of men and women differ on average in how well they perform on some of these skill tests, but do equally well on other tests. For example, women tend to score higher on certain verbal and memory tests, whereas men tend to score higher on spatial and mathematical tests. While these results are relatively uncontroversial, the question of whether men and women differ on average in g is a matter of debate among experts. Most recent studies unambiguously find that men as a population are more varied than women in g (i.e. they have a slightly higher variance and therefore there are slightly more men than women at the extremes of ability).

Determining whether men and women differ on average has been more difficult. It is easy to design an IQ test in which either males or females score higher on average, by selecting different tests or giving them different weights, so the question boils down to which weights the different tests should have for the g factor. For example, when the Stanford-Binet test was revised in the 1940's, preliminary tests yielded a slightly higher average IQ for women, a discrepancy attributed to a greater than usual emphasis on verbal ability. The test was subsequently adjusted to give identical averages for men and women.^[1]

Contents 1 History 2 IQ tests 2.1 Variance in IQ 3 Localization of General Intelligence 4 Gender-related Reversed Asymmetry of Intelligence 4.1 Specific abilities 5 SAT scores 6 Physical Brain Parameters 7 Hypotheses 8 Controversies 9 See also 10 References 11 Bibliography

[edit] History

The scientific study of the differences in mental aptitudes between men and women dates back at least as far as the mid-nineteenth century, when the question of women's voting rights arose in a number of countries. In Victorian England, for example, the philosopher John Stuart Mill argued that there were no differences between men and women, whereas the scientist Charles Darwin (in his Descent of Man) argued that women were by their nature inferior in respect to mental ability. Many of these early attempts were based on anecdotal data. However, some scientists, such as Paul Broca (1861), attempted to derive empirical results from various forms of anthropometry, namely the comparison of brain mass. With the development of psychology at the end of the nineteenth century, and the evolving focus on intelligence testing in the early twentieth century, further attempts were made by a variety of scientists to examine the mental differences between men and women.

This discussion has provoked controversy at various times, often because political implications were perceived to be attached to them. In the nineteenth century, as noted, whether men and women had equal intelligence was seen by many as a prerequisite for the granting of suffrage. Leta Hollingworth argues that: Women were not permitted to realize their full potential, as they were confined to the roles of child rearing and housekeeping. From the late twentieth century onwards, sex differences in intelligence have been discussed to determine whether disproportionate employment or payment favouring men is a manifestation of sexism or simply a reflection of innate aptitudes.^[2]

[edit] IQ tests

According to Jackson and Rushton, a scientific consensus existed during the early 20th century that there are no sex differences in overall intelligence.^[3] They attribute this consensus in part to early work by Cyril Burt^[4] and Lewis Terman^[5] who found no sex differences in the first IQ tests.

More recent studies have shown inconsistent results in comparing the overall IQ performances of men and women: In 1995, Hedges and Nowell demonstrated only statistically insignificant differences in average IQ between men and women using data published in several large representative studies published up until that year.^[6] A 1995 study performed by the American Psychological Association in response to the book The Bell Curve (which investigated intelligence differences between different social classes) also showed no difference in average IQ between sexes.^[7]

In 1999, a study by Richard Lynn [2] analyzing data from a number of published tests (such as the standardized g-loaded Wechsler Adult Intelligence Scale-Revised) found that the mean IQ of men exceeded that of women by approximately 3-4 IQ points.^[8] A meta-analysis, conducted in 2004, examining sex differences on the Standard and Advanced Progressive Matrices (comprising various g-loaded tests of non-verbal reasoning) also found that men exceeded women by an average of 5.0 IQ points.^[9] In a 2006 study, Jackson and Rushton extracted a g factor from 145 items on the 1991 SAT, using responses from 46,509 males and 56,007 females.^[3] Their study findings showed that 17- to 18-year old males averaged 3.63 IQ points higher than did their female counterparts, and that the differences become more evident with increasing age. They postulated that this increasing discrepancy with age was due to the relative inactivity of women in higher age groups.

What these studies illustrate consistently, however, is greater variance in the performance of men compared to that of women (ie. men are more represented at the extremes of performance), and that men and women have statistically significant differences in average scores on tests of particular abilities, which even out when the overall IQ scores are weighted.

[edit] Variance in IQ

Hedges and Nowell (1995), in their meta-analysis of national ability surveys over a 32 year period, demonstrated significantly greater variance among men compared to women in most skilled tasks and executive abilities.

Deary et al. (2003) performed an analysis of an IQ test administered to almost all children in Scotland at age 11 in 1932 (>80,000).^[10] The average IQ scores by sex were 162.69 for girls and 163.36 for boys. The difference in mean IQ was not significant. However, the standard deviation was 14.1 for girls and 14.9 for boys, a difference which was statistically significant. Because of the difference in variance between the sexes, however, girls are in excess by 2% in the middle IQ range of 90–115. At the extreme IQ ranges, 50–60 and 130–140, boys make up 58.6% and 57.7% of the population (gaps of 17.2% and 15.4%) respectively. That is, boys were overrepresented amongst the lowest and highest IQ groups.

Deary et al. (in press) also compared IQ scores from 1292 pairs of opposite-sex siblings from the US National Longitudinal Survey of Youth.^[11] Siblings were used to control for background factors that differ between families. They describe finding a 1 IQ point sized difference in mean score favouring scores favoring males, which was significant in this sample. They also describe finding larger differences in variance, with nearly twice as many males as females scoring in the top 2% (the IQ equivalent of scores above 130).

An extensive study published by the British Journal of Psychology in 2005 used research based on IQ tests given to 80,000 people and a further study of 20,000 students.^[12] It demonstrated that men on average scored five points ahead on IQ tests, a difference which remained statistically significant despite the study design accommodating for gender-specific abilities. The study also found a much higher proportion of men in the higher IQ brackets. "There are 3 men to each woman with an IQ above 130 and 5.5 men for each woman with an IQ above 145" according to Dr Paul Irwing, the paper's lead researcher. He goes on to say: "These different proportions of men and women with high IQ scores may go some way to explaining the greater numbers of men achieving distinctions of various kinds, such as chess grandmasters, Fields medallists for mathematics, Nobel prizewinners and the like." [3]

Consistent with these findings, men have been shown to predominate in many high IQ societies. In Mensa the male-to-female ratio is approximately 2:1, based on US demographic data in 2008. [4]

[edit] Localization of General Intelligence

The localization of the determinants of human performance have been a subject of intense debate for over a century. In 1904, Spearman put forward the idea that measures of performance or success in diverse cognitive tests show a pattern of almost universal positive correlation ^[13];^[14];^[15],^[16]. He postulated the hypothesis of a general intelligence or g factor making some contribution to success in diverse forms of cognitive activity.

Subsequently, two contrasting postulates on cortical localization of intelligence were put forward. The first proposes that, processing of any task receives contribution from a large set of components or information-processing functions ^[17],^[18],^[19]; ^[20];^[21];^[22]. Therefore, any two tasks are likely to share at least some components, giving rise to universal positive correlation. The proposed hypothesis assumes that, general ability is diffusely represented on the whole cortex, and injury to any region of the brain produces an intellectual decrement. On the other hand, the second, so-called hierarchic hypothesis postulates that, in the course of evolution some area of the brain has gained a dominant role in sustaining general intelligence ^[23].

Intelligence tests have classified by task complexity, into tests at the center and tests at the periphery ^[24]. Psychometric tests including Raven's Progressive Matrices (RPM) ^[25] and other complex reasoning tests were classified as tests at the center, while simpler tests were placed at the periphery. Applying this construct, irrespective of the views held by both hypotheses, the centrality of the RPM emerges in either case. Furthermore, it has been suggested that RPM is a good test of intelligence, and should account for a great deal of the reasoning in other tests ^[26].

Prabhakaran and others ^[27], have suggested that the neural substrate for intelligence lies within prefrontal cortex and post-rolandic structures. ^[28]. Recently, positron emission tomography studies have shown selective recruitment of lateral prefrontal cortex in one or both hemispheres ^[29].

[edit] Gender-related Reversed Asymmetry of Intelligence

Studies of cerebral lateralization of intelligence may implicate one hemisphere, or both, if general abilities were diffusely represented. Recent experiments using functional transcranial Doppler sonography ^[30], have provided insights to the gender-related hemispheric localization of general intelligence. These findings have been confirmed in studies using functional magnetic resonance imaging ^[31], and magnetic resonance spectroscopy ^[32], and have further elucidated the difference in localization of general intelligence in men and women. Overall, the results of the experiments by Njemanze (2005), using functional transcranial Doppler during RPM tasks of general intelligence suggest that, for successful resolution of RPM tasks, women used a left hemisphere strategy while men used the right hemisphere.

This implies that general intelligence is associated with neural systems within one hemisphere that are accessible to a variety of cognitive processes. These findings support the hierarchical hypothesis, and agree with results of recent studies, using positron emission tomography ^[33] and fMRI ^[34]. Similarly, the results are consistent with earlier studies in patients with lesions in the postrolandic structures ^[35]. In studies using RPM tasks, the areas of regional CBF activation found in young subjects ^[36] comprised inferolateral temporal cortex including the fusiform gyrus bilaterally, and the middle temporal gyrus on the left, portions of the left medial temporal cortex including the parahippocampal gyrus, the left inferior parietal lobule, and the cerebellum.

Gender-related differences in cognitive tasks have been a subject of intense debate. The Vandenberg mental rotation tests depicts rotation in three-dimensional space, and typically yields one of the largest established cognitive gender differences favoring males. Furthermore, processing in three dimensions is not a necessary condition for large sex difference on tests of mental rotation ^[37]. On the other hand, superior performance by women on task requiring object location memory has challenged the traditional view that men excel on all spatial tasks ^[38]. Njemanze (2005) explained these gender-related differences, by presuming that general intelligence is related to other lateralized hemispheric cognitive functions ^[39] including some visuospatial skills. His findings that men used a right hemisphere strategy may well explain the observation in numerous reports, that the average level of spatial talents usually requiring general intelligence is higher in men than women ^[40]. This may suggest that, in men, ipsilateral right hemisphere localization of general intelligence facilitates visuospatial problem-solving. On the other hand, in women, transcallosal pathways are implicated in left hemisphere strategy of visuospatial processing. It is plausible that, reported gender-related hemispheric advantage is dependent on whether task processing strategies implicate either the intra-hemispheric or transcallosal pathways.

Njemanze (2005) postulated that, successful RPM problem-solving employs a discrete knowledge strategy (DKS), that selects neural pathways represented in one hemisphere. While unsuccessful outcome implicates a non-discrete knowledge strategy (nDKS). RPM paradigm is by itself a working memory function ^[41]. This suggests that the DKS model may have a correlate in mnemonic operations. In other words, DKS model may have a discrete knowledge base (DKB) of essential components needed for task resolution, while for nDKS, DKB is absent, and hence a 'global' or bi-hemispheric search occurs. Future studies with RPM in patients with memory deficits may provide useful insights.

Tranel and colleagues (2005) found a systematic effect of gender on the pattern of left-right asymmetry in ventromedial prefrontal cortices (VMPCs) using fMRI. In men, there were severe defects following unilateral right VMPC damage, but not following left-sided damage. In women, there were defects following unilateral left VMPC damage; following right-sided damage, however, defects were mild or absent. The findings suggest that men and women may use different strategies to solve similar problems--e.g. men may use a more holistic, gestalt-type strategy, and women may use a more analytic, verbally-mediated strategy.

In a study, Jung and colleagues (2005) examined twenty-seven normal control subjects (17 male, 10 female) to determine whether N-acetylaspartate (NAA), a metabolite found primarily within neurons, is related to intelligence as assessed by the Wechsler Adult Intelligence Scale-III. Of the three white matter regions studied (i.e., left frontal, right frontal, left occipito-parietal), they found that a model including only left occipito-parietal white matter predicted intellectual performance, providing regional specificity to previous findings of NAA-IQ relationships. Moreover, they found that a complex combination of left frontal and left occipito-parietal NAA strongly predicted performance in women, but not men. Overall, these studies support the reversed gender-related asymmetry first postulated by Njemanze (2005). ←

[edit] Specific abilities

• Verbal/mathematical ability: Hedges and Nowell (1995) demonstrated strong average advantages for men in math and science and typically male vocations, and moderate to strong average advantages to women in reading. This trend was also found in a 2001 report by Richard J. Cooley of the ETS.[5].
• Spatial abilities: large differences favoring males are found in performance on visual-spatial tasks (eg. mental rotation) and spatio-temporal tasks (eg. tracking a moving object through space).^[42] The male advantage in visual-spatial tasks is approximately 1 standard deviation, and becomes experimentally discernible at puberty.^[43]
• Memory: Women show greater proficiency and reliance on landmarking for memory skills.^[44] Studies by H. Stumpf and Richard Lynn have also demonstrated statistically significant medium- and short-term memory advantages in women, respectively.
• General knowledge: A study by Richard Lynn suggests that men show greater ability than women in maintaining broad general knowledge^[45]
• Education: In the United States and Great Britain, women outnumber men at colleges and universities, except at technical institutions which emphasise mathematics and science such as MIT and Caltech, where men predominate.^[46][47] An analysis of 2007 figures by Universities UK shows that, even given this overall imbalance, men continue to achieve a significantly greater proportion of first class degrees than their female counterparts.
• Academia: Men outnumber women in tenured faculty positions in mathematics and science, and women outnumber men in tenured faculty positions in humanities fields.^{[citation needed]}

[edit] SAT scores

The SAT is a voluntary, standardised test taken by many American college applicants. It is administered by the Educational Testing Service, which keeps track of the gender of test-takers and releases SAT scores by gender. In 2001, men scored 533 and 509 in the math and verbal tests respectively, while women scored 498 and 502 respectively. These discrepancies reflect gender differences in scores since the first implementation of the SAT (see table.)

Following concerns about the persistent gender disparity in SAT scores, a newly-designed SAT was implemented as of 2005 which placed much greater emphasis on writing abilities; areas in which women have historically shown greater strength^{[citation needed]} . The figures in 2006 show men scoring 505, 536 and 491 respectively in reading, math and writing, respectively, while women scored 502 in all 3 categories. [6]

Rosser (1989) claimed that there were four potential areas for sex bias through testing^[48]:

1. Test questions refer to more men than women, and women are shown in situations of lower status.
2. Test questions refer to contexts more familiar to men.
3. Test validity under-predicts women's academic capabilities and over-predicts men's.
4. Tests that under-predict women's capabilities are used to restrict their educational opportunities.

As with any standardized test, there will always be general speculation as to the accuracy of SAT scores in predicting one's general cognitive ability. The most commonly cited discrepancy is the occasional tendency for adept test takers to score very highly, while failing to demonstrate a corresponding high academic aptitude in other areas. Conversely, those who fare poorly in standardized testing commonly achieve high marks in other modalities of grading, evidence which might suggest the prescient purpose of standardized testing is decreasing. An alternative explanation is that subjective or biased grading by teachers at secondary and collegiate institutions may contrast with more objective scores retrieved through the identity-blind automated process employed by the SAT.

[edit] Physical Brain Parameters

See also: Craniometry, Brain size and intelligence, and Neuroscience and intelligence

In 1861, Paul Broca examined 432 human brains and found that the brains of males had an average weight of 1325 grams, while the brains of females had an average weight of 1144 grams.

In studies concerning intelligence, it has been suggested that the ratio of brain weight to body weight is more predictive of IQ levels, rather than actual brain weight. While men's brains are an average of 10-15% larger and heavier than women's brains, some researchers propose that the ratio of brain to body size does not differ between the sexes.^[49][50] However, some argue that since brain to body size ratios tend to decrease as body size increases, a sex difference in brain weight ratios still exists between men and women of the same size. A 1992 study of 6325 Army personnel found that men's brains had an average volume of 1442 cm³, while the women averaged 1332 cm³. These differences were shown to be smaller but to persist even when adjusted for body size measured as body height or body surface, such that women averaged 100g less brain mass than men of equal size.^[51]

An alternative proposal is the measurement of gray matter or white matter volume in the brain as an indicator of intelligence; the former used for information processing, whereas the latter consisting of the connections between processing centers. Neuroimaging studies, such as MRI and CT have demonstrated loss of gray matter volume in conditions associated with cognitive impairment, such as Alzheimer's disease, frontotemporal dementia and senile dementia.^[52] In 2005, Haier et al. reported that compared with men, women show more white matter and fewer gray matter areas related to intelligence.^[53] Using brain mapping, it was shown that men have more than six times the amount of gray matter related to general intelligence than women, and women have nearly ten times the amount of white matter related to intelligence than men.^[54] They also report that the brain areas correlated with IQ differ between the sexes. In short, men and women apparently achieve similar IQ results with different brain regions.^[55]

Despite these findings, there still remains no clear relationship between physical brain measurement and functional capacity. Some have suggested^{[citation needed]} that physical studies of the brain in predicting intelligence are largely arbitrary due to the inherent neuroplasticity of the organ and the multitude of ways that brain function can be influenced by experiential and hormonal influences.^[56]

[edit] Hypotheses

The importance of testosterone and other androgens as a cause of sex differences has been a subject of study. Adult women who were exposed to unusually high levels of androgens in the womb due to a condition called congenital adrenal hyperplasia score significantly higher on tests of spatial ability.^[57] Girls with this condition play more with "boys' toys" and less with "girls' toys" than unaffected controls.^[58] Many studies find positive correlations between testosterone levels in normal males and measures of spatial ability.^[59] However, the relationship is complex.^[60][61]

It is possible that sexual dimorphism may exist in regard to intellectual abilities in humans. Men may have evolved greater spatial abilities, possibly as a result of certain behaviors, such as navigating during a hunt, that they were more likely to be involved in during humans' evolutionary history.^[62] Similarly, women may have evolved to devote more mental resources to gathering food, as well as understanding and tracking relationships and reading others' emotional states in order for them to be able to better understand their social situation.^[62]

Another possibility is the effects of socialization. Girls are sometimes discouraged from studying math or science. Similarly, boys are sometimes discouraged from displaying empathy, or from spending excessive time reading for pleasure^{[citation needed]}.

According to Diane F. Halpern, the above two hypotheses are not mutually exclusive; some combination of the two may be at work. She wrote in the preface of her 2000 book Sex Differences In Cognitive Abilities:

At the time I started writing this book it seemed clear to me that any between sex differences in thinking abilities were due to socialization practices, artifacts, and mistakes in the research. After reviewing a pile of journal articles that stood several feet high, and numerous books and book chapters that dwarfed the stack of journal articles, I changed my mind. The literature on sex differences in cognitive abilities is filled with inconsistent findings, contradictory theories, and emotional claims that are unsupported by the research. Yet despite all the noise in the data, clear and consistent messages could be heard. There are real and in some cases sizable sex differences with respect to some cognitive abilities. Socialization practices are undoubtedly important, but there is also good evidence that biological sex differences play a role in establishing and maintaining cognitive sex differences, a conclusion I wasn't prepared to make when I began reviewing the relevant literature.

Some observed differences in the variability of skills between the sexes can be explained genetically: many brain-related genes are located on the X chromosome, of which women have two copies and men only one. A mutation in one of these genes, whether positive or negative, will thus have a higher impact in males than in females (where the second, presumably non-mutated copy will mitigate the effect of the mutated one)^[63][64].

[edit] Controversies

In January 2005, Lawrence Summers, president of Harvard University, unintentionally provoked a public controversy when MIT biologist Nancy Hopkins leaked comments he made at a closed economics conference at the National Bureau of Economic Research.^{[65] [66] [67]} In analyzing the disproportionate numbers of men over women in high-end science and engineering jobs, he suggested that, after the conflict between employers' demands for high time commitments and women's disproportionate role in the raising of children, the next most important factor might be the above-mentioned greater variance in intelligence among men than women, and that this difference in variance might be intrinsic,^[68] adding that he "would like nothing better than to be proved wrong." The controversy generated a great deal of media attention, forced Summers to resign, and led Harvard to commit $50 million to the recruitment and hiring of women faculty.^[69]

In May 2005, Harvard University psychology professors Steven Pinker and Elizabeth Spelke debated "The Science of Gender and Science".^[70]

In 2006, Danish psychologist Helmuth Nyborg was temporarily suspended from his position at Aarhus University after publishing a paper in Personality and Individual Differences that showed an 8 point IQ difference in favour of men.^[71]

In July 2006, Stanford University neurobiologist Ben Barres, a transsexual man, wrote a provocative piece in Nature on his own experiences as both a male and female scientist.^[72] Barres argued that prior to transition, he had succeeded as a female despite pervasive sexism. Barres wrote that numerous studies show female scientists are consistently rated lower than their male counterparts with the same levels of productivity and credentials.

[edit] See also

• Race and intelligence
• Neuroscience and intelligence
• Height and intelligence
• Sexual dimorphism
• Sex and crime
• Gender differences
• Ruth Hubbard

[edit] References

[edit] Bibliography

• Born, M. P., Bleichrodt, N. & van der Flier, H. (1987). "Cross-cultural comparison of sex-related differences on intelligence tests". Journal of Cross-Cultural Psychology 18: 283–314. 
• Haier RJ, Benbow CP. (1995). "Sex differences and lateralization in temporal lobe glucose metabolism during mathematical reasoning". Dev Neuropsychol. 11: 405–414. 
• Lynn, Richard, with P.Irwing and T.Cammock (2002). "Sex differences in general knowledge". Intelligence 30: 27–40. 
• Lynn, Richard (1999). "Sex differences in intelligence and brain size: a developmental theory". Intelligence 27: 1–12.