Sex and intelligence

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 tests, particularly mental spatial rotations.

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 higher variance and therefore there are more men than women at the extremes of ability). Previously the opposite had been thought as evidenced by the variability hypothesis.

However, 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.

The primary reason for expecting that men will have higher average g than women is the male advantage in brain size. Resolving this question requires the use of sophisticated statistical techniques to extract g from the results of IQ tests. Some studies find an average male advantage in g, but most do not.

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. Leta Hollingworth has argued that women were not permitted to realize their full potential, as they were confined to the roles of child rearing and housekeeping.

The findings have 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. In the late-twentieth century, whether men and women had different aptitudes is often taken to reveal whether disproportionate employment or payment of men is a form of sexism or simply a reflection of innate aptitudes.

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 509 out of 800 on the verbal portion while women scored 502 out of 800.

The difference, however, is more pronounced and consistent on the math segment of the SAT. In 2001, men scored 533 while women scored 498. This difference tends to appear year after year. The National Assessment of Educational Progress, another standardized test used in the US, finds a small advantage for males in mathematics (282 vs. 280 points in 12th grade).

Jackson and Rushton extracted a g factor from 145 items on the 1991 SAT, using responses from 46,509 males and 56,007 females. They write that their study found that 17 to 18-year old males averaged the equivalent of 3.63 IQ points higher than their female counterparts. They also write that male-female differences in g are found throughout the score distribution, at every socioeconomic level, and across several ethnic groups.

Allegations of SAT bias
Rosser claimed that there were four potential areas for sex bias through testing:
 * 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.

However, many of these hypotheses are difficult to test, and there is no direct evidence women's lower scores are due to any of these factors.

As with any standardized test, there will always be general speculation as to the efficiency of the scores in predicting one's cognitive ability to begin with. 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 terms of grading. Conversely, those who fare poorly in standardized testing commonly achieve high marks, evidence which might suggest the prescient purpose of standardized testing is decreasing. An alternative explanation is that the subjective assignment of grades by teachers and professors at secondary and collegiate institutions may itself reflect bias as a subjective process which contrasts with the identity-blind automated process employed by standardized tests such as the SAT.

IQ tests
When the Stanford-Binet test was revised in the 1940's, preliminary tests yielded a higher average IQ for women; the test was consequently adjusted to give identical averages for men and women.

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

A 1994 study by H. Stumpf and Douglas N. Jackson based on medical school application test scores showed that men averaged IQs about 8.4 points higher than women, while women averaged memories about 7.5 points higher than men.

A 1999 study by Richard Lynn, found that the IQ difference between men and women is typically about 3-4 IQ points, while women usually maintain short-term memory advantages over men of about 2 IQ points. In a 2005 study published in the British Journal of Psychology which attracted media attention in the wake of the January 2005 controversy at Harvard (below), he and Paul Irwing analyzed existing studies to report that university men have an average IQ between 3.3 and 5.0 points higher than that of university women. In Nature, intelligence-test designer Steve Blinkhorn argued in reply that Lynn and Irwing's analysis was critically flawed, for example by deliberately excluding a large contrary study that made up almost 45% of the subjects in the meta-analysis ; in subsequent correspondence in the same journal, Blinkhorn claimed that their use of meta-analysis was "methodologically inappropriate, statistically bizarre and unsuited to the estimation of subpopulation parameters."

Variance in IQ
Evidence against differences in overall average IQ scores between men and women has come from several very large and representative studies. However, these studies did find that the scores of men show greater variance than the scores of women, and that men and women have some differences in average scores on tests of particular abilities, which tend to balance out in overall IQ scores.

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). The average IQ scores by sex were 100.64 for girls and 100.48 for boys. The difference in mean IQ was not significant. However, the standard deviation was 14.1 for girls and 14.9 for boys. This difference was statistically significant. In the sample studied, 49.6% are girls and 50.4% are boys. 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. It is generally observed that males tend to hit the most positive and negative performance results of many tests.

Deary et al. (in press) compared IQ scores from 1292 pairs of opposite-sex siblings from the US National Longitudinal Survey of Youth. Siblings were used to control for background factors that differ between families. They describe finding a 1 IQ point sized difference in mean scores favoring males, which was significant in this sample. They 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).



Diagram example
The average scores of young men and women in mathematics, for example, will be close, but there will be more men than women in the very low scores and in the very high scores. In this sense, the red bell curve in the diagram can be thought of as representing women, compared to men in green. There is evidence to suggest that forms of autism may be essentially extreme expressions of certain typically male characteristics. The diagram is illustrative of what it means to have a difference in variance, and as such shows much greater difference in variance between the red and green curves than the difference that studies have found between men and women.

Specific abilities
A 2001 report by Richard J. Coley of the ETS found that females often outperformed males on various measures of verbal ability, while males tended to outperform females on measures of mathematical and spatial ability. 


 * Spatial abilities: large differences favoring males are found in performance on visual-spatial tasks (e.g., mental rotation) and spatio-temporal tasks (e.g., tracking a moving object through space). The male advantage in visual-spatial tasks is approximately 1 standard deviation. The difference starts at twelve to sixteen years of age.
 * Quantitative abilities: females have an advantage in quantitative abilities in the early years of school. This trend reverses before puberty, after which time males maintain an advantage.
 * Verbal abilities: a range of differences, some large, favoring females are found in performance on verbal tasks. Males also show higher levels of dyslexia and other reading disabilities. The incidence of stuttering is also higher among males.
 * Memory: Several studies have shown women are better at certain types of memory.


 * General knowledge: A study by Richard Lynn showed that men have more general knowledge than women.
 * Education: In the United States, women tend to outnumber men at colleges and universities, except at technical institutions that emphasize math and science such as MIT and Caltech, where men predominate.
 * Academia: Men outnumber women in tenured faculty positions in math and science. Women outnumber men in tenured faculty positions in humanities fields.
 * High IQ societies: In all "high IQ societies" men outnumber women; e.g., in Mensa the male-to-female ratio is 2:1.
 * Boys tend to have a higher incidence of behavioral problems in schools which may affect academic achievement.

Diagram example
These are examples of different averages between two statistical populations, represented by the pink and red hills in the diagram above. The diagram shows only general concepts of how curves might compare. It does not represent specific experimental data.

Wage differences
Hedges and Nowell (1995) performed a meta-analysis of national ability surveys that cover a 32-year period. Their primary conclusion is that male scores show greater variance (more men than women at the extremes of ability) in most abilities. The use of representative samples gives them reassurance that these differences in variance are true, and not the result of differential selection by sex. Their second finding is that average differences in most abilities are small. Exceptions include moderate to strong average advantages for men in math and science and typically male vocations, and moderate to strong average disadvantages to men in reading. They suggest the male advantage in measures of typical male vocations is not predictive, but that the other strong differences are. Thus, they claim to be concerned about the relative disadvantage of men in writing and the disadvantage to women in science and math. For example, they mention that male advantage in math and science may cause inequality in income between men and women and the underrepresentation of women in these academic fields, and likewise that men may suffer greater rates of illiteracy that will push more of them towards unemployment.

Controversy
Another study performed by the American Psychological Association in response to the book The Bell Curve, which investigated the difference in intelligence between different social classes (strongly correlated with race in the U.S.), determined (as did the authors of the book) that the studies available in 1995 showed no major difference between males and females in regard to IQ scores.

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. 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,, 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.

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

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. 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.

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.

Brain size
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. 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. (Ankney 1992). The differences are smaller but persist when adjusted for body size measured as body height or body surface (Ankey, 1992).

In 2005, Haier et al. reported that compared with men, women show more white matter and fewer gray matter areas related to intelligence. 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. Gray matter is used for information processing, while white matter consists of the connections between processing centers. 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.

Although women may have smaller brains than men, they have greater neuron density in their prefrontal lobe, which is involved in planning, judgment, and language. .

Reasons for differences


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. Girls with this condition play more with "boys' toys" and less with "girls' toys" than unaffected controls. Many studies find positive correlations between testosterone levels in normal males and measures of spatial ability. However, the relationship is complex.

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. 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.

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.

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."

The 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).