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 cognition, which take a variety of forms. 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 current scientific consensus is that there are average overall differences in specific abilities and men have a wider spread in intelligence.

IQ tests

According to Jackson and Rushton, during the early twentieth century, the scientific consensus held that gender plays no role in intelligence. They attribute this consensus in part to early work by Cyril Burt[4] and Lewis Terman, who found no sex differences in the first IQ tests. 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.

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) shows no difference in average IQ between sexes. Other studies done in the mid-1990s have concluded that the IQ performances of men and women differ small. Analyzing data from 2,404 individuals, the “California Verbal Learning Test” concluded that “When mediating variables were controlled, gender differences tended to disappear on tests for which there was a male advantage and to magnify on tests for which there was a female advantage.” In his book, Developmental Influences on Adult Intelligence: The Seattle Longitudinal Study, K. Warner Schaie concludes that there are few gender differences in spatial competencies.

A 1999 study by Richard Lynn in which he analyzed 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. Lynn’s 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. Steve Blinkhorn published a critique of the article in Nature, to which the original article’s authors wrote a response, also published in Nature.. In another study by Rushton, males aged 17 – 18 years were found to have an average of 3.63 IQ points in excess of their female equivalents. Another study, but, found a 2-4 IQ point advantage for females in later life

A 2009 study measured the gap between 3-5 IQ points.

Variance in IQ

Studies consistently show greater variance in the performance of men compared to that of women (i.e., 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.[citation needed] Most modern IQ tests are weighted to even out overall sex differences in average score. Different weightings or tests other than IQ, for instance general intelligence factor, may but be used in defining intelligence. A study by Colom et al. in 2002 showed that the difference observed is in “ability in general”, not in “general ability”, and that the average sex-difference favoring males must be attributed to specific group factors and test specificity.

A 2005 study by Ian Deary, Paul Irwing, Geoff Der, and Timothy Bates,[18] focusing on the ASVAB scores of 1,292 pairs of opposite sex siblings, showed twice as many males as females in the top and bottom 2% of scores, demonstrating a significantly higher variance in male scores. The study also found a very small (d’ ≈ 0.07, or about 7% of a standard deviation) average male advantage in G (factor).

Specific abilities

See also: Sex-related differences in spatial cognition
In a 2008 study[19] paid for by the National Science Foundation in the United States, researchers found 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.”[20] But, the study indicated that, while on average boys and girls performed similarly, boys were overrepresented among the very best performers as well as among the very worst.

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, and becomes experimentally discernible at puberty. A minority of opinions are known to differ on this issue: In his book, Developmental Influences on Adult Intelligence: The Seattle Longitudinal Study, K. Warner Schaie concludes that there are few sex differences in spatial competencies.

Memory: Women show greater proficiency and reliance on distinctive landmarks for navigation while males rely on an overall mental map.[24][25] Studies by H. Stumpf and Richard Lynn have also demonstrated statistically significant medium- and small-term memory advantages in women.
A study examining sex differences in performance on the California Verbal Learning Test found that males performed better on Digit Span Backwards and on reaction time, while females were better on small-term memory recall and Symbol-Digit Modalities Test.

History

In the nineteenth century, whether men and women had equal intelligence was seen by many as a prerequisite for the granting of suffrage.[citation needed] 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 favoring men is a manifestation of sexism or simply a reflection of
innate aptitudes.

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 1,325 grams, while the brains of females had an average weight of 1,144 grams. Other differences that have been established include greater length in men of myelinated axons in their white matter (176,000 km compared to 146,000 km);[27] and 33% more synapses per mm3 of cerebral cortex.

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.[29][30] But, 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 6,325 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.

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. In 2005, Haier et al. reported that, compared with men, women show more white matter and fewer gray matter areas related to intelligence.[33] 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.[34] They also report that the brain areas correlated with IQ differ between the sexes. In small, men and women apparently achieve similar IQ results with different brain regions.

Despite these findings, there still remains no clear relationship between physical brain measurement and functional capacity. Some have suggested[who?] 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 the stimulating quality of the environment and hormonal influences.

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.[37] Girls with this condition play more with “boys’ toys” and less with “girls’ toys” than unaffected controls.[38] Many studies find positive correlations between testosterone levels in normal males and measures of spatial ability.[39] But, the relationship is complex.

It is possible that sexual dimorphism may exist in regard to intellectual abilities in humans.[citation needed] 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.[42] 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 know their social situation.

Another possibility is the effects of socialization. Girls are sometimes discouraged from studying math or science.[citation needed] Similarly, boys are sometimes discouraged from showing empathy, or from spending much 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 vital, but there is also excellent 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 started reviewing the relevant literature.

Some observed differences in the variability of skills between the sexes can be clarified 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).[43][44]
[edit]Controversies

In January 2005, Lawrence Summers, president of Harvard University, unintentionally provoked a public controversy when several attendees discussed with reporters some statements he made during his lunchtime presentation at an economics conference at the National Bureau of Economic Research.[45][46][47] These attendees included MIT biologist Nancy Hopkins, University of California Santa Cruz chancellor designate Denice D. Denton, former deputy director of the National Science Foundation, Anne C. Petersen, former executive director of the Association for Women in Science, Catherine Didion, chemistry professor at the University of Oklahoma, Donna J. Nelson, and Sheila Tobias, a feminist author and proponent of women in science. 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 vital factor might be the above-mentioned greater variance in intelligence among men than women, and that this difference in variance might be intrinsic,[45] adding that he “would like nothing better than to be proved incorrect.” The controversy generated a fantastic deal of media attention; it contributed to the resignation of Summers the following year,[48] and led Harvard to commit $50 million to the recruitment and hiring of women faculty.[49]
In May 2005, Harvard University psychology professors Steven Pinker and Elizabeth Spelke debated “The Science of Gender and Science”.

In 2006, Danish psychologist Helmuth Nyborg was temporarily suspended from his position at Aarhus University, deemed guilty by some of scientific misconduct in relation to the academic documentation after publishing a paper in Personality and Individual Differences that showed an 8-point IQ difference in favor of men. His work was reviewed by an investigative committee.