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. 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".

Background

Determining whether men and women differ in average IQ has been 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 1940s, 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. [Quinn McNemar, The Revision of the Stanford-Binet Scale, Houghton Mifflin Company, 1942.]

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.

According to Jackson and Rushton, a scientific consensus existed during the early 20th century that there are no sex differences in overall intelligence.cite journal | author=Douglas N. Jackson and J. Philippe Rushton | title=Males have greater g: Sex differences in general mental ability from 100,000 17- to 18-year-olds on the Scholastic Assessment Test | journal=Intelligence | volume=34 |issue=5 |month=September-October |year=2006 |page=479-486 | doi=10.1016/j.intell.2006.03.005] They attribute this consensus in part to early work by Cyril Burt [Burt and Moore, 1912 Burt, C. L., and Moore, R. C. (1912). The mental differences between the sexes. Journal of Experimental Pedagogy, 1, 273–284, 355–388.] and Lewis Terman [Terman, 1916 L.M. Terman, The measurement of intelligence, Houghton Mifflin, Boston, MA (1916).] 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.cite journal | author=Larry V. Hedges; Amy Nowell | title=Sex Differences in Mental Test Scores, Variability, and Numbers of High-Scoring Individuals | journal=Science | year=1995 | volume=269 | pages=41–45 | doi=10.1126/science.7604277 | pmid=7604277]

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. ["Myths of Gender: biological theories about women and men" By Anne Fausto-Sterling 1992 ISBN 0465047920]

IQ tests

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. [ [Intelligence: Knowns and Unknowns http://www.lrainc.com/swtaboo/taboos/apa_01.html] ]

A 1999 controversial [ [http://www.guardian.co.uk/education/2005/nov/06/research.gender The Guardian] "Who has the bigger brain?"] [ [http://news.bbc.co.uk/2/hi/uk_news/education/4183166.stm BBC] 'Men cleverer than women' claim] 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. [cite journal | author=Lynn, Richard | title= Sex differences in intelligence and brain size: a developmental theory | journal=Intelligence | year=1999 | volume=27 | pages=1–12 | doi= 10.1016/S0160-2896(99)00009-4] 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. [cite journal | author= Lynn, R., & Irwing, P. |year=2004 |title=Sex differences on the Progressive Matrices: A meta-analysis |journal=Intelligence |volume=32 |page=481−498 |issue=5 | doi=10.1016/j.intell.2004.06.008]

Other recent studies have concluded IQ performances of men and women differ little.cite journal | author=Larry V. Hedges; Amy Nowell | title=Sex Differences in Mental Test Scores, Variability, and Numbers of High-Scoring Individuals | journal=Science | year=1995 | volume=269 | pages=41-45] [ [Intelligence: Knowns and Unknowns http://www.lrainc.com/swtaboo/taboos/apa_01.html] ] Analyzing data from 2,404 individuals "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." [ [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W4M-49N9FHG-1&_user=10& Gender differences in cognitive abilities: The mediating role of health state and health habits (2002)] Anthony F. Jorm, Kaarin J. Anstey, Helen Christensen and Bryan Rodgers - Abstract: Gender differences were examined in performance on the California Verbal Learning Test (immediate and delayed recall), Digit Span Backwards, Symbol–Digit Modalities Test, Spot-the-Word, and simple and choice reaction time. The data came from a community survey involving 2404 people aged 20–24 years, 2530 aged 40–44 years, and 2551 aged 60–64 years. When mediating variables were controlled, sex 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 spacial competencies. [ [http://books.google.com/books?id=1rDh9EUwy1oC&pg=PA68&vq=gender+difference&dq=%22gender+difference%22+intelligence&source=gbs_search_s&sig=ACfU3U2vwfJIJQLrB1jN4DLbhaKq_GuXWQ#PPA171,M1 Developmental Influences on Adult Intelligence: The Seattle Longitudinal Study] ]

Variance in IQ

Studies consistently show 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. Colom et alii (2002), for example, show that the difference observed is in "ability in general", not in "general ability" and that the average IQ sex-difference favoring males must be attributed to specific group factors and test specificity.

A 2005 Edinburgh University was reported in the Daily Telegraph newspaper. The article states, "An all-male team of psychologists at Edinburgh University has discovered that there are twice as many males as females in the brightest two per cent of the population... There are clearly exceptions: Carol Vorderman, the television presenter, is reported to have an IQ of more than 150, whereas the national average is 100." [ [http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2007/09/24/eamen124.xml Men: either very clever or really stupid] The Daily Telegraph]

pecific abilities

*In a 2008 study 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 boys in high school performed better than girls in math 20 years ago, 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." [ [http://www.nytimes.com/2008/07/25/education/25math.html?pagewanted=all Math Scores Show No Gap for Girls, Study Finds] New York Times, July 25, 2008] However, 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. [ [http://online.wsj.com/article/SB121691806472381521.html Boys' Math Scores Hit Highs and Lows ] The Wall Street Journal, July 25, 2008]
*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).] The male advantage in visual-spatial tasks is approximately 1 standard deviation, and becomes experimentally discernible at puberty. [Siann (1977)] 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 gender differences in spatial competencies. [ [http://books.google.com/books?id=1rDh9EUwy1oC&pg=PA68&vq=gender+difference&dq=%22gender+difference%22+intelligence&source=gbs_search_s&sig=ACfU3U2vwfJIJQLrB1jN4DLbhaKq_GuXWQ#PPA171,M1 Developmental Influences on Adult Intelligence: The Seattle Longitudinal Study] ]
*Memory: Women show greater proficiency and reliance on landmarking for memory skills. [ [http://www.sciam.com/article.cfm?articleID=00018E9D-879D-1D06-8E49809EC588EEDF Sex Differences in the Brain] : Men and women display patterns of behavioral and cognitive differences that reflect varying hormonal influences on brain development- By Doreen Kimura May 13, 2002.] Studies by H. Stumpf and Richard Lynn have also demonstrated statistically significant medium- and short-term memory advantages in women.
*A study examining gender 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 short term memory recall and Symbol-Digit Modalities Test. advantage. [ [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W4M-49N9FHG-1&_user=10& Gender differences in cognitive abilities: The mediating role of health state and health habits (2002)] Anthony F. Jorm, Kaarin J. Anstey, Helen Christensen, Bryan Rodgers]

Physical Brain Parameters

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. [Kimura, D. 1999. Sex and Cognition. MIT Press. ISBN 9-780262-611640] [cite journal |author=Ho KC, Roessmann U, Straumfjord JV, Monroe G |title=Analysis of brain weight. I. Adult brain weight in relation to sex, race, and age |journal=Arch. Pathol. Lab. Med. |volume=104 |issue=12 |pages=635–9 |year=1980 |month=December |pmid=6893659 |doi= |url=] 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. [cite journal|author=CD Ankney |title=Sex Differences in Relative Brain Size: The Mismeasure of Woman, Too? |journal=Intelligence |volume=16 |year=1992 |pages=329-336 | doi=10.1016/0160-2896(92)90013-H]

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. [cite journal |author=Richards BA, Chertkow H, Singh V, "et al" |title=Patterns of cortical thinning in Alzheimer's disease and frontotemporal dementia |journal=Neurobiol. Aging |volume= |issue= |pages= |year=2008 |month=February |pmid=18261828 |doi=10.1016/j.neurobiolaging.2007.12.019 |url=] In 2005, Haier "et al." reported that compared with men, women show more white matter and fewer gray matter areas related to intelligence. [cite journal | author=Haier RJ, Jung RE, Yeo RA, et al. | title=The neuroanatomy of general intelligence: sex matters | journal=NeuroImage | year=2005 | volume=25 | pages=320–327 | pmid=15734366 | doi=10.1016/j.neuroimage.2004.11.019] 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. [cite journal |author=Haier RJ, Jung RE, Yeo RA, Head K, Alkire MT |title=Structural brain variation and general intelligence |journal=Neuroimage |volume=23 |issue=1 |pages=425–33 |year=2004 |month=September |pmid=15325390 |doi=10.1016/j.neuroimage.2004.04.025 |url=http://www.ucihs.uci.edu/pediatrics/faculty/neurology/haier/pdf/82.pdf] 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. [ [http://today.uci.edu/news/release_detail.asp?key=1261 Intelligence in men and women is a gray and white matter: Men and women use different brain areas to achieve similar IQ results, UCI study finds] by Irvine, Calif. , January 20, 2005]

Despite these findings, there still remains no clear relationship between physical brain measurement and functional capacity. Some have suggested Fact|date=April 2008 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. [cite journal |author=Faverjon S, Silveira DC, Fu DD, "et al" |title=Beneficial effects of enriched environment following status epilepticus in immature rats |journal=Neurology |volume=59 |issue=9 |pages=1356–64 |year=2002 |month=November |pmid=12427884 |doi= |url=http://www.neurology.org/cgi/pmidlookup?view=long&pmid=12427884]

Hypotheses

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.Geary, D. (1998). "Male, female: The evolution of human sex differences". Washington, DC: American Psychological Association.] 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.

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) [cite news | author=Nicholas Wade |year=2007 |title=Pas de Deux of Sexuality is Written in the Genes | publisher=The New York Times | date=10 April 2007 | url=http://www.nytimes.com/2007/04/10/health/10gene.html] [Ounsted and Taylor (1979)] .

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. [ [http://www.boston.com/news/education/higher/articles/2005/01/17/summers_remarks_on_women_draw_fire?mode=PF Original Boston Globe story] reporting the remarks of Larry Summers at a January 2005 conference] [ [http://www.president.harvard.edu/speeches/2005/nber.html Transcript] of Summers' remarks at NBER Conference on Diversifying the Science & Engineering Workforce] [ [http://www.president.harvard.edu/speeches/2005/womensci.html Summers' initial response to controversy] ] 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 important factor might be the above-mentioned greater variance in intelligence among men than women, and that this difference in variance might be intrinsic, [ [http://www.boston.com/news/local/articles/2005/01/17/summers_remarks_on_women_draw_fire/ Summer's Remarks on Women Draw Fire] 2005 January 17] 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. [ [http://www.thecrimson.com/article.aspx?ref=507771 University Will Commit $50M to Women in Science] , Harvard Crimson, 2005 May 16]

In May 2005, Harvard University psychology professors Steven Pinker and Elizabeth Spelke debated "The Science of Gender and Science". [ [http://www.edge.org/3rd_culture/debate05/debate05_index.html Edge: The Science Of Gender And Science ] ]

In 2006, Danish psychologist Helmuth Nyborg was temporarily suspended from his position at Aarhus University deemed guilty of scientific misconduct in relation to the academic documentationFact|date=October 2008 after publishing a paper in "Personality and Individual Differences" that showed an 8 point IQ difference in favour of men. [cite journal| author=Nyborg H | year=2005 |title=Sex-related differences in general intelligence g, brain size, and social status | journal=Personality and Individual Differences |volume=39 |pages=497-509 | doi=10.1016/j.paid.2004.12.011 | month=August | issue=3]

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, Ben (13 July 2006). [http://www.nature.com/nature/journal/v442/n7099/edsumm/e060713-04.html Does Gender Matter?] "Nature"] 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.

ee also

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

References

Bibliography

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