Sex and intelligence

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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, which are specially designed to measure cognitive ability, usually test a variety of skills.

The population 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 test, 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 studies unambiguously find that men as a population are more varied than women in g (i.e. there are more men than women at the extremes of ability). For instance, a 2005 study which was published in the British Journal of Psychology concluded that there are twice as many men with an above-average IQ of 125 than there are women. [2]

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.

Contents

[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. 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.[1]

[edit] SAT scores

Average SAT scores (recentered) by sex, 1972 - 2003. Source: [1], Table A
Average SAT scores (recentered) by sex, 1972 - 2003. Source: [1], Table A

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.

Jackson and Rushton extracted a g factor from 145 items on the 1991 SAT, using responses from 46,509 males and 56,007 females.[2] 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.

[edit] Allegations of SAT bias

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Rosser (1989 http://sq.4mg.com/IQincome.htm#G) claimed that there were four potential areas for testing bias in the SAT:

  1. Test content in which many more men than women are referred to or depicted and where women are depicted, they are typically shown in lower status situations
  2. Test context in which questions are set in experiences more familiar to men (except when the situation involved driving to the grocery store, cooking food, or doing the dishes)
  3. Test validity in which women's academic capabilities are under-predicted while that of men are over-predicted
  4. Test use in which women's access to educational opportunities are diminished or restricted by an institution's reliance on test scores which under-predicts their abilities.

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.

[edit] IQ tests

According to Jackson and Rushton, a scientific consensus existed during the 20th century that there are no sex differences in overall intelligence.[2] They attribute this consensus in part to early work by Cyril Burt[3] and Lewis Terman[4] 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 IQ points higher than men.[5] This study, however, may be atypical.[citation needed]

A 1999 study by Richard Lynn [3], 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 [6] which attracted media attention in the wake of the January 2005 controversy at Harvard (below),[7][8] 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.[9]

[edit] Variance in IQ

Evidence against differences in overall average IQ scores between men and women comes from several very large and representative studies.[10] 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).[11] The average IQ scores by sex were 100.64 for girls and 100.48 for boys. The difference in mean IQ is 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 (a gap of 17.2% and 15.4%) respectively. That is, boys were overrepresented amongst the lowest and highest IQ groups. A study done on 11 year olds may not be representative of adult populations.[citation needed]

Deary et al. (in press) compared IQ scores from 1292 pairs of opposite-sex siblings from the US National Longitudinal Survey of Youth.[12] 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).

[edit] 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. [4]

  • 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).[13] The male advantage in visual-spatial tasks is approximately 1 standard deviation.
  • Quantitative abilities: female 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. [14]
  • General knowledge: A study by Richard Lynn showed that men have more general knowledge than women.[15]
  • 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.[16][17]
  • 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 many "high IQ societies" men outnumber women, e.g., in Mensa the male-to-female ratio is 2:1. However, these societies are self-selecting, with voluntary membership.
  • Boys tend to have a higher incidence of behavioral problems in schools which may impact academic achievement[16]

[edit] Evolution over time

Hedges and Nowell (1995) performed a meta-analysis of national ability surveys that cover a 32-year period.[10] 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.

A recent report by the National Academies in 2006 found that female performance in high school mathematics now matches that of males, and expressed that if biology were the basis of the change, then major evolution has occurred in the past decades. In addition, the report found that there were lower numbers of women in the mathematics and science fields than there should be even when studies based on biological differences, childbearing demands, and hormonal differences were looked at. The report concluded that women were the subject of unintended bias in these fields. National Academy

[edit] 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 races, 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. [18]

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. [19] [20] [21] 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[22], adding that he "would like nothing better than to be proved wrong". The controversy generated a great deal of media attention, forced Summers to make a number of apologies, and led Harvard to commit $50 million to the recruitment and hiring of women faculty.[23]

In May 2005, Harvard University psychology professors Steven Pinker and Elizabeth Spelke debated "The Science of Gender and Science".[24]

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. [25] 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 asked to vacate 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.Nyborg, Helmuth (2005). "Sex-related differences in general intelligence g, brain size, and social status". Personality and Individual Differences 39: 497-509. 

[edit] Brain size

See also: Craniometry and Brain size 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. 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[5]). The differences are smaller but persist when adjusted for body size (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.[26] They also report that the brain areas correlated with IQ differ between the sexes. They conclude that men and women apparently achieve similar IQ results with different brain regions.[27]

Although women may have smaller brains than men, they appear to have greater neuron density in their prefrontal lobe ([6]), which is involved in higher functions such as planning, judgement, and language, although men still have higher absolute grey matter volume than women in their prefrontal lobe ([7]).

[edit] Reasons for differences

The effect of Stereotype threat. Data from Linking Stereotype Threat and Anxiety by Jason W. Osborne* North Carolina State University, USA
The effect of Stereotype threat. Data from Linking Stereotype Threat and Anxiety by Jason W. Osborne* North Carolina State University, USA

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.[28] Girls with this condition play more with "boys' toys" and less with "girls' toys" than unaffected controls.[29] Many studies find positive correlations between testosterone levels in normal males and measures of spatial ability.[30] However, the relationship is complex.[31][32]

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.[33] 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.[33]

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.

[edit] See also

[edit] References

  1. ^ Myths of Gender: biological theories about women and men By Anne Fausto-Sterling 1992 ISBN 0465047920
  2. ^ a b Douglas N. Jackson and J. Philippe Rushton, Males have greater g: Sex differences in general mental ability from 100,000 17- to 18-year-olds on the Scholastic Assessment Test, Intelligence, Volume 34, Issue 5, September-October 2006, Pages 479-486.
  3. ^ 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.
  4. ^ Terman, 1916 L.M. Terman, The measurement of intelligence, Houghton Mifflin, Boston, MA (1916).
  5. ^ Stumpf, H. and Jackson, D. N. (1994). "Gender-related differences in cognitive abilities: evidence from a medical school admissions program". Personality and Individual Differences 17: 335–344. 
  6. ^ Paul Irwing, Richard Lynn, "Sex differences in means and variability on the progressive matrices in university students: a meta-analysis," British Journal of Psychology, 96(4):505-524, 2005 November.
  7. ^ BBC reporting Lynn & Irwing study, 2005
  8. ^ Guardian reporting Lynn & Irwing study and Blinkhorn's reply, 2005
  9. ^ Blinkhorn, S. Intelligence: a gender bender, Nature 2005 Nov 3;438(7064):31-2.
  10. ^ a b Larry V. Hedges; Amy Nowell (1995). "Sex Differences in Mental Test Scores, Variability, and Numbers of High-Scoring Individuals". Science 269: 41-45. 
  11. ^ IJ Deary, G Thorpe, V Wilson, JM Starr, LJ Whalley (2003). "Population sex differences in IQ at age 11: the Scottish mental survey 1932". Intelligence 31: 533–542. 
  12. ^ Ian J. Deary, Paul Irwing, Geoff Der and Timothy C. Bates. Brother–sister differences in the g factor in intelligence: Analysis of full, opposite-sex siblings from the NLSY1979. Intelligence, In Press. DOI:10.1016/j.intell.2006.09.003
  13. ^ Intelligence: Knowns and Unknowns
  14. ^ 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.
  15. ^ Richard Lynn and Tatu Vanhanen IQ and Global Inequality. (2006).
  16. ^ a b Why Do Women Outnumber Men in College?
  17. ^ More black women than men in college, journal finds by Seung Hwa Hong, Daily Trojan (U. Southern California) 12/05/2002
  18. ^ [Intelligence: Knowns and Unknowns http://www.lrainc.com/swtaboo/taboos/apa_01.html]
  19. ^ Original Boston Globe story reporting the remarks of Larry Summers at a January 2005 conference
  20. ^ Transcript of Summers' remarks at NBER Conference on Diversifying the Science & Engineering Workforce
  21. ^ Summers' initial response to controversy
  22. ^ Summer's Remarks on Women Draw Fire, 2005 January 17
  23. ^ University Will Commit $50M to Women in Science, Harvard Crimson, 2005 May 16
  24. ^ http://www.edge.org/3rd_culture/debate05/debate05_index.html
  25. ^ Barres, Ben (13 July 2006). Does Gender Matter? Nature
  26. ^ Haier RJ, Jung RE, Yeo RA, et al. (2005). "The neuroanatomy of general intelligence: sex matters". NeuroImage 25: 320–327. 
  27. ^ 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
  28. ^ Resnick, S.M., Berenbaum, S.A, Gottesman. I.I., & Bouchard, T.J., Jr. (1986) Early hormonal influences on cognitive functioning in congenital adrenal hyperplasia. Developmental Psychology. 22. 191-198.
  29. ^ Berenbaum, S.A., & Hines, M. (1992). Early androgens are related to childhood sex-typed toy preferences. Psychological Science, 3, 203 206.
  30. ^ Janowsky, J.S., Oviatt, S.EC, & Orwoll, E.S. (1994). Testosterone influences spatial cognition in older men. Behavioral Neuroscience. I08, 325-332.
  31. ^ Gouchie, C., & Kimura, D. (1991). The relation ship between testosterone levels and cognitive ability patterns. Psychoneuroendocrinology. 16, 323-334.
  32. ^ Nyborg, H. (1984). Performance and intelligence in hormonally different groups. In G. .J. DeVries. J. DeBruin, H. Uylings, & M. Cormer (Eds.), Progress in brain research, 16I. 491-508). Amsterdam Elsevier Science Publishers.
  33. ^ a b Geary, D. (1998). Male, female: The evolution of human sex differences. Washington, DC: American Psychological Association.

[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. 

[edit] External links


Human group differences
Race and intelligence | Sex and intelligence
Health and intelligence | Height and intelligence
Race and crime | Sex and crime
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