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Sex and smarts: The sequel

— Three weeks ago, we took a look at research from University of Western Ontario psychologist J. Philippe Rushton - reporting that 17- to 18-year-old men had an advantage of several IQ points over women, based on an analysis of SAT scores. Virtually everything about the politically incorrect study could spark a controversy: How do you define "g," the measure of general intelligence that Rushton looked for within the SAT tests? Are g scores, or even IQ scores, a valid measure of intelligence? Are there statistical or societal factors that could distort the scientific results? And the questions go on and on....

Anyway, Rushton sent along an e-mail today that could shed more light, as well as heat, on the issue of gender and intelligence. The tone of the correspondence is a bit academic, but feel free to digest it and add your comments:

"What a difference a day makes. Just 24 hours ago, I was telling a newspaper reporter that my confidence in the finding that men average higher IQ scores than women would be greatly enhanced if new studies could be found in support. These new studies, from other than the 'usual suspects,' are now to hand.

"The finding of a male IQ advantage of 3.6 points on g from the SAT by Jackson and Rushton (2006), as earlier of 4 to 8 points by Richard Lynn, Paul Irwing, and Helmuth Nyborg, is validated by reaction time (RT) measures. The effect size of 0.24 favoring males found by Jackson and Rushton is matched by simple and choice reaction time (SRT and CRT) effect sizes of from 0.17 to 0.40.

"RT tasks are so easy that 9- to 12-year-old children can perform them in less than one second. Children with higher IQ scores perform faster than children with lower scores, because RT measures the efficiency of the brain's capacity to process information, which is the same ability measured by intelligence tests. SRT correlates with IQ about 0.20, while CRT correlates about 0.40 - in aggregate, RTs can correlate 0.70 with IQ (Jensen, 2006).

"In a meta-analysis of 72 effect sizes derived from 21 studies (N = 15,003) of SRT over a 73-year period, Silverman (2006) found both secular trends and an effect size favoring men of 0.17. Several small sample studies have also found that men average faster on CRT, such as the Bonn Longitudinal Study of Aging (Mathey, 1976; reviewed in Deary & Der, 2005).

"The most definitive evidence comes from two recent population representative studies by Deary and Der, carried out to examine aging effects.

"In the first, Deary and Der (2005) tested [more than] 500 16-, 36-, and 56-year-olds from the West of Scotland. Participants were retested eight years later, at which time they also took the g-loaded Paced Auditory Serial Addition Test (PASAT). Individual differences on the RT measures were stable over the 8-year period (r ~ 0.50), correlated with the PASAT scores (mean r ~ 0.25), and declined with age (CRT from age 20; SRT from age 50). Importantly, men scored higher on PASAT (d ~ 0.20) and averaged faster on RT, especially on one of the CRT measures (d ~ 0.40).

"In the second study, Der and Deary (2006) reanalyzed data for 7,130 adults in the UK's Health and Lifestyle Survey. Again they found CRT declined from age 20, SRT from age 50, and men consistently averaged faster.

"In conclusion, uncertainty over whether males really do average higher in general mental ability, a finding that has been missed for nearly 100 years, is considerably reduced."


Deary, I. J., & Der, G. (2005). Reaction time, age, and cognitive ability: Longitudinal findings from age 16 to 63 years in representative population samples. Aging, Neuropsychology, and Cognition, 12, 187-215.

Der, G., & Deary I. J. (2006) Age and sex differences in reaction time in adulthood: Results from the United Kingdom Health and Lifestyle Survey. Psychology and Aging, 21, 62-73.

Jackson, D. N., & Rushton, J. P. (2006). Males have greater g: Sex differences in general mental ability from 100,000 17- to 18-year-olds on the Scholastic Assessment Test. Intelligence, 34, 479-486.

Jensen, A. R. (2006). Clocking the mind: Mental chronometry and individual differences. Oxford: Elsevier.

Silverman, I. W. (2006). Sex differences in simple visual reaction times: A historical meta-analysis. Sex Roles, 54, 57-68.