Blog Post

AEIdeas

I featured a version of the chart above in this CD post back in 2010, and I thought it was worth re-posting because it reveals a very interesting statistical phenomenon: the measured variability of male intelligence has been found to be greater than the variability of female intelligence. That’s not saying that men are more intelligent than women on average, but that the dispersion of male intelligence around the mean is greater than the dispersion of female intelligence around the mean. Statistically, it’s saying that the standard deviation of male intelligence is greater than the standard deviation of female intelligence.

The graph displays the IQ test results from the article “Population Sex Differences in IQ at Age 11: The Scottish Mental Survey 1932,” based on “80,000+ children—almost everyone born in Scotland in 1921—tested at age 11 in 1932.”

Main conclusion of the article: “There were no significant mean differences in cognitive test scores between boys and girls, but there was a highly significant difference in their standard deviations (Prob. < .001). Boys were over-represented at both the low and high extremes of cognitive ability.”

As the chart shows, girls outnumbered boys for test scores in the middle – between 90 and 115, while boys outnumbered girls both for: a) low IQ scores below 90 and b) high IQ scores above 115. Further, the share of boys increased going out towards both ends of the distribution, so that boys represented 57.7% of the highest IQ scores of 140 (136 boys for every 100 girls) and boys represented 58.6% of the lowest IQ scores of 60 (142 boys for every 100 girls).

The authors speculate that their findings might “explain such cognitive outcomes as the slight excess of men achieving first class university degrees (the highest academic distinction in the British undergraduate degree classification system), and the excess of males with learning difficulties.”

This evidence also supports what former Harvard President and economist Larry Summers said, which really shouldn’t have been controversial, but which led to his firing as president in 2006 for being “sexist”:

It does appear that on many, many different human attributes- height, weight, propensity for criminality, overall IQ, mathematical ability, scientific ability – there is relatively clear evidence that whatever the difference in means – which can be debated – there is a difference in the standard deviation, and variability of a male and a female population.

It’s unfortunate that in today’s “politically correct” environment on college campuses, citing a known, statistical fact that was confirmed empirically back in 1932 can be grounds for getting fired as a college president for being insensitive, controversial, and sexist. I’m convinced that most of those complaining about what Larry Summers said had never taken a basic class in statistics, or if they did take a stats class, they didn’t understand the statistical concept of “standard deviation.”