Population sex differences in IQ at age 11: the Scottish mental survey 1932

There is uncertainty whether the sexes differ with respect to their mean levels and variabilities in mental ability test scores. Here we describe the cognitive ability distribution in 80,000+ children—almost everyone born in Scotland in 1921—tested at age 11 in 1932. 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 (P<.001). Boys were over-represented at the low and high extremes of cognitive ability. These findings, the first to be presented from a whole population, might in part explain such cognitive outcomes as the slight excess of men achieving first class university degrees, and the excess of males with learning difficulties.