ABSTRACT-The idea that general intelligence may be more variable in males than in females has a long history. In recent years it has been presented as a reason that there is little, if any, mean sex difference in general intelligence, yet males tend to be overrepresented at both the top and bottom ends of its overall, presumably normal, distribution. Clear analysis of the actual distribution of general intelligence based on large and appropriately population-representative samples is rare, however. Using two population-wide surveys of general intelligence in 11-year-olds in Scotland, we showed that there were substantial departures from normality in the distribution, with less variability in the higher range than in the lower. Despite mean IQ-scale scores of 100, modal scores were about 105. Even above modal level, males showed more variability than females. This is consistent with a model of the population distribution of general intelligence as a mixture of two essentially normal distributions, one reflecting normal variation in general intelligence and one reflecting normal variation in effects of genetic and environmental conditions involving mental retardation. Though present at the high end of the distribution, sex differences in variability did not appear to account for sex differences in high-level achievement.
Perspectives on Psychological Science -November 2008 -In PressThe variability hypothesis, which posits that general intelligence may be more biologically variable in males than in females, has a long history in both scientific and political writings. In recent years, it has received renewed attention as an explanation for the presence of greater numbers of males than females in technology, engineering, and the highest levels of scientific research. As is often the case in areas of research involving demographic group differences, much of the literature has been emotionally charged, and the empirical data have often been ambiguous. Even when observed results have seemed clear, researchers have raised issues involving adequacy of overall sample size and differences in relative selectivity of male and female samples, relevant experiential background, participant responses to the testing situation, and rates of physical and