Sex Differences in Variability for Cognitive Measures: Do the Ends Justify the Genes? (Commentary on Johnson et al., 2009).

Theories about the origin of cognitive sex differences must address differences in three portions of ability distributions: low-tail variability, high-tail variability, and mean values. In addition, genetic theories must provide evidence that these three types of differences are (at least in large part) caused by alleles that are located on the X chromosome. It is well established that there are more mentally retarded males than females, and this disparity is attributable to genes located on the X chromosome. By contrast, there are no known "intelligence genes" that can provide a parallel explanation for differences in variability in the high ability tail of distributions. Mean differences between males and females also defy any X-linked hypothesis about average intelligence because females and males excel on different cognitive measures. Thus, we conclude that X-linked genetic explanations of cognitive sex differences can only be substantiated as a causal explanation for the excess of males diagnosed with mental retardation.