This article proposes that a complete account of cognitive evolution may have to accommodate a domain-general source of variance in mental abilities accounting for differences among primate taxa. Deaner, van Schaik, and Johnson [Deaner, R.O., van Schaik, C.P. and Johnson, V.E. (2006). Do some taxa have better domain-general cognition than others? A meta-analysis. "Evolutionary Psychology, 4," 149-196.], in a meta-analysis of experiments testing the performance of different primate genera on various cognitive tasks, found a good fit to a model where the different genera differ along a single dimension of domain-general mental ability. Moreover, the examination of the literature undertaken in this article shows that the rank of a genus on this dimension predicts its brain size, recency of common ancestry with man, and life history strategy. The molecular evolution within the primate order of genes implicated in brain size coincides with this pattern and thus provides some support for the phylogenetic inference that there has been directional selection for general cognitive ability in the lineage leading to "Homo sapiens." Taken as a whole, these data suggest a generality of "g" (or something like it) even wider than has been supposed.