We review the literature on the relation between whole brain size and general mental ability (GMA) both within and between species. Among humans, in 28 samples using brain imaging techniques, the mean brain size/GMA correlation is 0.40 (N = 1,389; p < 10(-10)); in 59 samples using external head size measures it is 0.20 (N = 63,405; p < 10(-10)). In 6 samples using the method of correlated vectors to distill g, the general factor of mental ability, the mean r is 0.63. We also describe the brain size/GMA correlations with age, socioeconomic position, sex, and ancestral population groups, which also provide information about brain-behavior relationships. Finally, we examine brain size and mental ability from an evolutionary and behavior genetic perspective.