Determining how the human brain differs from nonhuman primate brains is central to understanding human behavioral evolution. There is currently dispute over whether the prefrontal cortex, which mediates evolutionarily interesting behaviors, has increased disproportionately. Using magnetic resonance imaging brain scans from 11 primate species, we measured gray, white and total volumes for both prefrontal and the entire cerebrum on each specimen (n = 46). In relative terms, prefrontal white matter shows the largest difference between human and nonhuman, whereas gray matter shows no significant difference. This suggests that connectional elaboration (as gauged by white matter volume) played a key role in human brain evolution.