In an attempt to reveal factors associated with neocortical development in monkeys and apes (anthropoids), relationships between the relative size of the neocortex and differences in ecology and social structure were examined for 24 genera of 11 subfamilies. Relative sizes of the neocortex (RSNs) in a given group were assessed as the difference between actual neocortical volume and the volume expected from an allometric relationship between neocortical volume and the volume of the rest of the brain. We found that RSNs are related to diet and social structure: frugivorous anthropoids had higher values of RSNs than folivorous anthropoids, and polygynous anthropoids had significantly higher values of RSNs than monogynous anthropoids. Furthermore, RSNs were positively correlated with the size of the troop. These results suggest that development of the neocortex is associated with both diet and social structure in anthropoids.