Our capuchin monkeys participated in an international collaborative effort involving 58 researchers from all around the world. This team tested 567 individuals belonging to 36 species in two problem solving tasks measuring self-control. Across species, differences in absolute brain volume best predicted performance on these tasks.

In addition, within the 22 primate species that were tested, dietary breadth also predicted cognitive performance, whereas social group size did not. Thus, this study suggests that (i) increases in absolute brain size provided the biological foundation for an improvement in self-control and that (ii) species difference in feeding ecology is the potential selective pressure favoring this skill.