The evolution of the neocortex in primates has been associated with social complexity, but the relationship between neocortex evolution and components of social complexity such as sexual selection and mating systems is not well studied. I examined the evolutionary relationship between relative neocortex size and the intensity of male–male competition for mates among various primate mating systems using bootstrapped estimates of least-squares regression parameters. A significant negative evolutionary relationship was found between relative neocortex size and the level of male–male competition for mates associated with various mating systems. The largest relative neocortex sizes among primate species were associated with monogamy This negative evolutionary relationship suggests that monogamy may require greater social acuity and abilities for deception and manipulation, and promote selection for larger brains.