The “Machiavellian intelligence” hypothesis states that intense social living, which is exhibited in certain species of primates, leads to selection for increased social skill, which is a function of neocortical enlargement. Neocortex size has been found to vary with different types of mating systems and social group size, and thus it is hypothesized that there is a social origin for the neocortical enlargement of primates dependent on the degree of social manipulation and inter-group competition.
This study looked at absolute neocortex volume and the ratio of neocortex volume to the remainder of the brain in 18 species of the major primate groups. Social cognition was measured as “tactical deception,” in which primates exhibited manipulative behavior of others within the social group without the use of force. Absolute neocortex volume was found to be a reliable predictor of the use of deception; however, there was no significant correlation between the use of deception and social group size, in contrast to the results of previous studies. The authors concluded that primate tactical deception is generally a function of rapid and extensive learning in monkeys and prosimians, and is only based on understanding of mechanism in a few records of great apes. Thus, the learning ability of primates seems to be dependent on neocortical enlargement, which is subject to the selective pressures of social sophistication.