The social-brain hypothesis refers to a quantitative relationship between social-group size and neocortex volume in monkeys and apes. This relationship predicts a group size of approximately 150 for humans, which turns out to be the typical size of both social communities in small-scale societies and personal social networks in the modern world. This constraint on the size of social groups is partly cognitive and partly temporal. It gives rise to a layered structure in primate and human social groups that, in humans, reflects both emotional closeness in relationships and the frequency of contact. These findings have potentially important implications for the way in which human organizations are structured.
These differences in functionality may well reflect the role of mentalizing competences. The optimal group size for a task may depend on the extent to which the group members have to be able to empathize with the beliefs and intentions of other members so as to coordinate closely, as well as manage conversations without suppressing anyone. If this demand is high (as in a practical task with a specific goal), then the work group may have to be smaller. The efficiency of such a group may then be influenced by heterogeneity in the mentalizing competences of its members.