Humans are social animals, and our species has evolved some unique ways of enforcing the bonds of friendship. Robin Dunbar, an evolutionary psychologist at the University of Oxford, studies the behavioral mechanisms behind the number and nature of such relationships. His work suggests social cohesion and long-term bonding among primates—Homo sapiens included—are the keys to their evolutionary success.
Primate societies are held together by unspoken contracts grounded in “social grooming,” whether in the form of physical affection or nonphysical activities such as storytelling. Primates possess limited time for this, but the more time individuals invest in such activities the more relationships they can maintain—which, in part, explains how big or small social groups are, Dunbar says.
His earlier research from 1992 revealed a correlation between brain size and social group size, suggesting humans can only comfortably maintain about 150 stable friendships whereas nonhuman primates can only sustain about 50.
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