Primates have really big brains relative to their body size if you compare them to other animals, even other mammals. The sheet of cells that forms the outer layer of mammal brains (the neocortex) is greatly enlarged. This neocortex is the newest part of the brain to evolve, and beneath it is a primitive brain similar to a reptile's brain (the medulla). As the center for memory, creative thinking, spatial reasoning, language, and conscious thought, the neocortex is what makes primates smart. A human being has the highest ratio of neocortex to medulla of any animal. Much of primate intelligence comes to play in social and learning behavior. Scientists believe the extreme development of the neocortex may be an adaptation for the advanced cooperation (e.g. for hunting) that takes place in primate societies. Sustained social interactions require that primates remember a lot of information about each other.

Most primates live in social groups. While group size ranges from a few to over 100 individuals (depending on species), the composition of a primate group is pretty stable. The hub of the group tends to be females with offspring. Some groups have just one male, the harem leader, who monopolizes females by chasing other males away from a territory. Other groups have multiple males, and may range over areas too large to defend. Regardless, dominance hierarchies are often established in primate groups. The strongest individuals get better access to food or other resources. Primates are constantly picking through each other's fur (grooming). While grooming removes parasites and cleans fur, it may also help maintain social hierarchies. Scientists have found that in some primate societies, subordinate females will groom dominant females in exchange for extra food.