Dunbar's number

Evolutionary anthropologist Robin Dunbar expands on "Dunbar's number," his theory that the maximum number of stable relationships a person can maintain is approximately 150. Time to delete a few hundred Facebook friends?
We are the product of our evolutionary history, and that history colors our experience of everyday life -- from the number of friends we have to how religious we are. Renowned evolutionary anthropologist Professor Robin Dunbar visits the RSA to explain how the very distant past underpins all of our current behaviors, and how we can best utilize that knowledge.
Did you know that you have just 150 friends, acquaintances and relatives? And that this is a natural size for villages all over the world? Now known as "Dunbar's Number," it defines the feasible boundaries of our social lives. Dunbar's investigations show us that we inherited the social side of our brains from our mother, and the emotional side from our father; why many women see the world in four or even five different colors, but men only ever have the conventional red, green and blue; and why facial symmetry has everything to do with voter choices in elections. - RSA
Robin Ian MacDonald Dunbar is a British anthropologist and evolutionary biologist, specializing in primate behavior. He is best known for formulating Dunbar's number, roughly 150, a measurement of the "cognitive limit to the number of individuals with whom any one person can maintain stable relationships".