What's Your Christmas Card List Got to Do With the Development of the Human Brain? - Boing Boing:
The discovery has its origin in studies that compared the size of non-human primate social groups with the animals' brain size. The idea is that larger social networks are good things: Offering physical protection against enemies, shared strength and ingenuity to accomplish difficult tasks and a safety net in case you, personally, don't hunt or gather up enough food. But managing those networks takes brain power. If you don't have enough, your clique can't ever get very big.
But say you're the one guy primate with a slightly larger brain and, thus, slightly bigger social network. You'd have a better chance of surviving adverse conditions. And, you'd have a better chance of meeting women who'd be interested in your monkey butt. The fact that a larger brain means a larger social network was probably one of the evolutionary pressures that turned humans into the big-brained species we are today, Dunbar said.
In the early 1990s, Dunbar applied the ratio between primate brain size and social network size to modern humans. By his calculations, 150 people is about the largest social network each human can maintain. You might know more folks than that, but the 150 will be the ones you really have an important relationship with--the ones you really care about.