Actually, Dunbar's Co-evolution of neocortex size, group size and language in humans is worth a good read -- there's more in it than the magic number 150 I referred to yesterday. The argument goes roughly like this: the maximum cohesive social group of 150 comes from the size of the primary network - intense friends - which is itself correlated with the neocortex size (in relation to the brain) and the proportion of time spent on social grooming. These three figures seem well matched across different primates.
However, if you look at humans we maintain a group size (of about 150) which matches our neocortex size but which is way over the time budget we allow for it (we'd need to spend about 42% of our time on one-on-one grooming, but it doesn't seem we do).
Dunbar contends that humans evolved vocal grooming (language) as a more efficient form of bonding. Assuming that our closest ancester, the chimpanzee, has hit the time budget limiting factor, and that our extra efficiency has all come about with the transition to vocal grooming, this means language is 2.8 times more efficient for bonding than the mechanism nonhuman primates use. That is, "a speaker should be able to interact with 2.8 times as many other individuals as a groomer can. Since the number of grooming partners is necessarily limited to one, this means that the limit on the number of listeners should be about 2.8. In other words, human conversation group sizes should be limited to about 3.8 in size (one speaker plus 2.8 listeners)." (Which makes sense if you think about the different qualities of a conversation with three versus four participants. A study is quoted to back this up.)
And for supporting evidence, showing that disparate human characteristics are coherent with this: "It turns out that there is, in fact, a psycho-physical limit on the size of conversation groups. Due to the rate at which speech attenuates with the distance between speaker and hearer under normal ambient noise levels, there is a physical limit on the number of individuals that can effectively take part in a conversation. Sommer (1961), for example, found that a nose-to-nose distance of 1.7m was the upper limit for comfortable conversation in dyadic groups; this would yield a maximum conversation group size of five individuals with a shoulder-to-shoulder spacing of 0.5m between adjacent individuals standing around the circumference of a circle."
All extremely amusing.