Grooming Gossip and the Evolution of Language: Robin Dunbar matches neocortex size among primates (neocortex being the thinking part of your brain) with the size of the group the monkeys live in and finds that the bigger the group, the bigger the brain, because a big group means lots of friends, enemies and alliances to keep track of, which takes a lot of thinking about.
The biggest neocortexes and the biggest group sizes are the higher primates - chimps, baboons etc - and anyone who’s seen The Jungle Book knows that the highest primates are us (“I wanna walk like you, talk like you…”). If you carry on plotting the graph past chimps you get to humans and our gigantic, mad-scientist sized brains, and a neocortex size that correlates with a group size of about 150.
This group size of 150 matches all sorts core human organisational units from Witness-style religious community the Hutterites to the basic unit for every army in the world, the company.
It also matches this: “The teen group was remarkable in that almost all the participants had between 100 and 150 names in the automatic dialing registers of their mobile phones”.
(pg161, Hyper-coordination via mobile phones in Norway, Richer Ling and Birgitte Yttri in Perpetual Contact, Mobile Communication, Private Talk, Public Performance James E. Katz and Mark Aakhus (Eds), Cambridge University Press, 2002).