Dunbar's Number

http://online.wsj.com/article/SB10001424052748704422204576130602460527550.html

 

http://www.economist.com/node/13176775

 

http://www.youtube.com/watch?v=ppLFce5uZ3I

 

 

 
Robin Dunbar
How Many Friends Does One Person Need?: Dunbar's Number and Other Evolutionary Quirks
Faber & Faber, London, 2010