(Taken from [The Magic of 150], which references R.I.M Dunbar, "Neocortex size as a constraint on group size in primates," Journal of Human Evolution (1992), vol. 20, pp. 469-493.)
This motivates why traditional villages have a natural bound of perhaps 30 families (WikiPedia:Village): with three generations per family, that easily reaches 150 people. Everyone knows everyone else, and the community can tackle major problems with a BarnRaising event. Add more people, and other dynamics must come to the fore.
See also TheTippingPoint.
Not sure which page - this, or TheTippingPoint - is better for our PatternLanguage. They're essentially the same idea, but I feel that it's easier to refer to this in an argument and have the major point be recalled: TheTippingPoint makes me think of swings. Plus, the book is about a good deal more than just DunbarsNumber. -- ChrisPurcell