In a book called Evolutionary Psychology: A Beginner’s Guide (2005), Robin Dunbar, Louise Barrett and John Lycett addressed this issue.  The book drew on additional research that can be found in Grooming, Gossip and the Evolution of Language, by Robin Dunbar (1997).

We don’t have limited numbers of friends and acquaintances merely because we choose to have such limited numbers.  Rather, as explained in these two works, physiological limitations constrain human social choices. We are limited in the number of acquaintances we can have because we are physiologically limited.  This is another example that those who claim to explain human animals without the benefit of careful science do so at their own risk.

Human societies are complex social environments.  Archaeologists have determined that pre-modern humans lived in small-scale hunter gatherer societies “characterized by very small, relatively unstable groups, often dispersed across a very large area.”  Only after agriculture was developed (10,000 years ago) did large permanent settlements become possible Living in groups gives members huge advantages such as reduced predation risk (we benefit from the “many eyes” advantage and large groups of individuals deter most predators).

Group living comes with costs, too.  We have conflicts over limited resources, such as food and mates.  Group living stresses immune systems too.  The menstrual cycles of female primates are disrupted.  In order to obtain necessary food, humans need to travel further each day. Associating with large groups of people also has a huge mental cost.  In order to live safely within large groups, …