I never knew this. (Via xkcd.) Wikipedia defines Dunbar's number:
Dunbar's number, which is very approximately 150, represents a theorized cognitive limit to the number of individuals with whom any one person can maintain stable social relationships, the kind of relationships that go with knowing who each person is and how each person relates socially to every other person. Group sizes larger than this generally require more restricted rules, laws, and enforced policies and regulations to maintain a stable cohesion. Dunbar's number is a significant value in sociology and anthropology. It was proposed by British anthropologist Robin Dunbar, who theorized that "this limit is a direct function of relative neocortex size, and that this in turn limits group size ... the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained."
In the context of the impending Super Duper Tuesday showdown, I can't help but thinking of this in terms of politicians. Various famous political figures are occasionally described as having uncanny abilities to connect quickly with a wide variety of people, remember faces, and convince casual acquaintances that they are your best friend. Perhaps their neocortices have the unusual ability to maintain relationships (or at least appear to) with far more than the conventional 150?
