The header says it all. The good and the bad of having a large brain.
Groups of 150 are about what we would expect when comparing the size of human brains to those of our relatives. Community size in primates is linked to the size of the neocortex region of the brain, where all social cognitive processing occurs. Other primates, with smaller brains, live in smaller groups. Species, such as the baboon-like mandrill of Central Africa, that do gather in larger numbers only contain females and children in their “horde,” so these are not true mixed-sex social groups. If one extrapolates the relation between brain and group size in other primates, then humans with their very large neocortex extend the graph to a community size of 150.