[biomcgary]

Almost all mammals studied (and other vertebrates too) show stress (e.g., elevated cortisol levels, abnormally aggressive behavior) when exposed to high population density. This is true even when the population density is faked (e.g., studies placing pictures of con-specifics in the wild or playing animal sounds).

Obviously, there is also a lower bound for density, too (gotta find a mate). I think this explains why young people like cities, but once a person has a mate and progeny they move suburban or rural.