That must explain why Chicago is such a happy go lucky place but rural areas are virtual battlegrounds.
Ok, I admit, that was a wee bit snarky.
Actually, much of the urban violence that we see in places like Chicago or Detroit proves Nicholas Wade's point. If you take Aboriginal hunter gatherers or other tribal people with no history of urban life and force them into a city, they really can't function emotionally or culturally in such an environment. In contrast, cities that are populated by people with a long history of civilization and urban life tend to be no more violent than rural areas (i.e. most east Asian cities, most European cities before the mass influx if Middle Eastern and African tribal peoples).