If the maintenance of close group ties is essential to the social and mental well-being of humans, clearly groups can reach sizes beyond which the invisible ties between their members become so attenuated that for all practical purposes they cease to operate. Dysfunctions then set in.
There is evidence to suggest that beyond a maximum community size of 150,000, communities begin to experience an ever increasing amount of fragmentation, deviance, criminality, social stress, alcoholism and mental illness. In studies of cities in the USA, those with least stress averaged 116,000 people. In the USA studies have indicated that 68% of people do not know their next door neighbour.