As a species, we have spent more than 95 percent of our evolutionary history in small scale foraging societies in which no more than a few hundred people are dispersed over a wide geographical area. But ever since the Industrial Revolution, with its voracious demand for large labor forces, human communities have become increasingly concentrated in urban centres. In 1800, just 2 percent of the world's population lived in cities, rising to 13 percent in 1900 and 60 percent in 2000. While we can obviously manage to live in very large communities, our psychology is not really designed to handle the kind of densities and social pressures associated with urban centres.