There is an impression, both in the press and among some urban analysts that as cities become larger they become more densely populated. In fact, the opposite is overwhelmingly true, as Professor Shlomo Angel has shown in his groundbreaking work, A Planet of Cities. This conclusion arises from the fact that, virtually everywhere, cities grow organically so that they add nearly all of their population on the urban fringe, which has considerably less expensive land. As their physical form of cities (the urban area) expands, the residents per unit of developed area generally falls.