No feature of human activity is more reflective of environmental conditions than where people live. In the areas of densest populations, a mixture of natural and human factors has combined to allow maximum food production, maximum urbanization, and maximum centralization of economic activities. Three great concentrations of human population appear on the map--East Asia, South Asia, and Europe--with a fourth, lesser concentration in eastern North America (the "Megalopolis" region of the United States and Canada). One of these great population clusters--South Asia--is still growing rapidly and is expected to become even more densely populated during the twenty-first century. The other concentrations are likely to remain about as they now appear. In Europe and North America, this is the result of economic development that has caused population growth to level off during the last century. In East Asia, population has also begun to grow more slowly. In the case of Japan and the Koreas, this is the consequence of economic development; in the case of China, it is the consequence of government intervention in the form of strict family planning. The areas of future high density (in addition to those already existing) are likely to be in Middle and South Amerca and Africa, where population growth rates are well above the world average.