Since the beginning of the twentieth century, the world's population has increased threefold. The "demographic revolution" has not only changed the size of the population; it is radically transforming its age structure and its spatial distribution, with attendant problems and contradictions. Despite measurable advances in human welfare, such as increased longevity, more than one billion people--about a third of the total Third World population--live in poverty. Sharply diverging rates of population growth have been accompanied by increasing disparities in income and quality of life across...