Despite ongoing declines in fertility in many countries, the population of the world is experiencing a period of rapid expansion, and its size is expected to reach 10 billion by the end of the demographic transition. Three causes of this growth are identified and quantified: 1) fertility above the replacement level of two surviving children per woman, 2) continuing declines in mortality, and 3) population momentum resulting from a young age structure. A set of simple analytic expressions is proposed for estimating these factors from standard demographic indicators. Population momentum is shown to be the main cause of future growth in most countries and regions. Copyright 1999 by The Population Council, Inc..
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