To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.
为了做出更准确的人口预测，人口统计学家必须比现在更多地了解生育率的社会和经济决定因素。
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Demographers reckon that three quarters of humanity could be city dwelling by 2050, with most of the increase coming in the fast growing towns of Asia and Africa.
Working from what America already knows about itself, the bureau's experts and other demographers offer an irresistible, if clouded, crystal ball among their visions.