Research > Research Themes > Growth and Structure of Populations > Fertility

In an American Economic Review article, Greenwood developed models to explain the US baby boom in the context of a 200-year secular fertility decline. The secular decline in fertility is due to the relentless rise in real wages that increased the opportunity cost of having children. The baby boom is explained by an atypical burst of technological progress in the household sector that occurred in the middle of the last century. This lowered the cost of having children. Preston has prepared a paper for the Social Security Administration on the likely future of American fertility; the two most predictable demographic changes with strong associations with fertility—a greater proportion of ethnic Hispanics and a rise in educational attainment—essentially cancel one another out. Timing-induced depression in US period rates could last a long time. Smith, with former student J. Lundquist, in Journal of Marriage & the Family, considered how much of black-white differences in fertility behavior (timing of fertility, and fertility in and out of marriage) is due to the different opportunity structures confronting the two groups. Even after substantial adjustments for observable sources of differential selection into the military, African- and European-Americans within the US military have fertility behaviors far more similar to one another than to their civilian counterparts—Africa-Americans in particular—suggesting that when the “playing field” is leveled, racial differences will be small. But the playing field is not yet leveled: Furstenberg’s Baltimore Study has evolved into one of the longest running longitudinal studies of fertility and family formation among the disadvantaged. Beginning in 1966 with an interview of some 399 women registered for prenatal services at Sinai Hospital in Baltimore, the project has followed these women, their children, and grandchildren for 4 decades, culminating in Destinies of the Disadvantaged: The Politics of Teenage Childbearing, in which he chronicles the history of public concern over teen pregnancy, comparing political understandings of the phenomenon with the life experiences of his subjects. Work by Kohler and Behrman on the biodemography of fertility falls under the theme: Genes and gene-environment interactions; and Valeggia has a series of studies showing hormonal mediation between social and fertility practices, e.g., with P. Ellison, analyses of reproductive hormones (salivary estrogen and progesterone) show how access to market foods attenuated the effects of seasonality of resources, which in turn affected the fertility of the Toba population.