genes, bringing the loop full circle. This metaphor, articulated by Randy Bulatao, the director of this National Research Council study, may serve in place of any formal definition to indicate our subject’s scope.

In practice, in choosing topics for this volume, the organizing panel began with two areas in which sizable amounts of research familiar to demographers already exist. These areas are twin studies from behavioral genetics and anthropological studies of intergenerational transfers drawing on evolutionary theory. From these two starting points, represented in this volume by Chapters 2 and 7, the panel reached outward, seeking to include a selection of research pertaining to as many as possible of the links in the great loop. Of course, only a sampling is possible. The hope has been to survey and stimulate prospects for new initiatives in boundary-crossing research.

Biology and demography have always been intertwined, even before the days when Darwin was reading Malthus. Demographic studies over recent decades in the area of fertility reinforce the connections. Examples include work on proximate determinants by Kingsley Davis and Judith Blake (1956) and by John Bongaarts and Robert Potter (1983), the fertility exposure analysis of John Hobcraft and Rod Little (1984), the mathematical models of conception and birth of Mindel Sheps and Jane Menken (1973), and the insights into hormonal conditioning of gender roles and demographic outcomes in the 1994 presidential address of Richard Udry to the Population Association of America. (Any selection is necessarily personal.) But the biological and social sciences have both been changing so rapidly that they easily come to seem like strangers, needing fresh introductions to each other.

The most visible interdigitation of the new biology with the new demography, evoked by the name “biodemography” has been in the study of longevity. The Committee on Population published an early overview of that field, Between Zeus and the Salmon, in 1997 under the editorship of Wachter and Finch (National Research Council, 1997). The state of play in the areas of fertility and family formation on the occasion of the present volume differs from the state of play in the area of longevity. Students of longevity work with a definitive outcome—death. That contrasts with the many outcome measures and intermediate processes of transition summed up under a general heading like “Fertility Behavior” or “Family Formation.” The biodemography of longevity has been channeled in particular directions by a set of initiating discoveries, measurements of hazard curves tapering at extreme ages in Mediterranean fruit flies, Drosophila, and nematode worms. The biodemography of fertility has not gathered force from some canon of particular discoveries but has grown out of overlapping interests. For longevity the biodemographic agenda is closely tied to the practical imperative to forecast the pace of gains against old age mortality

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