ing returns? The deep theoretical questions in the demography of mortality and aging-including the proper framework for incorporating genetic variables and cofactors into demographic models-cluster around this very practical question of prediction, whose answer some of us may live to know.

Confronting this question, the in-house tools of traditional demography—accurate accounting of vital trends and descriptive modeling of variability across time and circumstances—are indispensable but inconclusive. Knowledge of detailed mechanisms is too patchy for causal models with aggregate implications. Thus demographers are thrown back on a search for analogues. Biology is our cornucopia of analogues.

I do not want to overstate the relevance of biology to demography. Biology will not settle demographic questions directly. Finding the causes behind the leveling out of fruit-fly hazard functions after 100 days will not disclose the causes behind any leveling out of human hazard functions after 100 years. Genes promoting survival at advanced ages may be found in nematode worms without giving us any right to expect usefully close counterparts in people. Darwinian theory, for all its triumphs, is a poor basis for predicting whether women's advantage in life expectancy over men will be increasing or decreasing in 2047.

Nonetheless, biology is definitive. Experiments with laboratory organisms, genetic mapping, natural history, and evolutionary theory are defining the intellectual landscape within which demographic arguments and forecasts gain or lose their appeal. Uncertainties are so great and mortality prediction is so much a matter of bets and guesses that the powerful analogies provided by biology are the best guides we have. These analogies offer a basis for implicit choices about what to regard as ad hoc and what to regard as general, what forms of models to try, what kinds of data to put in the foreground. Biological analogies raise or lower our comfort level with particular kinds of scientific explanations. It seems to me, as I shall describe, that the newest work in biodemography is lowering our comfort level with accounts involving limits to life expectancy and programmed senescence and enhancing our openness to models and hunches that treat life spans as highly plastic.

I begin by reviewing ideas from the evolutionary theory of longevity that have coexisted amicably with a pessimistic demographic stance in regard to open-ended further progress against old-age mortality. I then turn to new empirical results that are reviving an optimistic stance, to studies of the role of the elderly in nature, and to new theoretical departures. I conclude with a look at the immediate future and the knowledge we can hope to gain from further joint work in biodemography.

Stern Theories

The emphasis on limits and tradeoffs in biologists' discourse about longevity goes far back, and still predominates. In the 1960s, when a plateau seemed to be



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