through the life course and among populations in fecundability (the monthly capacity to reproduce) and in sexual activity, combined with behavior to limit conception and the study of the biology and behavior of the determination of whether conceptions result in a live birth. Practical measurement often falls far short of the ideal.

This approach of working outward from the proximate determinants through to broader determinants of fertility (and related behavior and biology) has been prevalent and perhaps explains the rarity of attempts to include genetic or evolutionary components explicitly. This view can be illustrated by posing the question of whether it is more useful to know that 40 percent of the variance in fertility in some population at some time is attributable to contraceptive behavior in a proximate determinants framework or whether the same fraction can be attributed to genetic variation in an ACE (Additive, Common, and Environment) model (a model that attempts to partition the variance in behavior into three components: an additive genetic component, a common or shared-environment component, and a nonshared environment element). There may be doubts about the pathways that determine contraceptive behavior, but we can be sure that the links with fertility are fairly direct; on the other hand, the genetic association reveals very little about pathways at all.

Yet the more interesting questions for the future involve exploring the pathways forward from an observed genetic association or backward from an observed pattern of contraceptive behavior. Of course, the interconnecting pathways are much closer to being established for the biological components of fertility through endocrinological and neural links to genetic markers than for the behavioral components. In view of the huge range of potential pathways it makes considerable sense for exploratory work to ensure the identification of and discrimination among elements in the paths that are proximal forwards from genetic variability (or markers), emphasizing brain and endocrine links to fertility, and among those that are proximal backward from fertility and the proximate determinants. These two threads should ultimately interplay and meet. Nevertheless, some conceptual and measurement progress is occurring concerning pathways from genetic or evolutionary origins through to human bonding (Miller and Rodgers, 2001) or childbearing motivation (Miller et al., 2000).

A fairly separate strand of theorizing about fertility and related behavior goes under a variety of labels, including the almost defunct sociobiology, evolutionary psychology, evolutionary anthropology, human behavioral ecology, and gene-environment coevolution; these fields often disagree substantially, but they all grapple with evolutionary aspects of human fertility and related behavior. A central challenge facing these approaches, as far as a contemporary demographer is concerned, is the extent to which they are relevant to our understanding of the demographic transition and

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