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Biodemography of Fertility and Family Formation

Kenneth W. Wachter

This volume explores the relevance of new developments in biology, genetics, and evolutionary anthropology to our understanding of human fertility behavior and family formation, under the rubric of “biodemography.” The biology of fecundity, infecundity, and contraception has long been integral to human population studies. But demographers are only beginning to assimilate findings and approaches from behavioral genetics, molecular genetics, neuro-endocrinology, and cross-species life history analysis and to place them in the context of evolutionary theory. With support from the National Institute of Child Health and Human Development, the National Research Council’s Committee on Population has brought together an interdisciplinary group to review this young field of science and reflect on promising opportunities for future research.

“Come, meet our offspring.” The parent who says something like that is more likely to be found in a cartoon than in a living room or shopping mall. But the biological term “offspring” summons up a world of interconnections between the having of human children and the deeply scripted life-history dances in which creatures all renew and perpetuate their kind. Conscious choice, conditioned within social and economic systems, is the preoccupation of demographers, along with tabulations of “fertility” in the narrow sense of numbers of children born. But conscious choice colors a few links in a wider loop of impulse and empowerment, in which genes are linked to proteins, proteins to hormones, neurons, and gametes, bodily directives to organized behaviors, bonding, courtship, nurture, and childbearing to the demographic outcome measure, net generational replacement, and finally through the filter of natural selection, back to the evolving



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Offspring: Human Fertility Behavior in Biodemographic Perspective 1 Biodemography of Fertility and Family Formation Kenneth W. Wachter This volume explores the relevance of new developments in biology, genetics, and evolutionary anthropology to our understanding of human fertility behavior and family formation, under the rubric of “biodemography.” The biology of fecundity, infecundity, and contraception has long been integral to human population studies. But demographers are only beginning to assimilate findings and approaches from behavioral genetics, molecular genetics, neuro-endocrinology, and cross-species life history analysis and to place them in the context of evolutionary theory. With support from the National Institute of Child Health and Human Development, the National Research Council’s Committee on Population has brought together an interdisciplinary group to review this young field of science and reflect on promising opportunities for future research. “Come, meet our offspring.” The parent who says something like that is more likely to be found in a cartoon than in a living room or shopping mall. But the biological term “offspring” summons up a world of interconnections between the having of human children and the deeply scripted life-history dances in which creatures all renew and perpetuate their kind. Conscious choice, conditioned within social and economic systems, is the preoccupation of demographers, along with tabulations of “fertility” in the narrow sense of numbers of children born. But conscious choice colors a few links in a wider loop of impulse and empowerment, in which genes are linked to proteins, proteins to hormones, neurons, and gametes, bodily directives to organized behaviors, bonding, courtship, nurture, and childbearing to the demographic outcome measure, net generational replacement, and finally through the filter of natural selection, back to the evolving

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Offspring: Human Fertility Behavior in Biodemographic Perspective 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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Offspring: Human Fertility Behavior in Biodemographic Perspective in the new century. For fertility the specifically biodemographic agenda does not center on a short list of policy questions but ranges widely across a spectrum of concerns. Side by side with these differences go a host of commonalities. Most striking are new forms of data presenting opportunities for both areas. These opportunities are described in the Committee on Population’s volume Cells and Surveys edited by Finch, Vaupel, and Kinsella (National Research Council, 2000). That volume takes up a range of biological indicators—genetic, hormonal, and functional—that will progressively become available in conjunction with detailed survey information on socioeconomic background and demographic choices for population-based samples. Along with data opportunities, the different branches of biodemography share a reliance on evolutionary theory and on the clues to the human evolutionary environment that anthropology provides. Reproductive fitness in the face of natural selection is measured as a joint function of age-specific rates of survival and fertility. Both figure together in attempts to give mathematical expression to the implications for fitness of age-specific patterns of social support, intergenerational transfers, and constrained resources. Similarly, posited trade-offs between survival and fertility and associated pleiotropic genes (genes with multiple effects) may help explain how demographic schedules come to have the shapes they do. Biodemography directly encompasses the study of fecundity and the physiology of reproductive success. This field is already vast and active. The Committee on Population decided not to try to cover it, inevitably superficially, in this volume but to concentrate attention on behavior. Here “fertility” denotes the outcome measure of children born, incorporating all the interventions of social and individual choice, while “fecundity” denotes the biological capacity for childbearing. Naturally, no clean separation can be made, but fecundity and infecundity, reproductive disorders, reproductive technology, and the physiology of contraception are kept in the background so as to give space to incipient work on behavioral pathways. Ellison’s (2001) On Fertile Ground covers aspects related to fecundity that are given less attention here, and Sarah Hrdy’s (1999) Mother Nature serves as a bridge between these complementary emphases. RESPONSIBLE RESEARCH Behaviors relating to fertility and family are central to the creation of life and the well-being of individuals, families, and societies. Understandably, research on these behaviors can be controversial and how societies use scientific knowledge about these behaviors is even more so. Scientists need to be sensitive to the strongly-held and often diverse values that surround fertility and family issues.

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Offspring: Human Fertility Behavior in Biodemographic Perspective Although ethical issues in the responsible conduct of research are not the explicit subject of this volume, they pervade and need to inform all of the topics treated here, and the contributors to this volume held extensive discussions of these issues among themselves. A number of topics specific to genetic studies are treated in Michael Rutter’s chapter. Several general principles emerging from the discussions are also worth emphasizing for the sake of productive learning and exchange. It is important for us as scientists to ensure that we represent our science accurately, avoiding the temptation to draw less than fully warranted conclusions that may attract attention but also foster misunderstandings of what we know and what we do not know. It is also important that researchers not leap from scientific findings to policy prescriptions. Science does not tell us what we ought to do. It can inform decisions, but it cannot take the place of social, political, and moral choice. At the same time, scientists are called on to abandon the arrogant fiction that science can be a “thing apart” from the rest of society. This call involves, on the one hand, recognizing the extent to which our own perspectives and values may influence the science we do and how we interpret it. It involves, on the other hand, recognizing the potential power of scientific “facts” to influence the larger society. It is incumbent on all of us as scientists to do what we can to keep the facts straight, challenging assumptions and methods openly and often, fostering active debate over interpretations of findings and of their implications, and taking these debates outside the boundaries of scientific discussion if necessary to correct misinterpretations. This volume is organized around contributing disciplines. It moves from work in behavioral genetics to areas of molecular genetics, neuroscience and endocrinology, and comparative integrative biology. These are followed by treatments based on evolutionary theory and anthropology, rounded out by perspectives from economic and social demography. BEHAVIORAL GENETICS In Chapter 2, Michael Rutter begins with an overview of what is being learned and what can be learned from behavioral genetics about genetic influences on fertility. It is reasonable to expect some genetic transmission of components of tastes, skills, and impulses that work themselves out in courtship, family planning, and family building. Rutter cites evidence for moderate heritability of traits like social attachment and pair bonding and outcomes like age at first sexual intercourse and marriage dissolution and for number of children. Measures of heritability encapsulate information about genetic influences on differences among people in a population. But Rutter cautions firmly against inferring genetic determinants of differences

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Offspring: Human Fertility Behavior in Biodemographic Perspective in levels of traits from group to group and against readily ascribing changes in levels of traits over time to the same factors that modulate individual differences. The stock in trade of behavioral genetics is the use of twin studies. Rutter spells out the strong assumptions underlying twin study designs and offers caveats, as do Kamin and Goldberger (2002). For demographers interested in courtship, marriage, and family formation, another issue arises, not taken up in Chapter 2, namely, the specialness of identical twins with respect to social experiences where physical appearance matters. Chapter 3 by Kohler and Rodgers provides an illustrative case study of how current behavioral genetic approaches are being applied in practice to fertility behavior. (A fuller set of examples is found in Rodgers, Rowe, and Miller [2000] and Rodgers and Kohler [2002].) Kohler and Rodgers draw on the Danish twin registry and an associated 1994 survey for fertility and education histories of 2,729 pairs of same-sex twins born over 17 years between 1953 and 1970. They analyze within-pair correlations in numbers of children born to the twins. Their chapter puts on view a whole range of statistical tools employed by practitioners. Kohler and Rodgers find rapid changes across birth cohorts in correlations for female identical (monozygotic) twins not mirrored by correlations for sororal (dizygotic) twins and show that educational variables partially account for these changes in a statistical sense. Their analysis raises issues discussed further by John Hobcraft in Chapter 12. Kohler and Rodgers interpret their findings as evidence for underlying genetic effects that become visible in family formation choices when given freer rein by social and individual circumstances. In this case, the enabling factor is female education. Measured heritabilities are posited to change rapidly, not because genes change but because conditions change to let the genes have their say. A debate runs through the chapters of this volume over the prospective uses of heritability estimates and twin-pair correlations from behavioral genetics. It would be eminently desirable if such estimates could aid in picking out traits, variables, and outcome measures most likely to reflect direct and potent genetic propensities. If suitable traits can be singled out and specified, capabilities are in hand to search for “quantitative trait loci,” that is, for portions of chromosomes with detectable effects on quantitative traits. Any such guidance would also be welcome for designing studies of gene expression and ultimately for neuro-endocrine analyses. But it is not clear that heritability estimates can serve these purposes. In Chapter 2, Rutter writes, “within a very broad range, variations among traits in their level of heritability have little or no meaning with respect to theory, policy, or practice.” He goes on to say that “evidence that there is a significant heritability for fertility is completely uninformative with respect

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Offspring: Human Fertility Behavior in Biodemographic Perspective to causal mechanisms.” Rutter sees the search for causal mechanisms as primary and the assessment of genetic influence as secondary, while acknowledging the value of behavioral genetic studies for “a better understanding of the nature-nurture interplay in the development of socially relevant behaviors.” Rutter’s chapter can be read as a call for behavioral genetics in a new key. It is possible to go beyond statistical path diagrams and measures of association. Broader opportunities are now opening up for including what Rutter calls “discriminating and sensitive measures of the environment” in behavioral genetic studies. Techniques for direct monitoring of environmental exposures and stimuli for individuals are in rapid development. They dovetail with approaches for making use of biological indicators of hormonal and other physiological responses, as described in the National Research Council’s 2000 volume Cells and Surveys, already mentioned. Such measures are particularly relevant for studies of fertility behavior, since hormonal signaling must play a role in so many pathways of influence. The National Longitudinal Study of Adolescent Health (“Add Health”), whose first wave of home interviews were conducted in 1995, is providing researchers with data on familial correlations for a wide range of traits plausibly bearing on fertility. Jacobson and Rowe’s (1999) study of depressed moods in adolescents is an example. Where the outcomes of genetic predispositions are likely being modulated by social trends and circumstances, self-selection of individuals into particular niches has to be distinguished from causal influence. Information on school settings and experiences, peer-group composition, and parental treatment, such as Add Health collects, can give a basis for separating out genetic effects from the consequences of self-selection, social homogamy, and friends and family networks. For the topics central to this volume, later waves of the Add Health study should provide more extensive observations of union formation and childbearing as respondents grow toward adulthood. Conclusions with regard to gene-environment interactions can be particularly convincing when genotypes are measured directly with molecular markers. Rowe at al. (2001) report one example involving numbers of marriages for women. Successes with such approaches are not yet frequent, but technology is improving. Supplementing twin and adoption studies, there are natural experiments taking place willy-nilly all around us which, with ingenuity, can be recognized and analyzed to advantage. The study of change over time stands to benefit from hard thinking about gene-environment correlations and feedbacks. Given such a range of opportunities, it is fair to expect that, for studies of fertility and family formation, the behavioral genetics of the future should bear little resemblance to the behavioral genetics of the past.

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Offspring: Human Fertility Behavior in Biodemographic Perspective NEUROSCIENCE AND ENDOCRINOLOGY If one said the word “biology” 25 years ago in demography, one would have been talking about the physiology of reproduction and contraception, occupying a couple of weeks in courses on fertility, or perhaps talking about deaths by cause. Since then, biology has spread across the intellectual landscape. This volume is itself an illustration of the phenomenon. Although sensible and cautious writers emphasize the complexity of causal pathways and the intricacy of interactions between genes and environments, the lure of biology is in dreams of explanatory power. Some diseases are present or absent thanks to one mutation in one gene. Hunts for “genes for X” and “genes for Y” sometimes succeed. When they do, they often point forward to the next steps in chains of causation. What can be true for diseases could be true for behaviors. In Chapter 4, Larry Young describes a genetic discovery that exemplifies the dream. A modest change in one gene appears to be responsible for the presence of pair-bonding behavior in a species of small animals called prairie voles living in the American Midwest. The change is found in males in the regulatory portion of a gene for receptors that respond to the hormone vasopressin. It provides for receptors in an area of the brain associated with rewards. The prairie voles that form pair bonds have occurrences of a particular sequence of 420 base pairs of DNA in their version of the gene, as compared to their neighbors, the montane voles of the Rocky Mountain states, which do not form pair bonds. The full account, involving the hormone oxytocin in females as well as vasopressin in males, reads like a detective story. What has been found is perhaps something like a switch in a wiring diagram in the middle of a network of interconnected circuitry. A lot of genes in combination make the voles’ social attachments possible, but one kind of change in one gene has the eventual effect of turning them on or off. We are not to imagine that enduring love—Petrarch seeing Laura in the church pew—could be nothing other than an extrapotent battery of vasopressin receptors in the poet’s brain. But it is clear that some complex behaviors have some clear-cut genetic controls. Triumphs like the one Young describes (only a handful are known so far) encourage us to look for dimensions of human partnering and procreative choice that might be modulated by identifiable genetic variations. Less widely heralded than genomics but of complementary importance for demographers have been the strides in endocrinology, both in techniques for measuring and monitoring hormone levels and fluctuations and in the unravelling of systems of endocrine signaling. In Chapter 5, Judy Cameron summarizes our present understanding of the hormonal control systems for reproduction in men and women. She emphasizes the directing

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Offspring: Human Fertility Behavior in Biodemographic Perspective role of the brain and the complexity of the endocrine “code.” The same hormone may stimulate a response at some levels or in some pulsatile patterns and suppress that response at other levels. Many hormonal influences on sexual function are paralleled by influences on sexual desire and behavior. For the biodemography of fertility, Cameron points to several priorities for interdisciplinary research. One involves effects of extensive artificial exposure to steroids and other endocrinologically active substances now occurring in developed societies through medication, self-medication, and consumer choice. Another follows from experiments documenting the effects of psychosocial stress on hormone secretion, plausibly with implications not only for individual health but also for social gradients. Finally, Cameron directs renewed attention to the question of nutrition and fertility. She notes that biomedical scientists drawing on laboratory data tend to regard energetic stress as “an important regulator of reproductive ecology,” posing a contrast, at least in emphasis, to the conclusions of demographers drawing on field data from populations. In the abstract, one might have imagined that it could be an advantage in evolutionary terms for the reproductive systems of humans and other primates to turn off easily and automatically in the face of hard times. Gains in fitness might be thought to accrue from saving on large physiological and energetic investments in infants when chances for their survival were low and from concentrating resources on procreation when chances for infant survival were high. In fact, however, evolution seems to have honed our reproductive systems to turn on whenever they minimally can. Perhaps evolutionary environments were too unpredictable for automatic regulation to be worthwhile. It is tempting to wonder how different our overpopulated modern world might have been had we come to it endowed with built-in preventive checks on fertility. INTEGRATIVE BIOLOGY Happily, both the words “demography” and “population” have grown beyond their etymological roots in words for people and now apply to other species. The mathematical core of demography carries over directly. The species that have figured prominently up to now in the biodemography of longevity, including species of flies and worms, have little in the way of social structure. Now comprehensive demographic data on long-lived species with social structure are beginning to come into being. In Chapter 6, Jeanne Altmann and Susan Alberts treat us to an overview of the remarkable demography of populations of baboons from Amboseli, Kenya, based on observations collected over some 30 years. After setting the life course patterns of humans within the context of other mammals and other pri-

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Offspring: Human Fertility Behavior in Biodemographic Perspective mates, they present original data on the pooled age-specific fertility and survival rates of baboon populations and describe the variations of reproduction and survival with abundance of food and dominance status. The matrilineal dominance hierarchies that Altmann and Alberts describe give a tantalizing analog to the demographer’s staple explanatory category, or socioeconomic status. Among the Amboseli baboons, within each group daughters tend to inherit their mother’s rank. Rank matters. Low-ranking females have lower fertility. But occasionally, under favorable conditions members split off into a new group, with fewer members to outrank them. Possibly these social arrangements help regulate aggregate group fertility and population growth, with group fission supplying a mechanism for relaxation of constraints. The account by Altmann and Alberts is a telling example of social behavior altering demographic parameters in another primate. Behavioral flexibility is not unique to humans. Reproductive effort includes both mating and parenting, and both components involve complex social strategizing in other primate species as well as our own. Investment in social relationships has demographic payoffs. It turns out that the patterns of response to food availability in the Amboseli baboons are similar to some of the human patterns described by Carol Worthman in Chapter 10. Altmann and Alberts calculate quantities called elasticities that describe the sensitivity of population growth to small changes in age-specific rates of fertility and survival and that often serve as measures of susceptibility to forces of natural selection. Growth is much more sensitive to a proportional change in infant or child survival per year than to a proportional change in fertility per year. This finding might be taken to underline the significance of parenting behavior and investment in offspring quality over and above the maximization of offspring ever born. However, overall comparative effects are more complicated to calculate. From a baboon mother’s point of view, improving average infant survival by some factor requires improving survival a dozen times over, for a dozen or so infants, whereas raising fertility in a year by the same factor is a one-time investment. Relative returns remain to be assessed. The power of the approach is the ability to perform parallel calculations of demographic trade-offs for humans and other primates, taking advantage of nature’s natural experiments. EVOLUTIONARY ANTHROPOLOGY The terms of discourse of genetics are particularly attractive for organizing thinking about behaviors because they not only lead forward from genes to physiological mechanisms but also backwards from genes to their evolutionary origins. We ask not only what a gene produces in the body but also where it comes from out of prehistory. For human fertility behavior,

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Offspring: Human Fertility Behavior in Biodemographic Perspective we are a long way from carrying out such a program but not so far from understanding how some modern behavior may be rooted in the early exigencies of lineage survival. Such an evolutionary perspective is introduced in Chapter 7 by Hillard Kaplan and Jane Lancaster. Underlying their approach is the idea that natural selection has less to do with specific sequences of behavior and more to do with “norms of reaction,” systems for adjustable responses to environmental stimuli. The celebrated diversity of human mating arrangements and reproductive norms and practices may reflect common evolutionary principles working themselves out in highly diverse ecological settings. Kaplan and Lancaster’s chapter is representative of a line of thought in which a kind of economic calculus takes its place alongside the traditional reproductive calculus in the reckoning of long-term fitness in the face of natural selection. This economic calculus involves constraints imposed by feasible age-specific production and consumption, returns to skill, social support, and resource transfers across generations. In Kaplan and Lancaster’s account, the modern concept of a quality-quantity trade-off in the demand for children has an analog for hominid foragers. Compared to chimpanzees, hominids came to concentrate on an ecological food niche, including hunted prey and extracted nutrients, which demanded and rewarded skill and learning. Prolonged juvenile training and dependence, protracted parental investment, larger brains, and longer life spans are seen as coevolving, driven by returns to investment in “embodied capital.” In this picture, evolution would have been equipping humans not so much with an instinct toward maximizing total fertility as with instincts for adjusting familial resource transfers in response to the available lifetime returns to such investments. Elements of this picture are subject to lively debate. An alternative reading of the paleoanthropological data would see the distinctive pacing of human life histories, with extended gestation, juvenile dependence, and training, as tending to precede rather than accompany the evolution of larger and more sophisticated brains. Views differ on how much weight to give to direct provisioning and how much to concomitant gains in status that could enhance mating opportunities. The area is one of active research. Having outlined their overall perspective, Kaplan and Lancaster proceed in Chapter 7 with a whirlwind tour through human history, from foragers on to horticulturalists, pastoralists, agricultural civilizations, and industrialized societies. They point out along the way relationships between ecological settings and observed outcomes for male-female complementarity in provisioning and parenting, mating and marital systems, and age-specific economic returns. The modern examples are drawn especially from a U.S. perspective. Like the chapters that follow, Chapter 7 provides an entrée to an extensive corpus of existing scientific work.

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Offspring: Human Fertility Behavior in Biodemographic Perspective Steven Gangestad, in Chapter 8, describes how various competing interests of the male and female members of a couple exist alongside their common interest in offspring. He intentionally divides the topic with Kaplan and Lancaster, focusing on conflicting stakes in parenting while they focus on complementarities. Biological processes in which male and female members of a species successively adapt to each other’s own gene-promoting stratagems, in a generation-by-generation dance of action and reaction, is called “sexually antagonistic coevolution.” Gangestad reports on experiments with Drosophila, dung flies, and finches showing such coevolution taking place. He reviews the theoretical implications for humans and describes observations of human behavior under controlled conditions that exemplify the predicted patterns. Among the surprises featured by Gangestad is an increase in a suite of “mate retention tactics” on the part of husbands specifically during the ovulatory phase of their wives’ cycles. During the same phase, wives report fantasizing more often about other males. The biological studies provide a context for consideration of contemporary social phenomena in conflicts over parenting effort and marital instability. Ben Campbell shifts the focus in Chapter 9 to adolescence, specifically reproductive maturation in boys. The topic goes beyond physical sexual maturation to include the endocrinological regulation of emotional and social development. Campbell advances a hypothesis of his own about the possible roles for two adrenal hormones acting progressively over an extended period from just before puberty into early adulthood. Campbell reflects on the evolutionary trade-offs implicated in this part of the human developmental program, on the gap in years between puberty and procreation in many human societies, and on the reversed order of peak growth spurt and menarche for girls compared to puberty and peak growth for boys. He considers refinements of the basic idea that timing of sexual maturation balances the benefits of earlier reproductive opportunities against the costs of risky male-male competition. The availability of biological indicators from populations with differing levels of nutritional advantage and differing norms governing risk taking, competition, violence, affiliation, and mating hold out the promise of testing hypotheses in this area in the coming years. In Chapter 10, Carol Worthman draws on three approaches—life history theory, reproductive ecology, and developmental psychobiology—and discusses their strengths and limitations and what is to be gained by bringing them together. It is a wide-ranging chapter, taking up a variety of aspects of fertility and family formation not fully treated elsewhere in the volume. Alongside genetic transmission, Worthman stresses the importance of biocultural inheritance. She features the costs of sociality in evolutionary settings, and their implications for established patterns of behavior. She

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Offspring: Human Fertility Behavior in Biodemographic Perspective challenges proponents of life history theory to have a more realistic appreciation of the ubiquity of multi-tasking, as it bears on allocations of time and energy between production and parenting. Finally, she describes ideas about genetic plasticity and environmental feedbacks in the shaping of neuronal connections and more generally in physiological and psychological growth after conception that go under the general heading of “developmental Darwinism.” DEMOGRAPHY The concluding chapters of this volume engage themes of the earlier chapters from demographic perspectives. In Chapter 11, David Lam explores intersections between rational choice models from economic demography and evolutionary approaches to human fertility transitions. Two fertility transitions are at stake—the classic “demographic transition” to near-replacement fertility still under way in many developing countries and the recent transition to below-replacement fertility most visible in a number of European nations. Lam develops the economic formalism for quality-quantity trade-offs and relates it to evolutionary treatments like the one by Kaplan and Lancaster in Chapter 7. For the ultimate goal of predicting fertility levels, quality-quantity frameworks remain in need of a basis for predicting how much child quantity should trade off against how much child quality, which neither approach as yet supplies. In modern settings, education provides a proxy for child quality. Lam illustrates rational choice theory with empirical results on education, income, women’s employment, and fertility in Brazil. It would be interesting, as John Haaga has pointed out in discussing chapters of this volume, to include in such a nexus of variables the gains in status that education may provide to women when entering the marriage market and when negotiating fertility decisions in marriage, as Basu (1999) has described. The human setting is an intriguing parallel to the importance of hierarchical status to evolutionary fitness among some primates illustrated in Chapter 6. At first encounter, many people expect that evolutionary theory has to have a problem with below-replacement fertility. In principle, it need not. When the environment is transformed, appetites and responses that would once have tended to maximize an individual’s descendants may operate instead to reduce them. There are often evolutionary explanations for presently mal-adaptive traits. Even moderate fertility mediated by quantity-quality trade-offs appears on the face of it mal-adaptive in the Darwinian sense. In Chapter 11, Lam reflects on how odd the demographic dynamics would have to be in the modern world for smaller numbers of high-quality children in one generation to pay off in greater numbers of grandchildren in the next generation.

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Offspring: Human Fertility Behavior in Biodemographic Perspective But it would not be so odd for investments in child quality, status, and command over resources to empower lineages to survive severe rare bouts of hard times and mortality generations apart. In this respect, in the very long run, history and prehistory might not prove so different as they seem. In Chapter 12, John Hobcraft reviews the prospects for cross-fertilization across the whole range of approaches represented in this volume, from behavioral genetics, experimental biology, endocrinology, evolutionary anthropology, and demography. He underlines the opportunities while recognizing barriers. Returning to the topics with which the volume begins, he scrutinizes problems of analysis that hinder general acceptance of results from the application of behavioral genetics to fertility behavior. Hobcraft draws attention to the greater readiness of evolutionary anthropologists and behavioral ecologists to incorporate ideas from demography than of demographers to embrace evolutionary perspectives. In his view, “a central challenge facing these approaches . . . is the extent to which they are relevant to our understanding of the demographic transition and of changes in fertility behavior, especially to very low levels of fertility.” He expresses a degree of skepticism about the ability of evolutionary theory to make testable predictions. These sections of Chapter 12 provide lively counterpoint to the ideas espoused in Chapters 7, 8, and 9. Hobcraft goes on in Chapter 12 to describe a framework, developed in association with Kathleen Kiernan, for examining the decision to become a parent, with its long-term implications and uncertainties, in the English and European contemporary settings of low fertility. The biological factors discussed in this volume interact with a broad range of environmental demographic determinants, economic trade-offs, tastes and preferences, gender roles, the calculus of self-interest and joint interest, societal support, and future security. Granting parallels between this framework and behavioral ecology and life history analysis, Hobcraft describes the challenges facing a research agenda for uncovering genetic and neuro-endocrine sub-structures for fertility behaviors that are now so largely matters of conscious choice. Is below-replacement fertility a puzzle? Hobcraft balances conflicting views on this central question. Evolution could have arranged for humans to act to maximize the numbers of descendants primarily through our evolved sex drives. In that case, with sexual satisfaction disconnected from the procreation of children by contraception, the lowest of low fertility would be no puzzle. On the other hand, evolution could have emphasized a whole repertoire of innate drives toward pair bonding, nurturing, and parental commitment, originally operating toward assuring the survival of offspring and now constituting instinctive incentives for childbearing. Many of the chapters in this volume present evidence for the salience of such evolved pathways of influence in other species and our own. Are we to

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Offspring: Human Fertility Behavior in Biodemographic Perspective regard such inborn impulses as typically being satisfied, in the modern world, with the birth and rearing of a single child? Hobcraft points to changes in mothers’ brains during pregnancy and to physiological and psychological reinforcements to nurturing instincts during pregnancy and after birth, which one might interpret as part of an evolved set of mechanisms promoting further births. Is human nature pro-natal? This term may stand as short-hand for some of the larger issues through which the biodemography of fertility behavior and family formation impinges on the practical demography of policy forecasts. The question makes sense when our evolutionary heritage is viewed not as a built-in program of behavioral directives but as a sounding-board for responses to ever-changing environmental stimuli. For Third World countries, trade-offs between child quantity and quality are seen as propelling transitions away from high fertility. Demographers are struggling to foresee the degree of ease and evenness with which such transitions may proceed across the whole of the globe. Biodemographers are asking about the deep structure of quality-quantity transfers, their evolutionary status and origins, and plausible long-term roles. For the developed world, below-replacement fertility can be seen as a logical concomitant of new gender roles and economic opportunities. Will it be sustained or will it prove a temporary aberration, incompatible in the long term with the instincts with which evolution has endowed us? DIRECTIONS FOR FUTURE RESEARCH The biodemography of fertility behavior and family formation is at a stage where lines of communication are being opened up across research domains, ideas are being floated, and groundwork is being laid. The field is not at the point of offering definite competing answers to a handful of sharply-posed questions. It follows that a wide diversity of research directions deserve consideration. Some have been mentioned in earlier sections and are spelled out in a number of the chapters that follow. This volume is not a consensus report, and all that can be offered in this introductory chapter is a personal selection. Progress depends on data. Exciting opportunities for the future are likely to emerge from the gathering of biological indicators in demographic surveys, a prospect frequently mentioned in this chapter. We may learn whether the conscious reasons offered by individuals for wanting children, or for wanting or not wanting further children, sort themselves out by hormonal levels or other physiological measurements. Linkages between psychosocial stress and reproductive hormones are increasingly well understood from laboratory studies described in Chapter 6. Longitudinal surveys with biological markers are proving their worth in studies of long-term

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Offspring: Human Fertility Behavior in Biodemographic Perspective health and successful aging (e.g., National Research Council, 2001) and provide precedents for longitudinal studies of successful marital bonding, family planning, and parenting enriched by coordinated endocrinological and questionnaire-based measurements of stress and resilience. In the area of behavioral genetics, a need has been described for new starts, moving beyond heritabilities and correlations and emphasizing (to borrow Michael Rutter’s phrase) “discriminating and sensitive measures of the environment.” Such new approaches might be able to exploit the individual-level time-series measurements of environmental conditions and exposures that are becoming practical. Both evolutionary theory and endocrinological investigations presented, for example, in Chapters 8 and 9 suggest that traits implicated in family formation outcomes, like risk-taking, may be more amenable to genetic analysis than fertility outcomes themselves. Separating genetic influences from self-selection may be aided by accumulating longitudinal data sets like the Add Health Study. Molecular genetics has its own impetus and will take its own directions. Biodemographers are not for the most part qualified to suggest promising experiments, but they need to be poised to assimilate new molecular discoveries like those presented in Chapter 4. Such discoveries change the conceptual framework for thinking about subconscious sources of decisions and reactions. The suggestions about the kinds of reward circuitry involved in pair-bonding in one other species as described in Chapter 4 might well suggest novel survey designs for demographic fertility surveys. In the wake of advances in endocrinology, as discussed in Chapter 5, studies seem timely to relate psychosocial stress to hormone secretion and to reexamine links between nutrition and fertility. The ongoing studies of the demography and social interactions of other primates in the wild, illustrated in Chapter 6, are likely to be critically fruitful sources of new ideas and perspectives for biodemography throughout the foreseeable future. In this area, the interests of biologists and the interests of demographers are already well coordinated and mutually reinforcing. A rich variety of topics from evolutionary anthropology treated in Chapters 7, 8, 9, and 10 holding out promise for future research have been discussed in earlier sections. In the near term, comparative studies of the dynamics of grandparental provisioning and intergenerational support for offspring within an evolutionary framework appear to be at the top of the agenda, bringing together economists, sociologists, mathematical demographers, and anthropologists on common ground. One gap in coverage in this volume needs to be borne in mind. Despite strenuous efforts, the organizing panel was not successful in recruiting a contribution dealing directly with brain research. Some topics from neuroscience are treated in Chapters 4 and 5, and their relevance is discussed in

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Offspring: Human Fertility Behavior in Biodemographic Perspective Chapter 10, but a comprehensive overview would have been desirable. Rapid advances in brain-imaging techniques are being made. Hamer (2002) cites recent examples in which genetic influence on measured brain activity have proved far stronger than the downstream influences on behavior. Expanding knowledge of the role of the brain and the whole central nervous system in the physiology of sexual response and reproduction is likely to have long-term implications for the understanding of fertility behavior. A theme that runs through many of the chapters of this volume is the salience of pair-bonding, parent-child bonding, and nurturance as they take their place beside sex and procreation as constituents of fertility behavior. Sexuality research has been a continuing priority within the social science community. Bonding research appears to deserve complementary emphasis. There is much to build on, including the discoveries from molecular genetics reported in Chapter 4, the endocrine pathways described in Chapter 5, the comparative perspective from other primates in Chapter 6, and the evolutionary tradeoffs analyzed in Chapters 7, 8, 9, and 10. As Chapters 11 and 12 make clear, demographers have strong incentives to supplement economic and social theories with an understanding of the biological dimensions of the choices of couples to stop at one child or to go on to further children. Through such pathways, biodemography impinges on the ever-present question of the persistence or transience of below-replacement fertility. All these research priorities depend on the continuing development of communication and interaction between biological and social scientists. It is interesting to note how some specialists in fields of anthropology, genetics, and integrative biology have adopted ideas and approaches from demography. The mathematical core of demography makes it accessible as well as useful to others, and broader ideas are bound up with the mathematics and may be carried over in the process. Making genetics, neuro-endocrinology, integrative biology, and evolutionary perspectives accessible to demographers may be a larger challenge. But the chapters of this volume illustrate the extensive common ground that already exists around the study of fertility behavior and family formation and the promise of future cooperative biodemographic research. REFERENCES Basu, A. 1999 Women’s education, marriage, and fertility in South Asia: Do men really not matter? Pp. 267-286 in Critical Perspectives on Schooling and Fertility in the Developing World. Committee on Population. C. Bledsoe, J. Casterline, J. Johnson-Kuhn, and J. Haaga, eds. Commission on Behavioral and Social Sciences and Eduation. Washington, DC: National Academy Press.

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Offspring: Human Fertility Behavior in Biodemographic Perspective Bongaarts, J., and R.G. Potter 1983 Fertility, Biology, and Behavior: An Analysis of the Proximate Determinants. New York: Academic Press. Davis, K., and J. Blake 1956 An analytical framework. Economic Development and Cultural Change 4:211-235. Ellison, P.T. 2001 On Fertile Ground. Cambridge, MA: Harvard University Press. Hamer, D. 2002 Rethinking Behavior Genetics. Science 298:71-72. Hobcraft, J.N., and R.J.A. Little 1984 Fertility exposure analysis: a new method for assessing the contribution of the proximate determinants to fertility.differentials. Population Studies 38:21-45. Hrdy, S.B. 1999 Mother Nature: A History of Mothers, Infants, and Natural Selection. New York: Pantheon. Jacobson, K.C., and D.C. Rowe 1999 Genetic and environmental influences on the relationships between family connectedness, school connectedness, and adolescent depressed mood: Sex differences. Developmental Psychology 35:926-939. Kamin, L.J., and A.S. Goldberger 2002 Twin studies in behavioral research: A skeptical view. Theoretical Population Biology 61:83-95. National Research Council 1997 Between Zeus and the Salmon: The Biodemography of Longevity. Committee on Population. K.W. Wachter and C.E. Finch, eds. Commission on Behavioral and Social Sciences and Eduation. Washington, DC: National Academy Press. 2000 Cells and Surveys: Should Biological Measures Be Included in Social Science Research? Committee on Population. C.E. Finch, J.W. Vaupel, and K. Kinsella, eds. Commission on Behavioral and Social Sciences and Eduation. Washington, DC: National Academy Press. 2001 New Horizons in Health: An Integrative Approach. Committee on Future Directions for Behavioral and Social Sciences Research at the National Institutes of Health, B.H. Singer and C.D. Ryff, eds., Board on Behavioral, Cognitive, and Sensory Sciences. Division of Behavioral and Social Sciences and Education. Washington, DC: National Academy Press. Rodgers, J.L., and H.-P. Kohler, eds. 2002 The Biodemography of Human Reproduction and Fertility. Boston: Kluwer Academic Publishers. Rodgers, J.L., D.C. Rowe, and W.B. Miller, eds. 2000 Genetic Influences on Human Fertility and Sexuality: Theoretical and Empirical Contributions from the Biological and Behavioral Sciences. Boston: Kluwer Academic Publishers. Rowe, D.C., C. Stever, D. Chase, S. Sherman, A. Abramowitz, and I.D. Waldman 2001 Two dopamine genes related to reports of childhood retrospective inattention and conduct disorder. Molecular Psychiatry 6:429-433. Sheps, M.C., and J.A. Menken 1973 Mathematical Models of Conception and Birth. Chicago: University of Chicago Press. Udry, J.R. 1994 The nature of gender. Demography 31(4):561-573.