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Reflections on Demographic, Evolutionary, and Genetic Approaches to the Study of Human Reproductive Behavior

John N. Hobcraft

Demographers have long specialized in documenting, measuring, and trying to explain levels, trends, and differentials in human fertility. Traditional demographic approaches are nevertheless lacking in several respects. First, much of the explanatory literature neglects potential biological pathways, except in the very narrow reproductive biology area. The papers in this volume amply demonstrate the need to consider the roles of genetic, cognitive, and neuroendocrine pathways in shaping reproductive behavior. Second, far too little attention is paid to the fact that births initiate the process of parenthood, rather than being an end in themselves, and that reproductive choice and evolution have been shaped by the need to ensure survival and nurture of children. Exceptions to this omission have included some of the economics literature, which fairly unsuccessfully evaluates the benefits of children as a source of family production in traditional societies and as old age insurance (see Das Gupta, 1993, for a good overview). In a contemporary low-fertility context, Hobcraft and Kiernan (1995) proposed a framework for examining the decision processes and constraint involved in becoming a parent in Europe (see also Hobcraft, 1993, for an emphasis on child welfare above and beyond child survival). This issue of parenthood rather than fertility per se is also well illustrated by Worthman’s chapter in this volume.

The long-standing recognition of the critical importance of both reproductive biology and behavior and their interplays in determining human fertility has been especially clear in the various treatments of the proximate determinants of fertility (Bongaarts and Potter, 1983; Davis and Blake, 1956; Wood, 1994). In theoretical essence these studies examine variations



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Offspring: Human Fertility Behavior in Biodemographic Perspective 12 Reflections on Demographic, Evolutionary, and Genetic Approaches to the Study of Human Reproductive Behavior John N. Hobcraft Demographers have long specialized in documenting, measuring, and trying to explain levels, trends, and differentials in human fertility. Traditional demographic approaches are nevertheless lacking in several respects. First, much of the explanatory literature neglects potential biological pathways, except in the very narrow reproductive biology area. The papers in this volume amply demonstrate the need to consider the roles of genetic, cognitive, and neuroendocrine pathways in shaping reproductive behavior. Second, far too little attention is paid to the fact that births initiate the process of parenthood, rather than being an end in themselves, and that reproductive choice and evolution have been shaped by the need to ensure survival and nurture of children. Exceptions to this omission have included some of the economics literature, which fairly unsuccessfully evaluates the benefits of children as a source of family production in traditional societies and as old age insurance (see Das Gupta, 1993, for a good overview). In a contemporary low-fertility context, Hobcraft and Kiernan (1995) proposed a framework for examining the decision processes and constraint involved in becoming a parent in Europe (see also Hobcraft, 1993, for an emphasis on child welfare above and beyond child survival). This issue of parenthood rather than fertility per se is also well illustrated by Worthman’s chapter in this volume. The long-standing recognition of the critical importance of both reproductive biology and behavior and their interplays in determining human fertility has been especially clear in the various treatments of the proximate determinants of fertility (Bongaarts and Potter, 1983; Davis and Blake, 1956; Wood, 1994). In theoretical essence these studies examine variations

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Offspring: Human Fertility Behavior in Biodemographic Perspective 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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Offspring: Human Fertility Behavior in Biodemographic Perspective changes in fertility behavior, especially to very low levels of fertility. For some proponents of an evolutionary perspective it seems that Darwinism (sometimes very broadly interpreted) has become a new form of quasi-religious belief (the frequent use of the term “ultimate cause” is indicative— e.g., Clark and Low, 2001), so that any regularity in human behavior is deemed by definition of evolutionary origin and any variation simply reflects different adaptive strategies. Well-written and lively accounts of plausible mechanisms, which either cannot be tested or regard testing as irrelevant, often involve writing history backward rather than providing useful insights into modern behavior. However, some serious attempts to make hypotheses more precise and testable do occur, although many pitfalls and areas of dispute often remain. A recent illustration of the difficulties is provided by the exchange regarding the Trivers-Willard hypothesis (see Freese and Powell, 1999, 2001; Kanazawa, 2001). To express such caution and skepticism does not mean that I dismiss these approaches out of hand or that I take the blinkered social science approach that reproduction is “all in the jeans.” The problem is rather to get a clearer and better-defined set of mechanisms and then to explore the extent to which they are relevant to modern reproductive behavior. For example, in a framework for understanding “becoming a parent in Europe,” Hobcraft and Kiernan (1995) posed the question of whether evolution predisposed us to have sex or to reproduce, since either mechanism would suffice to ensure regular conception in a traditional society. However, they have different consequences for understanding current fertility behavior, since the separation of sex and reproduction is achievable. How could we distinguish between these two plausible “determinants” of past reproductive behavior? If we are predisposed to reproduce in some direct pronatalist sense, achieving very low fertility would require stronger behavioral desires and controls to offset this innate tendency than if the pathway was through recreational sex (Diamond, 1997; Foster, 2000; Hobcraft and Kiernan, 1995; Morgan and King, 2001). An interesting issue to explore is the extent to which evolutionary and genetic approaches to understanding human fertility, which diverge considerably in their approaches at the moment, will come back together as we refine our understanding of the pathways involved in the complexities of human reproduction. We will almost surely find that the genetic associations with modern fertility behavior are mediated through other forms of behavior. For example, there is some evidence that risk taking is partly genetically mediated and risk taking is also associated with many aspects of reproductive behavior, such as early sex and pregnancy, lower and less effective use of contraception, early partnership and partnership breakdown; risk taking could also be associated with the biology of reproduction, for example, with a

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Offspring: Human Fertility Behavior in Biodemographic Perspective higher incidence of sexually transmitted infections and consequent fecundity impairment. Part of the genetic variability in fertility might thus arise from variation in risk taking that affected many outcomes, all of which are related to reproduction. BEHAVIORAL GENETICS MODELS OF REPRODUCTION One of the key features of behavioral genetics approaches to the analysis of human behavior is the increasing attention to the design of their studies (Plomin, 1994; Rutter et al., 2001). Moreover, there is increasing attention to interplays of nature and nurture, or gene-environment correlations and interactions. Teasing out these elements requires strong identifying and theoretical assumptions for the models used and is likely to demand longitudinal information too. By comparison with Reiss et al. (2000), for example, most behavioral genetics studies of fertility-related behavior seem fairly primitive, though it is of course early days. Most of the studies on fertility-related behavior to date have relied on standard ACE or DCE models (Additive and Dominant; the latter include a dominant instead of an additive genetic component). Since many fertility-related studies rely on samples of twins (and occasionally siblings or other close relatives), we begin here. Essentially siblings and especially twins are defined as having a common environment, and monozygotic (MZ) twins are 100 percent genetically related, whereas dizygotic (DZ) twins and siblings share half their genes. The nature of the models and their fitting means that any gene-environment correlations are swept into the additive genetic component and that all unexplained residual variance is swept into the nonshared environment term (see, for example, Rutter’s chapter in this volume; Rutter and Silberg, 2002; Turkheimer and Waldron, 2000; Rutter et al., 2001; and for an accessible account of the basic models and ideas, Plomin et al., 1997 and Plomin, 1994). For a lively discussion of the use of such models in a demographic context, see the comments on Morgan and King (2001) by Capron and Vetta (2001) and Kohler (2001). Since the majority of studies that have attempted partitions of variance in fertility using behavioral genetics models have been based on the information from the Danish Twin Registry (for an overview see Rodgers et al., 2001b), we shall begin by looking at these in some detail. These studies have all examined a variety of birth cohorts, including those of 1900-1923 (Kohler and Christensen, 2000), 1870-1910 and 1953-1964 (Kohler et al., 1999), 1953-59 (Rodgers et al., 2001a), 1953-1970 (this volume), and 1945-1968 (Kohler et al., 2001). No compelling rationale has been provided for these very different selections of birth cohorts, although data availability and selection issues undoubtedly play a part. The fertility-related outcomes considered have also varied, partly as result of censoring of

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Offspring: Human Fertility Behavior in Biodemographic Perspective experience for the later cohorts, but have included children ever born (completed or partial cohort fertility), childlessness or its converse of having had a least one birth, ever marriage, measures of early childbearing, and a retrospectively reported age at first attempt to get pregnant (collected in 1994). The twin registry was first established in 1954, when those born in 1870 were already 84 years old, and information was sometimes only obtained from proxy reports from the cotwin or even other relatives (see Kohler et al., 1999). Strong selection for survival is reflected in the small sample sizes for the earliest cohorts (around 600 twin pairs of each sex for the birth cohorts of 1870 to 1889, compared with about 1,400 for those of 1890-1910). Moreover, the reported fertility for these earliest birth cohorts seems very low and apparently quite out of line with the general population, suggesting either severe misreporting or strong selection of survivors for low fertility. No indication is provided concerning conditions about survival to any age for inclusion in the analysis—it is not clear that including twins or co-twins who died early in their reproductive careers is wise (it might be defensible for studying reproductivity but not fertility behavior). No comment is made concerning the lower correlation of children ever born for MZ twin pairs than for DZ twin pairs among men born between 1870 and 1889. Moreover, smoothed estimates for the ACE and DCE models are obtained using a sophisticated weighting procedure with a “bandwidth” that concentrates 76 percent of the weights in the 7 years centered on the birth cohort in question; no indication is provided of how the end cohorts were weighted, and all results for the 1953-1964 cohorts may well be susceptible to such “end” effects. I also have unanswered concerns about the possible interactions of taking weighted averages in circumstances where both the numbers of cases included in the analysis and the levels of fertility are changing rapidly over time. For the earlier birth cohorts the most striking finding, discussed at length by Kohler et al. (1999), is an apparent sharp increase in the genetic component of variation in completed fertility for women born during the 1880s, with a rapid return to lower levels for those born after that decade. Given that estimates were weighted over several years, it is possible that this decadal effect is an artefact of a single spike or singularity getting spread across most of the decade. In contrast, there is, if anything, a reduction in the genetic component for men born in this decade to zero or even negative estimates. One striking feature of the results that can only be examined impressionistically from Kohler et al. is the apparent almost exactly opposite movements in the genetic and shared-environment components over time. This seeming constancy of broad-range heritability (or of the nonshared-environmental component) over time is intriguing and could possibly suggest that the just-identified nature of ACE or DCE models

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Offspring: Human Fertility Behavior in Biodemographic Perspective when all pairs share a common environment (no twins reared apart) may lead to instability of the partition of the two components of the variance. A similar observation might be made with respect to the other paper that examines time trends in these components (Kohler et al., 2001), where there is a seeming shift from 50 percent of the variation in early fertility being accounted for by shared environment, with no genetic component, for the 1945-1952 birth cohorts to exactly the reverse for the 1962-1968 birth cohorts. Again, this raises questions as to whether such shifts are real or, at least in part, artefacts of near collinearity in the components. The very large standard errors for the variance components estimates (significance is often indicated at the 10 percent level) are possibly suggestive in this respect. There are a series of other difficult assumptions that have to be made: for example, the assumption that MZ and DZ twins do not alter their environments differentially (an example of the potentially large implications of such a difference is provided in a different context by Feldman et al., 2000). Since I am no expert on models of this type, these are raised as concerns rather than explicit criticisms. The results and their interpretation are nevertheless both intriguing and challenging enough to warrant further replication with other data sources. Kohler and his collaborators are quite comfortable with a very low genetic component of variation for the earliest cohorts considered, seeing this as being compatible with Fisher’s fundamental theorem of natural selection for a society with pretransitional fertility levels, since very little genetic variation would be expected for fitness components. Hughes and Burleson (2000) give the plausible candidates for such life history traits: juvenile survival, development rates, age-specific fecundity, mating success, and longevity. Implicit in the interpretation of Kohler and colleagues is that fertility of the cohorts born during the 1870s in Denmark was almost entirely determined by fitness components. The significant early shared-environment component is thus interpreted as reflecting differentials in childbearing success (which would also probably be reinforced by survival differentials to achieve reproductive success) associated with variations in fitness that are usually interpreted as being likely to be status related in most evolutionary discussions. The very rapid and short-term emergence of a substantial genetic component of variation in fertility of women (but not men) born during the 1880s is seen as being associated with the (first) demographic transition. Such a shift, of course, does nothing to challenge Fisher’s theorem. In most fertility transitions the fertility decline is achieved through fertility limitation within marriage, although it is often also accompanied by delayed marriage. More importantly (and in many ways problematic for evolutionary interpretations—see, e.g., Foster, 2002), the differentials in fertility that emerge during the early fertility transition are usually negatively associated

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Offspring: Human Fertility Behavior in Biodemographic Perspective with status, and this situation usually continues through the first demographic transition and often during the second one too. Since new and status-related behavior is coming into play, there is a real possibility that some of the new behavioral constraints on fertility will be related to genetic origins, although of course inevitably constituting gene-environment interplays. But there is also no obvious reason why shared environment should disappear as a source of variation at the same time. And why should these features be different for men than for women? To begin to answer such questions, we need some clarity about what pathways are likely to be involved in originating new genetic sources of variation that are associated with innovative behavior such as trying to prevent conception. Is the rise in genetic variability simply due to new behavior being adopted by the advantaged (including the better educated)? Presumably, both in light of Fisher’s theorem and the lack of a genetic component for preceding cohorts, we would be justified in concluding that the genetic traits now being expressed had little to do with the fertility-related fitness components but reflected new sources of variation in changing circumstances. In this respect the conclusion that “fertility motivation rather than fecundity is subject to genetic variation” (Kohler et al., 1999:281) is hardly surprising, although the particular example of the analysis of age at first try to have a child (with potentially serious recall problems) does not show unequivocally stronger genetic variation than either censored completed fertility or ever having had a birth for the 1953-1964 birth cohorts. Moreover, there are clearly interplays between behavior and motivation on the one hand and fecundity on the other: delaying childbearing until ages of reduced fecundity can have unintended consequences. The finding that genetic influences seem to be stronger for the transition from zero to one child than for “completed” fertility is puzzling, especially for the earlier cohorts, and the fact that such correlations could not arise across the generations (i.e., from parent to child, since parents cannot be infertile) is never addressed. Why is the increase in the genetic component of variation in fertility for women so short lived during the demographic transition (lasting less than a decade, even when spread out by the smoothing procedures)? The (over)interpretation of Kohler et al. (1999:281) is: Since fertility is generally a household decision reflecting the preferences of husband and wife, correlated fertility motivations among twins reveal themselves in the number of children a couple has, provided that the household decision includes the preference of the twin. The strong genetic influence on the number of children at the onset of the fertility decline and in contemporary cohorts therefore emphasizes the instrumental role of females in adopting conscious fertility control and in household bargain-

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Offspring: Human Fertility Behavior in Biodemographic Perspective ing about fertility: only if females have a strong influence on household decisions about fertility do genetic influences on fertility motivations reveal themselves in fertility outcomes. While it is probably undisputed that females in contemporary cohorts exercise a strong influence on household decisions the strong influence of females in cohorts born around 1885 (i.e. in cohorts facilitating the early fertility decline) may be more surprising. Implicit in such an argument would be the notion that women were genetically more effective at bargaining over remaining childless than over family size. There is no justification for why women born in the 1880s stood out in terms of genetically mediated bargaining power over fertility control, compared with those born in immediately subsequent decades. Nor is it readily apparent why women’s liberation on fertility decision making should not have shared-environment components and sources. An important feature of this extended citation is that it acknowledges that childbearing (and marriage) involves both partners. This carries several implications for future research on fertility, marriage, and divorce. First, twin studies will never permit us to address and disentangle genetic components of variation for the dyad involved. The only way to get leverage on this aspect will be through molecular genetics (or linking back through neuroendocrine protein pathways to genetic markers), and that will require much sharper elaboration of likely pathways and identification of markers for these pathways. Second, shared environment predominantly relates to shared family of origin effects and is a concept largely introduced as a means for separating out narrow-sense heritability (that nevertheless incorporates all gene-environment interplays). In particular, it always seems odd to pay exclusive attention to shared family-of-origin environment components for adult behavior, although nevertheless a source of important and interesting pathways to adult development. A full analysis in the context of fertility would surely involve separating out shared-environment components from the family of origin of both partners and for the couple themselves, in addition to examining assortative mating by genes and by childhood shared and nonshared environments, and the roles of genotype-genotype interplays. The study of such complex differentiation would both necessitate molecular genetic indicators and new tools of analysis, although again endocrine and neural pathways may also hold some promise. We have examined the studies of sources of variation in fertility over time in considerable detail because these are potentially extremely interesting and could lead to insights that go beyond the “black box” partitioning of variance that is usually the only result from such studies. The studies using the Danish twins are the most sophisticated yet in this context. The innovative attempt to use linkages for kinship in the National Longitudinal

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Offspring: Human Fertility Behavior in Biodemographic Perspective Survey of Youth (Doughty and Rodgers, 2000; Rodgers et al., 1999; Rodgers and Doughty; 2000) seem beset by considerable uncertainty in the linkage process and the inability to determine the zygosity of twin pairs fully. Some other twin-based studies of fertility-related behavior raise more puzzles than they answer. For example, McGue and Lykken (1992) used a postal sample of twins from the Minnesota Twin Registry, with an unreported response rate. For their samples of both men and women, MZ twins show a lower overall correlation in risk of divorce than do DZ twins; this inconvenient and challenging feature of their data was not discussed, nor did it preclude strong claims of results for genetic sources of variation in divorce, generally based on looking at conditional probabilities of one twin divorcing given that the other had, where MZ twins did have higher conditional probability. A more careful genetically informed study on divorce is based on the Colorado Adoption Study, which enables the authors to tease out some interesting results (O’Connor et al., 2000). PATHWAYS TO FERTILITY BEHAVIOR The availability of simple and cheap assays for biological markers, such as neuropeptides and endocrine proteins has led to a wide range of studies that look at links to fertility behavior. Many of these studies have looked at pathways that are involved in the biology of fertility, including puberty, menarche, ovulation, pregnancy, and lactational infecundability. Such work is very valuable in aiding our understanding of variations in some of the key proximate determinants of fertility. These findings with respect to hormonal mediation of family formation outcomes are ably reviewed in Cameron’s chapter in this volume. It is striking that a large body of evidence can be reviewed concerning sexual behavior and the biology of fertility, but there seems to be a paucity of evidence on endocrine or neuroendocrine pathways related to the other behavioral aspects of fertility, such as reproductive intentions or motivations, with the exception of nurturance or parental behavior. See Panksepp (1998) for a review of neuroscience findings, especially on sexual feelings and behavior and on nurturance and maternal behavior. There are, however, many other interesting and unanswered questions on the underpinnings of the biology of fertility that are relevant to fertility outcomes and possible sources of genetic variation. Why are there significant levels of involuntary infertility? What are the sources of the variation in the ending of the reproductive life span? Why do quite high fractions of couples experience early natural onset of secondary sterility and what are the sources of variability among couples? Are they genetic, gene-environ-

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Offspring: Human Fertility Behavior in Biodemographic Perspective ment interactions, or environmentally induced? In a similar vein, why does heterogeneity in fecundity need to be incorporated into biometric models of fertility? Is this simply to account for differences in behavior, especially coital frequency? Or are there genuine variations among fecund couples in their biological chance of conception? All of these are questions about fitness-related aspects of fertility. At least for the reproductive biology of fertility we do know quite a lot about which biological markers are involved and often how they operate. But fertility-related behavior, including pair bonding, coital frequency, contraceptive use, and abortion among others and fertility motivations are much less well understood. This is why serious attempts to outline the pathways involved or the ontogenies are badly required (see Miller and Rogers, 2001, on human bonding; and Foster, 2000, or Miller et al., 2000, on childbearing motivation). It is probably here that the greatest opportunities for geneticists, brain scientists, and endocrinologists to come together with demographers and those engaged in evolutionary anthropology or psychology and in behavioral ecology. It is clear that the latter group has much to learn from the harder sciences, which are already having a profound effect on their understanding of the determinants of fertility and fertility-related behavior. It is also likely that the greater knowledge of empirical patterns, differences, and regularities in such behavior among human populations will provide interesting insights and challenges for those engaged in the biology. Improving the extent of understanding and intellectual interchange among the disciplines involved in the study of (human) reproduction does seem an important way forward. Clarity of understanding of pathways through genes, brain, endocrine system, and environment and more especially the mutual interplays and feedbacks involved and the elucidation of conditional responses is vital. Assumptions of determinism or of ultimate causality for one domain should always be challenged, whether genetic, evolutionary, or social scientific/environmental determinism. Thus, for example, it is not enough to assume that evolution must be the source of all behavior because it is by definition the ultimate cause (see, e.g., Clarke and Low, 2001). Clarke and Low (2001:636) provide the interesting example that evolutionary scholars would never ask “why Americans want children, since genetic and lineage success are basic currencies.” Yet Foster (2000) clearly addresses such an issue from an evolutionary perspective. If we cannot address questions about decisions concerning childlessness or the lowest low fertility in an evolutionary perspective, does this make such concerns irrelevant for modern reproduction? Foster (2002) argues persuasively that an evolutionary explanation for the (first) demographic transition is not needed.

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Offspring: Human Fertility Behavior in Biodemographic Perspective PATHWAYS TO WHAT? As already indicated above, there is a considerable body of evidence to suggest that genetic, neural, and endocrinological pathways are involved in both sexual maturation and sexual behavior and that these pathways interplay with and can be reinforced by behavior. Several authors have drawn attention to the sufficiency of an evolved sex drive among humans, one of the relatively few species to enjoy recreational sex (Diamond, 1997), to have ensured fertility in traditional societies (Foster, 2000; Hobcraft and Kiernan, 1995; Morgan and King, 2001; Potts, 1997). Foster (2000) also argued persuasively that this sufficiency would make it highly unlikely that a separate set of evolved responses to ensure fertility per se would have been necessary. The modern separation of sexuality and fertility through contraception and abortion has perhaps thus demonstrated the “blind watchmaker” aspect of evolution in this context: a purposive evolution might have ensured a mechanism tied to a need to bear children. In this context, Hobcraft and Kiernan (1995) also posed questions as to why, if there was a strong innate disposition to be fertile, so many societies and religions found it necessary to develop strong pronatalist norms. In many cultures it is parents and other elders who maintain social controls over early access to sexual partners and who arrange marriages. Such control is linked to status, lineage, and inheritance patterns but is external to the individual involved and thus neither phenotypic nor genotypic. Equally, there is much evidence from human societies that considerable pressure for an early first birth within marriage is exerted by parents or parents-in-law or others in the older generations. Were humans, perhaps especially women, who disproportionately bore the physical and emotional costs of childbearing and child rearing, always wont to limit reproduction once their brains evolved enough to enable free choice? And is modern family limitation simply a reflection of the increased ability (and societal tolerance) to control fertility? There is also evidence related to human bonding and especially nurturance that is seen as relevant to fertility motivation (see Foster, 2000; Miller and Rodgers, 2001; and Morgan and King, 2001). Once again, the evidence extends, however incompletely, to genetic, neural, and endocrinological pathways and to their interplay with each other and reinforcement from environmental experiences. Demographers who have picked up on this literature differ in their interpretations as to how far the environmental mediation is lifelong and how far it is directly mediated through pregnancy, birth, and nurturance of one’s own child; there is also dispute about the extent to which early gender differentiation goes beyond a learned response. An “Occam’s razor” approach to evolution might see a direct

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Offspring: Human Fertility Behavior in Biodemographic Perspective response through pregnancy-mediated changes in the mother as sufficient in this respect. But there is a case to be made that such nurturance is part of a wider set of attributes required for successful long-term pair bonding to help keep mother and father together during the dependent development period for their offspring (see Panksepp, 1998; Miller and Rodgers, 2001; and Young, this volume). An unresolved puzzle concerning nurturance arises in the context of very low fertility, as discussed by Foster (2000). She makes a persuasive case that it is the phenotypic need to nurture that means most women wish to have at least one birth and this means that fertility is unlikely to go much lower than the extremes already seen in parts of Europe. Yet if nurturance is strongly reinforced by brain changes during pregnancy and by feedback to neural receptors after a child is born, how is it that this mechanism does not then serve to make it even more likely that a woman will want further births? Presumably there have to be series of negative feedbacks to offset the nurturance drive? On the whole, evolutionary anthropologists and evolutionary behavioral ecologists (see Symons, 1979; Wood, 1994; Campbell and Wood, 1994; Low, 2000; Ellison, 2001; and both Kaplan and Lancaster and Worthman, this volume) have been much more enthusiastic about incorporating (and sometimes co-opting) the ideas and work of demographers and related social scientists on fertility behavior into their perspectives than demographers have been to grasp evolutionary perspectives or new biological research1 (with a few notable exceptions, e.g., Udry, 1996). This fusion is challenging, necessary, and not yet fully successful, and demographers, with their rich understanding of empirical research on contemporary and historical populations, need to play their part in any new synthesis. HOW CAN WE EXPLAIN MODERN VERY LOW FERTILITY? One of the truisms of many evolutionary accounts of behavior, including fertility and mating, is a clear gender differentiation of roles: in hunter-gatherer societies men were the hunters and women the gatherers, enabling the latter to nurture the children. Mate selection and infidelity are also seen as driven by quite different concerns, with men often seen as having to be “trapped” into long-term bonding and nurturance, and the more facile projections to modern society retain a breadwinner/homemaker distinction (e.g., for a variety of perspectives, Baker, 1996; Batten, 1994; Birkhead, 2000; Buss, 1994, 1999; Etcoff, 1999; Miller, 2000; 1   Of course, this does not apply to the integration of biology and behavior in the analysis of the proximate determinants of fertility, where demographers have led the way (e.g., Davis and Blake, 1956; Bongaarts and Potter, 1983; Gray et al., 1993).

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Offspring: Human Fertility Behavior in Biodemographic Perspective Potts and Short, 1999; Ridley, 1993). A similar role differentiation by gender is a basic postulate of much modern microeconomics theory of the family (Becker, 1981). Yet, hardly surprisingly, many modern feminists regard such postulates as provocative. What is clear, however, is that there has been a major change in roles and independence of women and that this has had implications for partnership stability, childbearing and more especially child rearing, and the broader domestic division of labor. Adaptation of men and institutions to these changes is quite variable, and these changes and tensions have been seen by several commentators to be implicated in the so-called baby bust since the late 1960s (for a broad European perspective, see Hobcraft and Kiernan, 1995; on England and Wales, see Hobcraft, 1996; see also Folbre, 1994, 2001, and McDonald, 2000). But there is a much broader range of factors to be looked at in the context of very low fertility. One fundamental shift of emphasis has to be to examine the factors related to becoming a parent with its long-term ramifications, rather than births (see Worthman, this volume). The framework proposed and applied for European fertility by Hobcraft and Kiernan (1995) and further to England and Wales by Hobcraft (1996) picked out several key elements involved in the decision-making process about becoming a parent, most of which have seen profound changes in the past 40 years. The separation of sex and fertility, which was made highly reliable for the first time by modern contraception and greater access to safe, legal abortion, played a key part as a proximate determinant. Our framework allowed for pronatalist phenotypic and societal pressures, although not explicitly identifying nurture needs. The main discussion was about the series of individual, partnership, and societal constraints on becoming a parent: including the biology of reproduction; time and money constraints involved in nurture and development and their competition with other activities, including work, leisure, and domestic tasks; changing ideas, tastes, and preferences on gender, reproduction, and individual rights; and security prospects over the rearing of a child. We saw the following requirements and an assessment of their future stability as being explicitly relevant to decisions about whether to have a child at a particular point in time: having a partner, completing education or training, employment, housing, and security. In addition, we paid particular attention to gender differences in the employment domain and the domestic division of labor. The broad interpretation of historical trends and regional variations was embedded in this framework. Hobcraft (2002) provided a more detailed elaboration of measurement and conceptual issues that has influenced the proposed content of the Gender and Generation Programme under the aegis of United Nations Economic Commission for Europe (UNECE).

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Offspring: Human Fertility Behavior in Biodemographic Perspective In many ways such an interpretive framework and analysis could be seen as having close parallels to a behavioral ecology approach represented and extended by Kaplan and Lancaster (this volume) and to the life history approach provided by Worthman (this volume), though much less constrained by the need to place all short-term behavioral changes in an evolutionary perspective. But the wide range of human behaviors and developmental, life course, and contextual factors included in this framework point up the challenges in unpicking biological (in the broad sense, including endocrinological, neural, and genetic pathways and feedbacks) roles in determining fertility behavior. In most low-fertility societies, attempts to become pregnant are largely under individual or couple control and assisted reproduction is often possible. This conscious (and possibly rational) choice manifestly involves brain-environment interactions. But will we find very few genetic or neuroendocrine protein markers each linked to the wide range of factors considered by the actors involved in such choices, or perhaps more likely discover a plethora of differing links for different elements of behavior and choice? Or is such a route the wrong path to search, since all of these behavioral elements are just constraints that the human mind imposes, which are set against an overarching innate and socially reinforced urge to reproduce or nurture? These conceptual issues clearly matter in determining where to look, and we need to pay much closer attention to designing studies that might enable us to clarify these issues. Although human behavior is deeply affected by our genes and their interplay with environment, there is no reason to suppose that all behavior has to be adaptive or that the only genetically mediated pathways to modern fertility behavior have their origins with genes that evolved in relation to fertility behavior itself. The example of genetically mediated risk-taking behavior was given in the introduction, and several possible pathways linked to fertility were indicated. Of course, as stressed earlier, it is helpful to know the relevant pathways, both biological (in the broad sense) and behavioral, and the interactions and feedbacks within and between these domains. CONCLUSION Over the next few years we see a highly productive domain of research that links both fertility and fertility-related behavior backward through identifiable pathways and genetically mediated neural and endocrinological pathways forward to fertility-related behavior. As argued above, there is a huge and challenging research agenda, not the least of which will be analysis of (changing) partner-partner interplays and interactions. Moreover, we see the future of such research as being necessarily linked to molecular genetics, brain science, and endocrine systems, rather than being possible to tease out from twin or adoption studies. Nevertheless, we have discussed

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Offspring: Human Fertility Behavior in Biodemographic Perspective the insights that are being gleaned from twin and adoption studies and expect that they will provide further stimulus and insights. The novel analysis of time trends in the partitioning of components of genetic and shared-environment variance components is replicable with long-term demographic registers given modern computer linkage algorithms. But the longer-term horizon demands molecular linkages, especially if the partners are both to be considered. Demographers concerned with such issues (and we all should be) will have to think carefully about the design of studies in order to tease out the complexities involved and most of the advances will involve learning from or engaging with molecular geneticists, brain scientists, and endocrinologists. Demographers also need to understand and work more closely with the broader interpretative disciplines rooted in evolutionary perspectives. The research agenda is an exciting one in which rapid scientific progress needs a strong social scientific perspective and vice versa. Exemplified by the National Research Council and Institute of Medicine (2000) and National Research Council (2001) and by the present volume, this recognition of the need to collaborate across disciplinary boundaries is gradually occurring in a range of fields. REFERENCES Baker, R. 1996 Sperm Wars: Infidelity, Sexual Conflict and Other Bedroom Battles. London: Fourth Estate. Batten, M. 1994 Sexual Strategies: How Females Choose Their Mates. New York: G.P. Putnam. Becker, G.S. 1981 A Treatise on the Family. Cambridge, MA: Harvard University Press. Birkhead, T. 2000 Promiscuity: An Evolutionary History of Sperm Competition and Sexual Conflict. London: Faber and Faber. Bongaarts, J., and R.G. Potter 1983 Fertility, Biology, and Behavior: An Analysis of the Proximate Determinants. New York: Academic Press. Buss, D.M. 1994 The Evolution of Desire: Strategies of Human Mating. New York: Basic Books. 1999 Evolutionary Psychology: The New Science of the Mind. Needham Heights, MA: Allyn & Bacon. Campbell, K.L., and J.W. Wood, eds. 1994 Human reproductive ecology: Interactions of environment, fertility, and behavior. Annals of the New York Academy of Sciences 709. Capron, C., and A. Vetta 2001 Comments on “Why have children in the 21st century?” European Journal of Population 17:23-30.

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