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Energetics, Sociality, and Human Reproduction: Life History Theory in Real Life

Carol M. Worthman

Understanding the determinants of human fertility remains a matter of urgent practical as well as scientific concern. Many fields, including demography, economics, health sciences, and policy and political science, offer theories at varying levels of explanation and predictive power. Only one theory, evolutionary theory, offers an account at the ultimate level of design.

Since its formulation, however, evolutionary theory has challenged the efforts of anthropologists and evolutionary biologists to apply the grand theory to variation within particular species and populations, or among individuals. Ideally, these efforts involve dialectic between epistemological and empirical work, between model building and model testing. Yet even outstanding empiricists in the field emphasize the remaining challenges, reporting, for instance, “the realization that no current models that adequately explain fertility variation in traditional societies have withstood empirical scrutiny” (Hill and Hurtado, 1995:396). Both theory and the models derived from it therefore require further work.

This essay attempts to develop a fresh view of human fertility behavior and family formation by considering the intersection of three approaches— life history theory, behavioral and reproductive ecology, and developmental psychobiology. On the theoretical front, life history theory aims to integrate comparative, cross-taxonomic data into a framework comprising life course attributes such as the timing, pace, and forms of reproduction and reproductive effort. On the population level, human evolutionary and reproductive ecologists have sought to probe the value of adaptationist models for understanding variations in reproductive behavior and biology.



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Offspring: Human Fertility Behavior in Biodemographic Perspective 10 Energetics, Sociality, and Human Reproduction: Life History Theory in Real Life Carol M. Worthman Understanding the determinants of human fertility remains a matter of urgent practical as well as scientific concern. Many fields, including demography, economics, health sciences, and policy and political science, offer theories at varying levels of explanation and predictive power. Only one theory, evolutionary theory, offers an account at the ultimate level of design. Since its formulation, however, evolutionary theory has challenged the efforts of anthropologists and evolutionary biologists to apply the grand theory to variation within particular species and populations, or among individuals. Ideally, these efforts involve dialectic between epistemological and empirical work, between model building and model testing. Yet even outstanding empiricists in the field emphasize the remaining challenges, reporting, for instance, “the realization that no current models that adequately explain fertility variation in traditional societies have withstood empirical scrutiny” (Hill and Hurtado, 1995:396). Both theory and the models derived from it therefore require further work. This essay attempts to develop a fresh view of human fertility behavior and family formation by considering the intersection of three approaches— life history theory, behavioral and reproductive ecology, and developmental psychobiology. On the theoretical front, life history theory aims to integrate comparative, cross-taxonomic data into a framework comprising life course attributes such as the timing, pace, and forms of reproduction and reproductive effort. On the population level, human evolutionary and reproductive ecologists have sought to probe the value of adaptationist models for understanding variations in reproductive behavior and biology.

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Offspring: Human Fertility Behavior in Biodemographic Perspective On the individual level, developmental psychobiologists have unpacked the roles of rearing environments in temperament, sociality and parenting, and life history strategy. The evolutionary and functional analyses presented below suggest the need to expand current demographic models of human fertility behavior to include considerations of design and human development. Each level of analysis–evolutionary, ecological, and developmental–suggests that human reproduction involves much more than fertility, and identifies critical variables for successful human reproduction that merit more attention in demographic analysis. IMPORTANCE OF A BIOCULTURAL PERSPECTIVE For over 25 years, adaptationist accounts of human behavior (first under the rubric of sociobiology, then behavioral ecology) have been dominated by the calculus of cost and benefits reckoned against the bottom line of limited available resources. Reproduction occupies a central place in this calculus because fitness, or differential reproductive success, is the currency of adaptation. The logic is simple: Reproductive effort will be determined by the availability of finite resources, principally energy and time, balanced against competing demands for subsistence or survival. Human evolutionary or behavioral ecology proceeds from the assumption that “humans should have evolved fertility and mortality patterns that lead to highest contribution to the future gene pool, given the constraints provided by general human morphological, physiological, and social characteristics, and the environments in which our species lives” (Hill and Hurtado, 1995:13). This bold proposition—that human behavior has been shaped by selective pressures to optimize fitness—inspired a wave of empirical research, the best of which sought to test the validity of this claim and probe its ability to illuminate human behavior. This research builds on an older foundation of evolutionary biology to assess whether and how adaptive design may explain the inter- and intrapopulation variations in human reproductive function. The proposition that reproductive function itself reflects design constraints posed by evolutionary processes has yielded a series of novel hypotheses that have met with empirical support while also providing fresh perspectives on both adaptation and reproduction (Ellison, 1994; Wood, 1994). Yet even the best of these studies seldom cover beliefs, values, and schemas that inform behavior—that is, culture and experiential worlds (Borgerhoff Mulder, 1995). Research on impacts on fertility of workload, nutritional status, breastfeeding and supplementation, and maternal age rarely pursues the cultural dimensions woven into the biocultural dynamics (McDade and Worthman, 1998). Inattention to culture, human experience, and cognition (in behavioral

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Offspring: Human Fertility Behavior in Biodemographic Perspective and reproductive ecology) reflects gaps in evolutionary thinking that are only slowly being addressed in evolutionary psychology (Crawford and Krebs, 1998; Henrich, 2001; Henrich et al., 2001). But the gaps also mirror the absence of powerful dual-inheritance models that incorporate contemporary advances in developmental and behavioral biology. Such advances demonstrate that the distinction between biological and cultural modes of transmission is gratuitous, since inheritance operates through biocultural mechanisms across ontogeny (Oyama, 1985). Using this insight, evolutionary models, reformulated as biocultural inheritance models, could build in developmental psychobiology to provide a more complete picture of human reproductive behavior that incorporates social viability as a goal for offspring, an absolute prerequisite for successful human reproduction. The analysis here begins with an examination of life history design, particularly resource allocation over the life course to growth, reproduction, and maintenance, as effected through neuroendocrine-endocrine regulators (abbreviated as neuro-endocrine). In addition to the well-studied ecological effects on reproductive biology, less studied trade-offs with social effort are identified in foundational pathways for neuro-endocrine regulation. Thus, human physiology reflects the adaptive significance of social life and the demands for social competence and participation in that life. Then, pathways by which social ecology instructs ontogeny are reviewed, which leads directly to consideration of the environments of evolutionary adaptedness. Both sets of analyses—endocrine and epigenetic—delineate components of biocultural inheritance central to human reproduction. Thus, inheritance for humans is not appropriately viewed as dual, genetic and cultural. Rather, it has a dominant biocultural coevolutionary component that is not divisible into biological (genes) and cultural (memes) units of inheritance but runs through the mechanisms and components of epigenesis. (If we wish to model this space, we would do well to commence with close readings of Baldwin [1895] and his modern counterpart, Gottlieb [1991, 1998].) Biocultural inheritance and the redefinition of fertility outcomes to include viability of offspring imply possibilities for parallel changes in demographic thinking. Such possibilities are exemplified in the classic formulation of the “theoretical maximum” of fertility for humans (see Figure 10-1). The maximum is intended as a biological starting point from which the components that expand interbirth intervals pare away fertility potential to determine actual fertility. But this formulation, though merely theoretical, is patently absurd. Better models of proximate determinants of effective fertility may be constructed from schedules for actual populations, such as the Hutterites and Gainj, from which we could begin to derive general patterns of the kind represented in the top three boxes of the figure. This is the classic work of

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Offspring: Human Fertility Behavior in Biodemographic Perspective FIGURE 10-1 Scheduling in timing of reproductive events in diverse human populations (modified from Wood, 1994, which was based on Bongaarts and Potter, 1983). demographers, which has shaped present understanding of fertility and could generate new, expanded models of determinants to help us think about fertility in creative ways. This analysis shows, however, that models of fertility behavior cannot stop with birth or lactation but must move on to identify and incorporate the key components of development that drive the production of viable offspring or the achievement of effective fertility. LIFE HISTORY THEORY AND ENERGETICS Life history theory concerns the evolved design constraints that shape species-specific phenotypes across the life course and that underlie the striking contrasts in life histories across the animal kingdom and even within taxa. Theorists seek design features from comparative evolutionary biology that will explain fertility and mortality schedules—classic demographic concerns. Consider the contrasts among any conceivable set of organisms, such as coelenterata, insectivora, and mammalia or sardines, whales, mice, and albatrosses (Charnov, 1993; Promislow and Harvey, 1990). Rather than

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Offspring: Human Fertility Behavior in Biodemographic Perspective focusing on differential transgenerational genetic representation (fitness) as a basis of evolutionary analysis, life history theory concentrates on resource allocation as intrinsic to adaptive strategy. The life course is viewed as the product of an integrated suite of strategies for resource allocation, of which reproduction and its translation into fitness are a central feature. Energy and Time Resource allocation involves deploying energy and time across the spatiotemporal place carved out of entropy that comprises a life. The calculus is simple in principle but challenging to operationalize in detail. First, energy. This is actually better conceived as energy (e.g., calories) plus valuable, limited material resources (e.g., micronutrients). The acquisition and the disposition of resources are both components of life history strategy, but current life history theory attends little to the input or acquisition side (a focus of behavioral ecology; Smith and Winterhalder, 1992). The total energy captured by an organism is allocated among three domains: maintenance, growth, and reproduction. Maintenance includes (1) the metabolic cost of being alive, which covers processing and distributing food, air, and other inputs, the operation of membrane potentials, biosynthesis, resource recycling, and waste treatment; (2) materials and energy required for continuous maintenance and replacement, as in the constant turnover in bone and most other organs, cells, and cellular constituents, DNA checking and repair, and barrier maintenance through dermal and mucosal production; (3) healing from injury or unusual wear and tear; and (4) defense against, removal of, and cohabitation with micropredators (pathogens and parasites), as well as detection and evasion of macropredators (non- and conspecifics). In other words, maintenance involves all the investments that allay causes of mortality, intrinsic (e.g., aging) or extrinsic (e.g., accident and predation). The extent and quality of such investments in maintenance determine mortality schedules, or age-specific probabilities and causes of death. Growth includes increases in body size—in length, height, weight, and overall mass. Two issues with respect to growth are the distinction between determinate and indeterminate growers and the value of size. Many species confine growth to an early period of life, ceasing to grow when adult body size is attained; others grow throughout life. The contrast in resource allocation strategies is apparent. Body size within taxa increases through evolutionary time, prompting the inference that size itself may have adaptive value (Charnov, 1993). Reproduction comprises producing viable offspring that successfully reproduce. Life history strategy for accomplishing this may vary in many

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Offspring: Human Fertility Behavior in Biodemographic Perspective dimensions, including timing of first reproduction; adult size and proportionate size of offspring at birth; number of offspring per reproductive event and spacing between events, types, amounts, and duration of postnatal care of offspring (parental effort); sex determination and the relative contributions of males and females to reproduction; and the slope of reproductive value with age. Evolutionary analysis has focused, virtually from its inception, on the necessary reproductive effort (Darwin 1871; Maynard Smith, 1978; Trivers, 1972; Williams, 1985), which has various costs (Borgerhoff Mulder, 1992), basically including (1) the biological costs of producing a new life (gametes, gestation, parturition) and (2) the costs of sustaining new life (parental care, including lactation, provisioning, defense, and solicitude for physical and emotional security). Costs may be expanded to cover (3) mating effort—finding, recruiting, and keeping a mate. Taxa show widely divergent mating strategies, which may comprise a significant portion of reproductive effort, particularly for males. A further dramatic expansion of the costs that can be counted as reproductive effort comes from defining inclusive fitness to incorporate reproduction by others, discounted by degree of relatedness to ego (Hamilton, 1964). This adds to the list (4) the costs of inclusive fitness, such as any form of altruism (to benefit others at one’s own expense) as well as aspects of social behavior and environmental modification. Particularly for social species, and especially when culture is employed as a major adaptive strategy, there is a final component of reproductive effort: (5) socialization cost. Socializing the young includes nurturing them (attending to emotional-cognitive needs as well as material ones), instructing them or guiding their participation in adult activities while tolerating juvenile ineptitude; providing opportunities to obtain and practice knowledge, attitudes, behaviors, and skills required for viable sociality requisite to survival and reproduction; and even actively advancing their social prospects (Blurton Jones et al., 1989, 1992). Second, time. Like energy, time is a finite resource that must be apportioned among the same three domains as material resources and for reproduction, particularly, among the subdomains outlined above. The time invested in any reproductive event, weighed against life expectancy, determines reproductive potential or value. The intersection of energy and time drives rates of resource demand and use, and lies at the heart of energetics. Energetic balance can be adjusted by modulating the rate of throughput. Lower throughput, due to late and infrequent reproduction, involves lower reproductive demand per unit of time than early and frequent reproduction. Lower throughput therefore relaxes the constraint of resource availability but carries the risk of dying before completing reproduction.

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Offspring: Human Fertility Behavior in Biodemographic Perspective Trade-Offs Such considerations demonstrate a central principle of life history organization, namely, to establish optimal trade-offs among competing demands under consistently unpredictable circumstances. Modeling trade-offs is essential for understanding the design or the biological organization of the functions and capacities of the human organism. Life history strategies can be usefully viewed, on the level of the organismic design of a species, as a set of algorithms that negotiate the trade-offs to optimize spatiotemporal allocation of limited resources. One cardinal rule underlies the ubiquity of trade-offs: the allocation rule. Related to thermodynamic notions of energy conservation, the allocation rule states that consumable resources used for one purpose cannot be used for another. This rule will be critically examined below. In schematic terms, maintenance may be subtracted from resource intake to determine the net resources available for growth or reproduction, which is labeled productivity. In comparison to productivity estimated from juvenile growth rates in other determinate growers, the productivity of primates appears remarkably low, only 40 percent of that of mammals in general—and the productivity of humans is only 20 percent of that of mammals, because human children grow very slowly between the ages of 5 and 10 (Kaplan et al., 2000). Slow growth affects life history strategy in several ways. It reduces the burden of energetic demands for growth (given the species has relatively low juvenile mortality). This effect may be particularly important for easing the demands on human parents, who provision their young throughout juvenility (Bogin, 1997, 1999). Slow growth may also permit greater investments in the maintenance required to reduce juvenile mortality or to meet the high metabolic costs of large brain size. Finally, low productivity may be the product of increased developmental costs not reflected in growth (e.g., play) but required for attaining adult competence. Human Life History Strategy Humans are large, long lived, and obligately social primates that have altricial singleton births, spaced at 2 to 5 years, which are first breastfed and then provisioned well into the second decade. Young are carried, tended, defended, instructed, and systematically exposed to experiences or adult-driven ecologies aimed not only at teaching attitudes and techniques for survival but also developing social competence and coherence as well as adult productivity. Reproductive timing is set through distinctive physiological switches that inhibit gonadal function in childhood and activate it in puberty.

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Offspring: Human Fertility Behavior in Biodemographic Perspective Humans are determinate growers who grow slowly over a protracted period and then exhibit a growth spurt during a relatively late puberty. In puberty, adult height is achieved and energy previously used for growth becomes available for reproduction. Shifts in energy storage through changes in body composition (girls to fat, boys to muscle) attend this transition. After a phase of subfecundity, women enter a period of reproductive activity sustained over roughly two decades. Notably, they possess the unusual feature of programmed cessation of reproductive function—menopause—whereby ovarian potency declines and ceases decades before life ends. Reproductive aging leads to a remarkably ubiquitous average age at last birth of around 40 years (Bongaarts and Potter, 1983). The reproductive careers of men are not so curtailed, although they also show signs of reproductive aging. Challenges for Life History Theory Life history theory has excited interest because it applies formal and game theoretical models to comparative data; provides a life span, time-integrated framework; integrates across components of phenotype rather than focusing on specific features; identifies key cost-benefit trade-offs in the design of life history strategies; and thus suggests organic design criteria and evolutionary constraints. It promises to be generalizable, predictive, and hypothesis generating. Formerly, life history analysis was primarily based on species averages for the various life history parameters; variance was not included in formal analysis. However, the range of behavioral decisions used has been expanded to include state-dependent action (condition-, context-, or density-dependent) in models that can evaluate variations within and between individuals (Brommer, 2000). Notable limitations remain. These limitations do not impugn the importance and value of life history theory but should inform its application to fertility behavior. First, variations within taxa should be distinguished from variations across taxa. Life history theory is based on formal comparative analysis across taxa, yet the goal of the present volume is to address fertility behavior within one taxon, humans. The hierarchy of life history trade-offs derived from analysis encompassing macrotaxonomic variations (between classes or phyla—e.g. fishes versus mammals) does not necessarily generalize directly to variations within species. By definition, different taxa do not share evolutionary history and thus have different design features and life history strategies, so each taxon has a different hierarchy of allocation trade-offs, as allometric analysis confirms. Analysis involving restriction of the phyletic range (within rather than across orders or families) would narrow the sweep of organic design questions and help focus on the relevant variance to partition.

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Offspring: Human Fertility Behavior in Biodemographic Perspective Second, like evolutionary theory itself, the formal models of life history theory are highly abstract and aphysiological and do not support direct inferences about mediating mechanisms responsible for resource allocation or constraining life history design. A given life history constraint or trade-off can be met through diverse mechanisms. An additional empirical step is required to link life history to function in specific taxa. This step is abetted by identification of design patterns and constraints that do support testable hypotheses about organic design. For instance, comparative life history analysis identified adult mortality as the prime factor explaining variations among life history strategies. But a distinction between intrinsic and extrinsic sources of mortality (Charnov, 1993) proved difficult to sustain at the level of the functioning organism, though it generated a body of work that has illuminated relationships between these two sources of mortality (Ricklefs, 1998) as well as mechanisms of aging in general (Holliday, 1995; Kirkwood, 1981; Williams, 1957). A later section will deal with endocrine architecture and attempt to link this aspect of organic design and function with life history to show how trade-offs are embodied and shape fertility in humans. A third limitation of the life history literature concerns the allocation rule and the need for more attention to the trade-offs around sociality. Perhaps the allocation rule has not received adequate critical or empirical scrutiny. The rule may actually be “bendable,” particularly with regard to the fertility behavior of highly social primates, a possibility evaluated below. The following sections bring the strengths of life history analysis to bear on human fertility behavior while addressing limitations and expanding the scope of adaptationist thinking to substantially strengthen the analytic purchase on human reproduction. TRANSLATION AND TRANSMISSION OF EVOLVED DESIGN An integrated theory of reproductive ecology requires linkage of life history theory with phenotype and the mechanisms that produce life history, particularly reproductive careers. Phenotype comprises all the manifest features of an individual, including behavior. As in most species, human phenotypes adjust to environmental quality. The capacity for facultative adjustment can be gauged by the norm of reaction, or the phenotypic variation shown by a given genotype across diverse environments (Stearns and Koella, 1986). Cultural variations and the complexities associated with sociality lead to large reaction norms in human behaviors such as parenting practices or timing of marriage, but humans also exhibit substantial reaction norms in the biological bases of reproductive capacities and behavior.

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Offspring: Human Fertility Behavior in Biodemographic Perspective Age at Menarche One of the most thoroughly documented reaction norms for humans is age at menarche, which has been found to vary by nearly 50 percent across populations, from a low median age of just over 12 years in consistently well-nourished, healthy populations, to up to 18 years in persistently poorly nourished, less healthy ones (Eveleth and Tanner, 1990; Worthman, 1999a). One might argue that the correlation of environmental quality with age at menarche reflects the impact of environmental insult rather than adaptive biological response, but the case of girls adopted at different ages into radically improved circumstances contradicts this interpretation. Girls from disadvantaged south Indian populations adopted at age 3 or later had an earlier age at menarche (11.1 years) than those adopted at 2 years or less (11.8 years); Proos et al., 1991). Furthermore, recent epidemiological studies of the effects of early environment on systems that drive resource allocation (Barker, 1991, 1997; Clark et al., 1996; Fall et al., 1995) have documented that fetal programming of neuro-endocrine regulation alters developmental and health outcomes across the life span (Adair, 2001; Godfrey, 1998; McDade et al., 2001; Susser and Levin, 1999; Williams and Poulton, 1999). Secular trends and population variations in timing of reproductive maturation (indexed by menarche for girls) are related to variations in the timing of the onset of puberty in both sexes. Intensive investigation into these variations has identified maternal well-being, infant and child health, nutrition, and psychological well-being as salient ecological correlates. In teleological terms of life history theory (see Figure 10-2), maternal well-being (health, nutrition, and low stress), low juvenile mortality risk (indexed by morbidity), and sustained good nutrition reduce the resources needed for maintenance, with the net effect of increasing productivity. Greater productivity increases the energy available for growth (in height or weight, fat or muscle) or reproduction and reduces the marginal costs of reproduction. This allows faster growth and accelerated maturation, conducive to earlier puberty. Good conditions from gestation through childhood also signal that the risk or relative cost of reproduction likely will be low (the dashed line in the figure). At this point, limitations on the applicability of life history theory to intraspecific variations become clear, for life history predicts that adult mortality risk will be associated with earlier onset of the reproductive career (Charnov, 1993). Humans show the opposite pattern, with mortality risk associated with later puberty. As child health and survival improve in a population, child maturation accelerates, as evidenced by increased height for age and earlier age at menarche. Indeed, so close is this link that child

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Offspring: Human Fertility Behavior in Biodemographic Perspective FIGURE 10-2 Secular trend in timing of puberty in terms of life history. growth indices are used as a sensitive measure of population nutrition and health (World Health Organization, 1995). Two Mediating Mechanisms Phenotypic variations in reproductive behavior, such as variations in the timing of puberty, must originate somehow from evolved life history design. Two issues are involved, and lead to two distinctive but ultimately related sets of mediating mechanisms. First is design for information use: how is life history design translated into phenotype? A case will be made here for the place of neuroendocrine-endocrine systems in the translation process. Endocrine architecture provides the concrete physiological structure for realizing the intersection of constitutional genetic endowment of the individual, fixed at conception, with environmental inputs, demands, and chance. Second is transmission of life history design across generations. How can all the information that an organism needs to develop, survive, and succeed be communicated across generations? This is an enormous

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Offspring: Human Fertility Behavior in Biodemographic Perspective the dynamic between genes and environment that informs phenotype. More formally, epigenesis comprises nonlinear developmental processes, not reducible to genes or genetic programs, that involve interactions within and among many levels of the organism and its environment (Gottlieb, 1998; Maleszka et al., 1998). Some of the most exciting work in developmental and behavioral biology over the last two decades yields insights into the nature of these interactions, and has transformed scientific views of ontogeny and adaptive design. The problem of intergenerational transfer of information to guide ontogeny was apparently resolved by the (re)discovery of Mendelian genetics in 1900 (cited in International Human Genome Sequencing Consortium, 2001), and the blueprint analogy was reinforced by elucidation of DNA structure and function in the 1960s. But the genome simply cannot convey enough information to specify phenotype in its myriad details; indeed, initial human genome maps reported in 2001 gauge the number of protein-coding genes at 30,000 to 40,000 (International Human Genome Sequencing Consortium, 2001; Venter et al., 2001), merely double those in a fly or worm. The number of genes does not correspond to phenotypic complexity. As a crude example, the mammals mouse, whale, and human have a similar numbers of genes but possess 40, 200,000, and 85,000 million neurons, respectively (Miklos and Maleszka, 2000). Genes provide at least the minimum information to code essential proteins and establish basic body plans. But use of the genome involves a complex metaarchitecture that is demand or context driven, and one might even say that, in ecological terms, what reproduces is the environment (Bonner, 1974). The environment is implicated in epigenesis at many levels, including replication, gene expression, splicing, posttranslational modifications, and context-dependent protein interactions (Miklos and Maleszka, 2001). Much of ontogeny–particularly that of humans, with the high premium on plasticity and longevity–is designed to capture information from the environment to “instruct” development of the organism. Environmental context can convey far more information than genes can about what the organism needs for adequate functioning. Present knowledge of epigenesis rests on the well-studied nervous and immune systems. Epigenesis operates through (1) the proliferation of variants (e.g., neurons and neural connections, or unique cell lines) and production of transient redundancy, (2) the retention of variants that “work” best, and (3) the pruning of those that do not. Which variants work best is determined via processing of functional throughputs provided by contextual inputs or demands (e.g., visual representation or pathogen recognition; Changeux, 1985; Edelman, 1987; McDade and Worthman, 1999). The strategem of developmental Darwinism constitutes a potent means for production of phenotypic complexity honed to the circumstances in which the

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Offspring: Human Fertility Behavior in Biodemographic Perspective organism must operate, and it does so by creating ontogenetic expectancies for functional load and contextual inputs. Expectable Environments of Rearing The elegant epigenetic devices for environmental programming depend on the information provided by contextual inputs and therefore can evolve only when those inputs are highly reliable. These form the expectable environments of rearing (EER), the set of environments normally encountered during ontogeny. The EER encapsulates the fitness outcomes and selective pressures to which organic design is adapted. An overview of the human EER (see Table 10-1) integrates findings from developmental biology, developmental psychobiology, anthropology, and epidemiology to translate the epigenetic landscape into everyday human ecology. The listed features are nearly or entirely universal in human groups and have all been associated with variations in phenotypic outcomes, though of varying magnitudes. The impact of a component of the EER on phenotype depends on several factors: (1) its predictiveness of future conditions, (2) the significance of those future conditions for fitness, (3) the timeliness of the signal for mobilization of an adaptive response, (4) the feasibility of transducing the signal into ontogenetic responses, (5) the relative fitness value versus the cost of adjusting the phenotype to one cue versus another, (6) pleiotropic effects of phenotypic adjustment on other features with adaptive value, and (7) the time horizon for realizing costs versus benefits. This set of factors determines trade-offs around maintenance of plasticity versus reliance on context to inform ontogeny. Humans are thought to maintain an unusually high degree of plasticity in domains pertaining to information processing and behavior. Ecological circumstances, group composition, and social dynamics can range widely over the life course of an individual, so many early conditions will generalize poorly to later circumstances. Reproduction is separated from early development by over a decade or more and continues for several decades. Thus, although childhood environment strongly affects biological dimensions of life history (maturation and senescence rates), its effects on reproductive behavior (on partner relationships and infant and child care) are far more subtle than immediate ecological and cumulative sociocultural effects (Chisholm, 1993, 1996; Hill and Hurtado, 1995). IMPLICATIONS FOR RESEARCH AND POLICY The view of human reproduction as rooted in biocultural inheritance casts new light on current human affairs. It implies that when the fabric of

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Offspring: Human Fertility Behavior in Biodemographic Perspective TABLE 10-1 Humans’ Expectable Environments of Rearing Social ecology Child care   • Gestation: maternal stress, nutrition, activity   • Prolonged carrying (devices may be used for holding, carrying)   • Infant/child signaling-caregiver response (contingency, state regulation)   • Cosleeping   • Breastfeeding (and weaning)   • Variable caregiver competence   • Provisioning into adolescence with transition to productive in(ter)dependence Family   • Parents   • Coresident dependent siblings   • Privileged emotion communication and intersubjective regulation   • Resident in family into adolescence Social group   • Multiage, mixed-sex with changeable composition (mobility, mortality)   • Presence of kin and nonkin Pervasive language use, multiple registers (e.g., information exchange, narrative) Collaboration Sharing and exchange (socially and spatiotemporally displaced reciprocities) Contexts for play, practice, and exploration (risk taking)   • Multiage, mixed-sex play groups   • Tolerance of low productivity, incompetence   • Surveillance, safe spaces Participant observation in adult activities and competent performances   • Feedback on imitation, provision of instruction, guided participation Use of analogs and symbols (visual, acoustic) Tool use Fire and thermal buffering (possibly with clothing, coverings) Sanctions (physical, verbal, social) Bioecology Food constituents and gut activity Energetics (resource reliability and quality, maintenance costs) and metabolic regulation Exposures to pathogens, parasites, and dirt and immune development Sensory inputs, activity patterns, and brain development Perceived safety or security and vigilance (attention and arousal regulation) human social life and culture is repatterned or rent, fertility outcomes (in terms of effective fertility) are changed. We can expect a large measure of robustness in biocultural transmission for critical features such as language and for those under previous selection for retention of plasticity. Other areas of human developmental ecology may be more vulnerable, particularly (1) emotion and arousal regulation, particularly in affiliation, anxiety, violence and aggression, and toler-

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Offspring: Human Fertility Behavior in Biodemographic Perspective ance of novelty and threat; (2) modes of learning; (3) capacities for behavior change; (4) risk for psychopathology, particularly depression, suicide, and substance use; and (5) health and mortality risk associated with metabolic and eating dysregulation (diabetes, obesity), bioreproductive morbidities (polycystic ovarian disease, breast cancer), and hyperarousal and vigilance. Demographic and epidemiologic data worldwide suggest that human development, and hence human reproduction, currently face enormous pressures (not all unfavorable). These are well documented in recent surveys of world mental health (Desjarlais et al., 1995), the global burden of disease (Murray and Lopez, 1996), inequity and health (Chen and Berlinguer 2001; Dasgupta, 1993; Deaton, 2001; Gwatkin, 2000; Wilkinson, 1996), and globalization in virtually all domains (World Bank, 2001; Dollar and Collier, 2001). These pressures require attention because some may strain even the large evolved human capacities for plasticity, accommodation, and successful adaptation (Nesse and Williams, 1994; Trevathan et al., 1999). Such pressures may need to be met with thoughtful planning and action. The issues are biocultural. To get a sense of this, consider the well-documented worldwide shift in growth and maturation rates (Eveleth and Tanner, 1990; Worthman, 1999a) and concurrent dramatic transformations in the ecology of child rearing: changing family composition and maternal employment; widespread schooling of children; urbanization and increased densities; demographic effects of emerging diseases, specifically AIDS; and escalating rates of population dislocation. These formidable challenges raise the bar for effective theory that will support new hypotheses about human behavior, stimulate research, and inform policy. Pushing past dual thinking about the human condition and operationalizing biocultural inheritance models may provide the start to understanding the human implications of these challenges. ACKNOWLEDGMENTS Thanks are owed to Kristen Hawkes, Daniel Lende, and Ryan Brown for close reading, critique, and significant additions to this work, and to Randy Bulatao for heroic editorial input. REFERENCES Adair, L.S. 2001 Size at birth predicts age at menarche. Pediatrics 107:E59. Anderson, L.A., P.G. McTernan, A.H. Barnett, and S. Kumar 2001 The effects of androgens and estrogens on preadipocyte proliferation in human adipose tissue: Influence of gender and site. Journal of Clinical Endocrinology and Metabolism 86:4951-5956.

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