Offspring Human Fertility Behavior in Biodemographic Perspective (2003) / Chapter Skim
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7. An Evolutionary and Ecological Analysis of Human Fertility, Mating Patterns, and Parental Investment
7. An Evolutionary and Ecological Analysis of Human Fertility, Mating Patterns, and Parental Investment
Pages 170-223
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From page 170... ...
The first section of this chapter presents an introduction to life history theory and current thinking in evolutionary biology with respect to the three themes. Since the fitness consequences of alternative fertility and parental investment regimes depend on ecology and individual condition, both specialization and flexibility in life histories are considered.
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The chapter concludes with a discussion of the new research questions and approaches to research design suggested by this framework. THE THEORETICAL FRAMEWORK Fundamental Trade-Offs in Life History Theory Natural selection acts on variability in the traits of individual organisms within populations.
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recognized that these differences in relative parental investment affect the structure of mating markets and the characteristics of the more and less investing sexes. The more investing sex is selected to be choosy about when
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. Ecology and Life History Evolution Variations across taxa and across conditions in optimal energy allocations and optimal life histories are shaped by ecological factors, such as food supply, mortality hazards, and the effects of body size on both energy capture and mortality hazards (Charnov, 1993; Koziowski and Weigert, 1987; Werner, 1986~.
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On the other hand, the commitment to a large brain and the long period of development and exposure to environmental information necessary to make it fully functional place important constraints on the flexibility of the human life course and require specializations for a slow life history. In fact, consideration of brain- and learning-intensive human adaptation reveals shortcomings in existing biological theory and inspires the development of a more general approach to life history evolution, which is the focus next.
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Thus, the present-future reproductive trade-off can be understood in terms of optimal investments in own embodied capital versus reproduction, and the quantity-quality and mating-parenting trade-offs can be understood in terms of investments in the embodied capital of offspring versus their number. The central thesis of this chapter is that there are four major factors affecting the timing of reproduction in the life course, reproductive rates, and parental investment for each sex: ( 1 )
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also affects the likelihood of dying from environmental assaults. Models of embodied capital also show that ecological features or investments that increase the probability of survival to older ages also produce selection for greater investments in income-related embodied capi
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tel. These coevolutionary effects appear to have been particularly important in human life history evolution.
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Males, in turn, as their investments in offspring increase, are expected to exert choice with respect to variation in female quality and to compete with other males for access to the resources utilized in reproduction. Ecological variability affecting the variance among males and females in resource access or access to mates is expected to exert a significant influence on mating market dynamics and in male and female investments in parenting and mating effort.
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Compared to other primates, there are at least four distinctive characteristics of human life histories: (1) an exceptionally long life span, (2)
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These buffers against mortality also favor a longer juvenile period and higher investment in other mechanisms to increase life span. Thus, we propose that the long human life span, lengthening of the juvenile period, increased brain capacities for information processing and storage, intergenerational resource flows, and cooperative biparental investment in offspring coevolved in response to this dietary shift and the new production processes it entailed.
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ity profiles of the foraging populations suggest that this pattern is an evolved life history characteristic of our species. Figure 7-3 illustrates the differences between human foragers and wildliving chimpanzees.
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Role of Men in Human Reproduction Unlike most other mammals, men in foraging societies provide the majority of the energy necessary to support reproduction. After subtracting their own consumption from total production, women supply an average of 3 percent of the calories to offspring, with men providing the remaining 97 percent among the 10 foraging societies for which quantitative data on adult food production are available (Kaplan et al., 2001a)
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In contrast, nonhuman primate females do not store appreciable fat, and they increase work effort during lactation; as a result, they have increased risk of mortality (Lancaster et al., 2000~. The human specialization could not have evolved if women did not depend on men for most of their food provisioning throughout human history.
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have organized research into those design features. There is mounting evidence that human reproductive physiology is particularly specialized toward the production of high-quality, large-brained offspring.
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Another fundamental trade-off is between production and mortality risks, since foraging exposes people to predation, accidents, and getting lost. This suggests that parental psychology should be responsive to age-specific mortality rates and how they are affected by alternative activity profiles and on their short- and long-term consequences for production/productivity.
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found that the expected reproductive success of older women would have to be implausibly low to favor menopause. However, as Hill and Hurtado (1991, 1996)
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and women can be more economically self-sufficient. The next section treats major historical trends in reproduction, parenting, and mating practices associated with the domestication of plants and animals.
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However, since neither males nor females produce beyond subsistence needs and the means of production are held in common through usuiruct, there is little opportunity for major variance in resource holding. However, variance in male reproductive success does arise on the basis of their success in raiding, which brings certain males both high status and more wives (Chagnon, 1979~.
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Greater variance among men was possible on the basis of raiding and bride capture, but the social system itself is not stratified and individual men cannot amass or control access to resources relative to other men or pass them on to their sons. Extrasomatic Wealth and Tribal Pastoralism The Socioecological Context The domestication of animals, particularly large herd animals such as cattle, camels, and horses, proved to have a profound effect on human social and reproductive patterns.
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Generally, however, pastoralists show a wide range in fertility levels due to the many different kinds of pastoralism with respect to agricultural supplementation, disease risk, mobility, and so forth (Borgerhoff Mulder, 1992; Sellen and Mace, 1997~. Large-animal herding demands a high degree of complementarily between female processing and child care and male risk taking in herd management and defense.
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A male's ability to successfully control more resources translates directly into more wives and children (Borgerhoff Mulder, 1989, 1991, 1995~. One extraordinary result of extrasomatic wealth, particularly readily partible wealth, is the institution of a new pawn on the marriage market table bride wealth.
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The flow of stock through families that are both bride wealth receivers and givers helps maintain the system at the same time that it creates problems for families with unfavorable ratios of sons to daughters (Borgerhoff Mulder, 1998~. Finally and most significantly, there is suggestive evidence that for the first time humans began to reproduce at levels that may not maximize the number of descendants with the appearance of extrasomatic wealth and its inheritance.
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Variance in male resource holding was probably the greatest it has ever been in human history (Betzig, 1986, 1992a, 1992b, 1993~. The reason for this is that despotic males had enormous political and social control with the ability to eliminate rivals and their entire families through despotic edicts, to wage war to increase personal and state resource bases, to acquire slaves and war captives for labor and reproduction, and to determine political succession for favored sons.
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It also generated a great deal of concern about keeping the family estate intact and about the management of inheritance. Mating Patterns, Parental Investment, and Reproduction Despotic males are an extreme example of resource defense polygyny (Orians, 1969~; that is, as individuals they control access to the resource base for reproduction that females require and, with few competitors, polygynous marriages to them become the only family formation strategy option for many women.
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A second outcome of variance in male resource holding and male mating success is that there tends to be universal marriage for women, with only those most severely compromised by health or other personal qualities being unlikely to find a role as secondary wife or minor union. For access to the mating market, men must bring wealth, power, and land in order to be favorably placed or else get wives as high-risk booty in state warfare (Clarke and Low, 2001; Low, 2000~.
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Human evolutionary ecologists in collaboration with historical demographers have provided a unique record of the relationships between fertility, family formation strategies, and the socioecological context during the premodern and early modern periods of European history (see review in Voland,2000~. Their studies, based on heraldic or parish records of births, marriages, deaths, and inheritance of estates, can be used to directly link reproductive strategies with resource holdings.
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The desire to concentrate wealth also limits the reproductive success of noninheriting sons and daughters. This may be another example in which reproductive and parental investment behavior in response to extrasomatic wealth results in outcomes that do not maximize parental fitness.
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Throughout human history there has always existed a conflict between production and reproduction for women, a conflict that in fact troubles female mammals in general. Human women are especially caught in this conflict because they have multiple dependent young of differing ages and needs (Draper, 1992)
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The length of the productive adult life span, especially when time lost to morbidity is taken into account, has also increased significantly. Together, the two shifts in production processes and mortality rates favor increased human capital investment in a way that is reminiscent of the initial dietary shift leading to the hominid specialization discussed above.
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It also means that the net increase in embodied capital at each age is a function of both the quality of inputs and the capital acquired at younger ages. Moreover, those qualities tend to be correlated across inputs.
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, Reproduction and Parental Investment There is considerable evidence that in response to these conditions people in modern societies do not maximize fitness through their fertility decisions (see Kaplan and Lancaster, 2000, for a review)
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Public- and private-sector responses to these trends then begin to change the payoffs to such investments among the poorer and less educated sectors of society. Increased public investments in health and schooling decrease mortality rates and increase the efficiency of private investments in children's schooling for all sectors of society.
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This change in the costs of divorce inspired the reform of divorce laws and, ultimately, increases in the divorce rate. On the other hand, the greater importance of education in wage determination increases educational homogamy in the mating market and parental assessments about the level of investment children require.
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It has been pointed out that men who are less educated have higher mortality rates and greater rates of incarceration (Geronimus et al., 1999; Willis, 1987, 1994~. This tends to make sex ratios female biased and promotes single parenthood.
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In the United States, many women, especially ethnic minorities, who face a mating market where male unemployment is great, choose to reproduce at younger ages while they are still living with their mothers, so that they can receive maternal or grand-maternal assistance in child rearing (Burton, 1990; Geronimus, 1996~. The change in the family structure associated with teen childbearing over the second half of the 20th century was dramatic.
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Endemic warfare and raiding for women and livestock are the focus of the male life course, leading to expanded variance among men in resource holding and fertility based on the number of wives they can pay for or capture. At the same time that males must pay higher costs in bride wealth and in risk to access women, their families begin to help them by contributing a balloon payment at maturity to position them favorably on the mating market.
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The first is an even stronger correlation between extrasomatic wealth and reproductive success than already found among pastoralists and brings into focus female-female competition in the marriage market. Since extrasomatic wealth has the potential of being controlled by powerful resource holders, social stratification and the ensuing variance among men restructure the mating market.
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The same can be said for investment in offspring's embodied capital, which appears to include not only investments in schooling but also in goods associated with social training and social status, such as hobbies and sports, clothing, and toys. Much of these investments are commitments to favorable placements of children in the mating market.
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Clearly humans exhibit a great deal of flexibility in their family arrangements; the relationships between allocation decisions by sex, mating markets, and socioeconomic conditions are a new frontier for research. These considerations suggest a "two-pronged" research program.
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The framework introduced here is designed to provide guidance for such research. It will be critical to analyze and measure how resources are acquired, the impacts of investments on resource acquisition, mortality rates and the impacts of investments on mortality, complementarily of male and female inputs, and variation among and within individuals over time in access to resources.
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Is very low fertility in Eastern and Southern Europe due to the same costs and benefits, or are there different factors leading to similar outcomes? For the most part, world population projections are based on statistical trends from the past and guesses about convergence on stable total fertility rates (Lutz et al., 2001~.
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Economists are primarily concerned with conscious, rational decision-making processes, but such processes are only a subset of the regulatory mechanisms of controlling fertility. In addition, there is no generally accepted causal theory of human utility in economics: natural selection is a causal process that constrains the types of utility functions that may evolve.
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We also wish to thank Monique Borgerhoff Mulder for her careful and extensive suggestions for revision of a draft of this paper. REFERENCES Anderson, K., H
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2002 Learning, life history, and productivity: Children's lives in the Okavango Delta, Botswana. Human Nature 13:161-198.
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1995 Bride wealth and its correlates: Quantifying changes over time. Current Anthropology 36:573-603.
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1981 Paternal confidence and dowry competition: A biocultural analysis of ourdah. In Natural Selection and Social Behavior.
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1996 What teen mothers know. Human Nature 7:323-352.
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Human Nature 2:315-350. 1996 Ache Life History: The Ecology and Demography of a Foraging People.
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A test of an optimality model and a new theory of parental investment in the embodied capital of offspring. Human Nature 6:325-360.
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Human Nature 5:223-253. 2000 Why Sex Matters: A Darwinian Look at Human Behavior.
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Promislow, D.E.L., and P.H. Harvey 1990 Living fast and dying young: A comparative analysis of life history variation among mammals.
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Ogy and Sociobiology 46:82-94. 2001 Linking dispersal and resources in humans: Life history data from Oakham, Massachusetts (1750-1850)
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. Human Nature 6:33-49.
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