Parental investment and the optimization of human family size (original) (raw)
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An evolutionary perspective on human fertility
Population and Environment, 1999
This paper illustrates evolutionary approaches to population issues. Life history theory is a general theoretical framework that incorporates environmental influences, contextual influences, and heritable variation. In general, physically or psychologically stressful environments delay maturation and the onset of reproductive competence. Perceptions of scarcity also result in lower fertility by delaying reproduction or having fewer children-a phenomenon viewed as an adaptation to ancestral environments. The desire for upward social mobility is viewed as an evolved motive disposition affecting fertility decisions. The opportunity for upward social mobility typically results in delaying reproduction and lowering fertility in the interest of increasing investment in children. Variation in life history strategies is also influenced by genetic variation, but genetic variation interacts with cultural shifts in the social control of sexual behavior. Finally, I discuss the effects of between-group competition for resources on population issues. Immigration policy and group differences in fertility influence political power within and between societies, often with explosive results. Demographic expansion has often been an instrument of ethnic competition and is an important source of conflict in the contemporary world.
Life-history theory, fertility and reproductive success in humans
Proceedings of the Royal Society B: Biological Sciences, 2002
According to life-history theory, any organism that maximizes fitness will face a trade-off between female fertility and offspring survivorship. This trade-off has been demonstrated in a variety of species, but explicit tests in humans have found a positive linear relationship between fitness and fertility. The failure to demonstrate a maximum beyond which additional births cease to enhance fitness is potentially at odds with the view that human fertility behaviour is currently adaptive. Here we report, to our knowledge, the first clear evidence for the predicted nonlinear relationship between female fertility and reproductive success in a human population, the Dogon of Mali, West Africa. The predicted maximum reproductive success of 4.1 ± 0.3 surviving offspring was attained at a fertility of 10.5 births. Eighty-three per cent of the women achieved a lifetime fertility level (7-13 births) for which the predicted mean reproductive success was within the confidence limits (3.4 to 4.8) for reproductive success at the optimal fertility level. Child mortality, rather than fertility, was the primary determinant of fitness. Since the Dogon people are farmers, our results do not support the assumptions that: (i) contemporary foragers behave more adaptively than agriculturalists, and (ii) that adaptive fertility behaviour ceased with the Neolithic revolution some 9000 years ago. We also present a new method that avoids common biases in measures of reproductive success.
Developmental influences on fertility decisions by women: an evolutionary perspective.
Developmental environments are crucial for shaping our life course. Elements of the early social and biological environments have been consistently associated with reproduction in humans. To date, a strong focus has been on the relationship between early stress, earlier menarche and first child birth in women. These associations, found predominately in high-income countries, have been usefully interpreted within life-history theory frameworks. Fertility, on the other hand—a missing link between an individual’s early environment, reproductive strategy and fitness—has received little attention. Here, we synthesize this literature byexamining the associations between early adversity, age at menarche and fertility and fecundity in women. We examine the evidence that potential mechanisms such as birth weight, childhood body composition, risky health behaviours and developmental influences on attractiveness link the early environment and fecundity and fertility. The evidence that menarche is associated with fertility and fecundity is good. Currently, owing to the small number of correlational studies and mixed methodologies, the evidence that early adversity predicts fecundity and fertility is not conclusive. This area of research is in its infancy; studies examining early adversity and adult fertility decisions that can also examine likely biological, social and psychological pathways present opportunities for future fertility research.
Introduction: Evolutionary theory and the search for a unified theory of fertility
American Journal of Human Biology, 2002
Demography and evolutionary biology share common origins but have divergent emphasis on the role of theory in understanding population phenomena. A unified theory of fertility would be beneficial in explaining variation in demographic characteristics across geographic and temporal gulfs and in integrating disparate perspectives. The six papers in this thematic collection represent a nascent but vital field: human evolutionary demography. These papers examine the ways in which evolutionary theory can inform, strengthen, and focus research on topics of long-standing interest to demographers by explicitly modeling the relationship of socioecological variables to life history traits. The papers demonstrate that an understanding of human life history evolution and the use of evolutionary theory as an organizing framework can lead to a productive reassessment of five areas, which are of long-standing concern to demographers, and which conventional demographic approaches have had limited success in understanding. These are conflicts of interest between parents and children and between men and women, the allocation of resources to competing and/or alternative forms of investment in reproduction and parenting, resource flow within the household, demographic transitions and particularly the fertility transition associated with economic development, and variation in life history characteristics such as fertility and mortality across populations. Future research integrating models of trait-environment correlation with models of individual information processing and decision-making will help identify areas of focus, revitalize current models, and play a leading role in the development of a unified theory of fertility applicable across societies and times. Am.
Current Directions in Psychological Science, 2001
To search for genetic influence on human fertility differentials appears inconsistent with past empirical research and prior interpretations of Fisher's fundamental theorem of natural selection. We discuss Fisher's theorem and give reasons why genetic influences may indeed account for individual differences in human fertility. We review recent empirical studies showing genetic influence on variance in fertility outcomes and precursors to fertility. Further, some of the genetic variance underlying fertility outcomes overlaps with that underlying fertility precursors. Findings from different cultures, different times, different levels of data, and both behavioral and molecular genetic designs lead to the same conclusion: Fertility differentials are genetically influenced, and at least part of the influence derives from behavioral precursors that are under volitional control, which are themselves genetically mediated.