Data from the Kipsigis of Kenya are used to test two models for how parents invest in offspring, the Trivers-Willard and local resource competition/enhancement hypotheses. Investment is measured as age-specific survival, educational success, marital arrangements, and some components of property inheritance, permitting an evaluation of how biases persist or alter over the period of dependence. Changes through time in such biases are also examined. Despite stronger effects of wealth on the reproductive success of men than women, the survival of sons (...) and daughters is not related to parental wealth. However, a Trivers-Willard effect characterizes educational investment: poor families show a greater concern for daughters’ (vis-à-vis sons’) schooling than do rich families, a trend that has increased over time. In regard to models of local resource competition and enhancement, men’s reproductive success decreases with number of brothers and increases with number of sisters; this pattern of competition with same-sex sibs and cooperation with opposite-sex sibs is not found among women. As predicted from these observations, parents show reduced investment in sons with a large number of brothers, and increased investment in sons with a large number of sisters. By contrast, investment in daughters is entirely unaffected by number of sisters and is influenced only in subtle ways by number of brothers. Levels of investment in relation to sibship size (irrespective of siblings’ sex) are highest for younger children of large sib sets.Discussion of the results in relation to those from other studies leads to three conclusions. First, predictive models for how investment biases vary across societies must consider a broad range of socioecological factors constraining parental options and payoffs. Second, the timing of investment biases within societies will be affected by the value of children and the costs of parental investment. Third, measures of investment appropriate for between-sex and between-class comparisons need careful attention. Each of these issues is brought to bear on the question of why, in contrast to so many other parts of the world, sex preferences are so muted in Africa. (shrink)

Work was conducted among traditional, subsistence whale hunters in Lamalera, Indonesia, in order to test if strict biological kinship or lineage membership is more important for explaining the organization of cooperative hunting parties ranging in size from 8 to 14 men. Crew identifications were collected for all 853 hunts that occurred between May 3 and August 5, 1999. Lineage identity and genetic relatedness were determined for a sample of 189 hunters. Results of matrix regression show that genetic kinship explains little (...) of the hunters’ affiliations independent of lineage identity. Crew members are much more closely related to each other than expected by chance, but this is due to the correlation between lineage membership and genetic kinship. Lineage members are much more likely to affiliate in crews, but kin with r<0.5 are just as likely not to affiliate. The results are discussed vis-à-vis the evolution of cooperation and group identity. (shrink)

Household structure may have strong effects on reproduction. This study uses household demographic data for 59 women in a Caribbean village to test evolutionary hypotheses concerning variation in reproductive strategies. Father-absence during childhood, current household composition, and household economic status are predicted to influence age at first birth, number of mates, reproductive success, and pair-bond stability. Criterion variables did not associate in a manner indicative of r- and K-strategies. Father-absence in early childhood had little influence on subsequent reproduction. Household wealth (...) and alloparenting were positively associated with age at first birth. Alloparenting was negatively associated with reproductive success. Women in long-term conjugal unions had higher reproductive success than did single women. Number of adult male kin resident in the household was negatively associated with women’s number of mates. (shrink)

This study examines the family environments and hormone profiles of 316 individuals aged 2 months-58 years residing in a rural village on the east coast of Dominica, a former British colony in the West Indies. Fieldwork was conducted over an eight-year period (1988–1995). Research methods and techniques include radioimmunoassay of cortisol and testosterone from saliva samples (N=22,340), residence histories, behavioral observations of family interactions, extensive ethnographic interview and participant observation, psychological questionnaires, and medical examinations.Analyses of data indicate complex, sex-specific effects (...) of family environment on endocrine function. Male endocrine profiles exhibit greater sensitivity to presence of father than do female endocrine profiles. Father-absent males tend to have (a) low cortisol levels during infancy, (b) high or abnormal cortisol profiles during childhood and adolescence, and (c) high cortisol and low testosterone levels during adulthood compared with those of males raised with a resident father. These results indicate that early family environment has significant effects on endocrine response throughout male life histories. (shrink)

A historical survey of the inheritance practices of farming families in North America and elsewhere indicates that resource allocations among children differed through time and space with regard to sex bias and equality. Tensions between provisioning all children and maintaining a productive economic entity (the farm) were resolved in various ways, depending on population pressures, the family’s relative resource level, and the number and sex of children.Against a backdrop of generalized son preference, parents responded to ecological circumstances by investing in (...) offspring differentially within and between the sexes. Vesting the preponderance of family resources in one heir increased the likelihood of at least one line surviving across several generations, whereas varying degrees of parental investment in emigrating sons or out-marrying daughters might yield boom or bust harvests of grandchildren according to circumstances in more remote locales. Primogeniture (eldest son as primary heir) allowed early identification of heirs and appropriate socialization, as well as more time for parents to contribute to the heir’s reproductive success. Son bias and unigeniture decreased as numbers of children per family declined, as land became less critical to economic success, and as legal changes improved the resource-holding potential of females. We suggest that changing ecological conditions affected parental decisions regarding resource allocation among children at least as much as did changing ideologies of parent-child relations. (shrink)

This article introduces a new model of the relationship between growth and learning and tests a set of hypotheses related to the development of adult competency using time allocation, anthropometric, and experimental task performance data collected between 1992 and 1997 in a multiethnic community in the Okavango Delta, Botswana. Building on seminal work in life history theory by Hawkes, Blurton Jones and associates, and Kaplan and associates, the punctuated development model presented here incorporates the effects of both growth and learning (...) constraints on age-specific task performance. In addition, the payoff to investment in two forms of embodied capital, growth-based and learning-based, are examined in relation to features of the socioecology, including subsistence economy and family composition.The three main findings are:The development of adult competency in specific tasks entails a steplike relationship between growth- and experience-based forms of embodied capital in the ontogeny of ability acquisition.There is a trade-off between the acquisition of experience-based embodied capital in the form of skills and knowledge and immediate productivity among children. Time allocation to these alternatives is primarily determined by the short- and long-term costs and benefits to parents of investment in children’s embodied capital.The availability of laborers and the overall labor requirements of the household are major determinants of investment in alternate forms of embodied capital and resulting variation in children’s time allocation. The value of children’s labor to their parents is dependent upon the opportunity costs to engaging in other activities not only for the child in question but also for potential substitute laborers.These results have important implications for our understanding of the role of growth and learning in the evolution of the human juvenile period, as well as for our understanding of cross-cultural variation in child growth and development and patterns of work and play. (shrink)