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)

When wealth is heritable, parents may manipulate family size to optimize the trade-off between more relatively poor offspring and fewer relatively rich ones, and channel less care into offspring that compete with siblings. These hypotheses were tested with quantitative ethnographic data collected among the Karo Batak—patrilineal agriculturalists from North Sumatra, Indonesia, among whom land is bequeathed equally to sons. It was predicted that landholding would moderate the relationship between reproductive rate and parental investment on one hand, and the number of (...) same-sex siblings on the other, among boys but not girls. The predicted interaction effect was observed in interbirth intervals and immunizations, but only a trace of the effect was detected in age-five mortality. The study raises questions about the coevolution of human behavior and social structure. (shrink)

What explains variation in hunting success? This paper examines foraging success among Inuit hunters, paying particular attention to factors that account for differential returns in hunting. Although there are several possibilities for explaining hunting success, this study finds that birth order and age are important predictors of foraging returns. Furthermore, data on food sharing suggests that birth order has important effects on the distribution of food. That is, early-born hunters not only produce more food, they give much of that food (...) to their parents, who then distribute it to the hunter’s younger siblings. These findings are discussed within the context of local resource enhancement and the value of early-born sons to Inuit parents. (shrink)

We present a quantitative model of sex allocation to investigate whether the simple “rules of thumb” suggested by Trivers and Willard (1973) would really maximize numbers of grandchildren in human populations. Using demographic data from the !Kung of southern Africa and the basic assumptions of the Trivers-Willard hypothesis, we calculate expected numbers of grandchildren based on age- and sex-specific reproductive value. Patterns of parental investment that would maximize numbers of expected grandchildren often differ from the Trivers-Willard rules. In particular, the (...) optimum parental behavior is sensitive to population dynamics, type of parental investment, and, most important, relative ages of sons and daughters. It is doubtful whether a parent blindly following the simple Trivers-Willard rules would maximize numbers of expected grandchildren, on average. In addition, we show that sex-specific infanticide will almost never achieve the goal of maximizing expected numbers of grandchildren. (shrink)