Competition for resources is one of the main evolutionary explanations for dispersal from the natal area. For humans this explanation has received little attention, despite the key role dispersal is thought to play in shaping social systems. I examine the link between dispersal and resources using historical data on people from the small farming town of Oakham, Massachusetts (1750–1850). I reconstruct individual life histories through a variety of records, identifying dispersers, their age at dispersal, and their destinations. I find that (...) sex, father’s wealth and social status, and age at father’s death were all significantly associated with one or more dispersal variables. Birth order and number of siblings were not significantly associated with any of the dispersal variables. I also use wills and deeds to study transfers of land from fathers to sons. More stayers than dispersers acquired Oakham land from their fathers, but some sons who acquired Oakham land later dispersed. I discuss the causality underlying the relationship between dispersal and resource acquisition, as well as implications for a general understanding of human dispersal. (shrink)

Patterns of human kinship commonly involve preferential treatment of relatives based on lineal descent (lineages) rather than degree of genetic relatedness (kindreds), presenting a challenge for inclusive fitness theory. Here, we examine effects of lineage and kindred characteristics on reproductive success (RS) and number of grandchildren for 130 men and 124 women in a horticultural community on Dominica. Kindreds had little effect on fitness independently of lineage characteristics. Fitness increased with the number of lineal relatives residing in the community but (...) decreased beyond an apparently optimal lineage size, suggesting resource enhancement and competition among kin. Female-biased patrilineage sex ratio was positively associated with men’s fitness, while male-biased matrilineage sex ratio was positively associated with women’s fitness. Number of brothers in the community was negatively associated with men’s, but not women’s, fitness. Parents and number of sisters had no effect on either male or female reproduction; however, women with younger sisters had higher RS, suggesting benefits of kin support for childcare. In sum, imposed norms for lineage social organization may enhance lineal ancestors’ inclusive fitness at a cost to individual inclusive fitness. (shrink)

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)

The wealthy elite males of nineteenth-century Krummhörn (Ostfriesland, Germany) achieved an above-average reproductive success. Membership in the elite class was determined from a list of the 300 richest men in the Ostfriesland district compiled by authorities in 1812. The main components establishing the link between cultural success and reproductive success aredifferences in the number of offspring owing to differences both in time spent in fecund marriage (mating success) and in rate of reproduction;differences in the probabilities of one’s adult offspring marrying (...) locally vs. emigrating unmarried owing to differential ability to allocate resources that enhance the “social placement” of adult offspring; anddifferences in the probability of total reproductive failure (lineage extinction).Contrary to what might be expected, infant survivorship was lowest amongst the richest families. We conclude that to a great extent females’ reproductive decisions contribute to the greater reproductive success of the elite males. (shrink)

Sex differences in behavior are most interesting when they are the result of inherent differences in the operational rules motivating behavior and not merely a reflection of differing life history experiences. American men and women exhibit a few differences in testamentary patterns of property allocation that appear to be due to inherently different rules of allocation. Even when analyses control for resources and surviving kin configurations, women distribute their property among a greater number of individual beneficiaries than do men. The (...) most striking differences in property allocation between men and women occur within the nuclear family and reflect differences in reproductive life span and the resulting reproductive conflicts between spouses that can endure beyond death. (shrink)

Sociobiologists and feminists agree that men in patriarchal social systems seek to control females, but sociobiologists go further, using Darwin’s theory of sexual selection and Trivers’s ideas on parental investment to explain why males should attempt to control female sexuality. From this perspective, the stage for the development under some conditions of patriarchal social systems was set over the course of primate evolution. Sexual selection encompasses both competition between males and female choice. But in applying this theory to our “lower (...) origins” (prehominid ancestors), Darwin assumed that choices were made by essentially “coy” females. I argue here that female solicitation of multiple males (either simultaneously or sequentially, depending on the breeding system) characterized prehominid females; this prehominid legacy of cyclical sexual assertiveness, itself possibly a female counter-strategy to male efforts to control the timing of female reproduction, generated further male counter-strategies. This dialectic had important implications for emerging hominid mating systems, human evolution, and the development of patriarchal arrangements in some human societies. For hominid males who will invest in offspring, there would be powerful selection for emotions, behaviors, and customs that ensure them certainty of paternity. The sexual modesty that so struck Darwin can be explained as a recent evolved or learned (perhaps both) adaptation in women to avoid penalties imposed by patrilines on daughters and mates who failed to conform to the patriline’s prevailing norms for their sex. Other supposedly innate universals, such as female preferences for wealthy husbands, are also likely to be facultative accommodations by women to constraints set up when patrilines monopolized resources needed by females to survive and reproduce, and passed on intergenerational control of these resources preferentially to sons. (shrink)

Birth order has been examined over a wide variety of dimensions in the context of modern populations. A consistent message has been that it is better to be born first. The analysis of birth order in this paper is different in several ways from other investigations into birth order effects. First, we examine the effect of birth order in an egalitarian, small-scale, kin-based society, which has not been done before. Second, we use a different outcome measure, fertility, rather than outcome (...) measures of social, psychological, or economic success. We find, third, that being born late in an egalitarian, technologically simple society rather than being born early has a positive outcome on fertility, and fourth, that number of older siblings and sibling set size are even stronger predictors of fertility, especially for males. (shrink)

What is morality? How many moral values are there? And what are they? According to the theory of morality-as-cooperation, morality is a collection of biological and cultural solutions to the problems of cooperation recurrent in human social life. This theory predicts that there will be as many different types of morality as there are different types of cooperation. Previous research, drawing on evolutionary game theory, has identified at least seven different types of cooperation, and used them to explain seven different (...) types of morality: family values, group loyalty, reciprocity, heroism, deference, fairness and property rights. Here we explore the conjecture that these simple moral ‘elements’ combine to form a much larger number of more complex moral ‘molecules’, and that as such morality is a combinatorial system. For each combination of two elements, we hypothesise a candidate moral molecule, and successfully locate an example of it in the professional and popular literature. These molecules include: fraternity, blood revenge, family pride, filial piety, gavelkind, primogeniture, friendship, patriotism, tribute, diplomacy, common ownership, honour, confession, turn taking, restitution, modesty, mercy, munificence, arbitration, mendicancy, and queuing. These findings indicate that morality – like many other physical, biological, psychological and cultural systems – is indeed a combinatorial system. Thus morality-as-cooperation provides a principled and powerful theory, that explains why there are many moral values, and successfully predicts what they will be; and it generates a systematic framework that has the potential to explain all moral ideas, possible and actual. Pursuing the many implications of this theory will help to place the study of morality on a more secure scientific footing. (shrink)

Social support networks play a key role in human livelihood security, especially in vulnerable communities. Here we explore how evolutionary ideas of kin selection and intrahousehold resource competition can explain individual variation in daily support network size and composition in a south-central Ethiopian agricultural community. We consider both domestic and agricultural help across two generations with different wealth-transfer norms that yield different contexts for sibling competition. For farmers who inherited land rights from family, firstborns were more likely to report daily (...) support from parents and to have larger nonparental kin networks. Compared with other farmers, firstborns were also more likely to reciprocate their parents’ support, and to help nonparental kin without reciprocity. For farmers who received land rights from the government, middle-born farmers reported more nonparental kin in their support networks compared with other farmers; nonreciprocal interactions were particularly common in both directions. This suggests a diversification of adult support networks to nonparental kin, possibly in response to a long-term parental investment disadvantage of being middle-born sons. In all instances, regardless of inheritance, lastborn farmers were the most disadvantaged in terms of kin support. Overall, we found that nonreciprocal interactions among farmers followed kin selection predictions. Direct reciprocity explained a substantial part of the support received from kin, suggesting the importance of the combined effects of kin selection and reciprocity for investment from kin. (shrink)