Growing evidence suggests mothers invest more in girls than boys and fathers more in boys than girls. We develop a hypothesis that predicts preference for girls by the parent facing more resource constraints and preference for boys by the parent facing less constraint. We test the hypothesis with panel data from the Tsimane’, a foraging-farming society in the Bolivian Amazon. Tsimane’ mothers face more resource constraints than fathers. As predicted, mother’s wealth protected girl’s BMI, but father’s wealth had weak effects (...) on boy’s BMI. Numerous tests yielded robust results, including those that controlled for fixed effects of child and household. (shrink)

Relatives play an important role in human reproduction according to evolutionary theories of reproductive behavior, but previous empirical studies show large differences in the effects of kin on fertility outcomes. In our paper we examine the effect of co-resident kin and non-kin on the length of birth intervals over the reproductive life course of Dutch women born between 1842 and 1920. We estimate Cox proportional hazard models for parity progression based on the presence of kin and non-kin in the household (...) while controlling for a large number of individual and community-level characteristics. We find that couples living with their brothers experienced shorter birth intervals whereas couples residing with a widowed father had relatively longer birth intervals. The effects of these types of kin on reproduction were most pronounced up to the birth of the fifth child, but not thereafter. We found no effect for mothers or other types of kin. (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)

We examine the relationship between birth order and reproductive behaviors in a sample of Australian residents, accounting for personality, personal achievements, and family structure. Using generalized linear models and survival analyses we build predictive models for each reproductive measure and test those models on an independent data subset. Compared with functional firstborns (middle-borns more than 5 years younger than their next older sibling), male middle-borns and last-born females had younger ages of first sexual intercourse, and middle-born females had a younger (...) age at first pregnancy. There was no difference in females’ age at first birth. Male middle-borns tended to have an older average age at first birth. Controlling for age, both male and female middle-borns had fewer children. Overall, middle-borns differ more from functional firstborns than do last-borns. Given the significant but small effect sizes demonstrated in this study, we suggest that developmental characteristics that may facilitate middle-borns’ success within the natal family carry slight fitness costs during the reproductive years. (shrink)

Evidence suggests that humans might have neurological specializations for music processing, but a compelling adaptationist account of music and dance is lacking. The sexual selection hypothesis cannot easily account for the widespread performance of music and dance in groups (especially synchronized performances), and the social bonding hypothesis has severe theoretical difficulties. Humans are unique among the primates in their ability to form cooperative alliances between groups in the absence of consanguineal ties. We propose that this unique form of social organization (...) is predicated on music and dance. Music and dance may have evolved as a coalition signaling system that could, among other things, credibly communicate coalition quality, thus permitting meaningful cooperative relationships between groups. This capability may have evolved from coordinated territorial defense signals that are common in many social species, including chimpanzees. We present a study in which manipulation of music synchrony significantly altered subjects’ perceptions of music quality, and in which subjects’ perceptions of music quality were correlated with their perceptions of coalition quality, supporting our hypothesis. Our hypothesis also has implications for the evolution of psychological mechanisms underlying cultural production in other domains such as food preparation, clothing and body decoration, storytelling and ritual, and tools and other artifacts. (shrink)

Considerable research on helpers-at-the-nest demonstrates the positive effects of firstborn daughters on a mother’s reproductive success and the survival of her children compared with women who have firstborn sons. This research is largely restricted to agricultural settings. In the present study we ask: “Does ‘daughter first’ improve mothers’ reproductive success in a hunting and gathering context?” Through an analysis of 84 postreproductive women in this population we find that the sex of the first- or second-born child has no effect on (...) a mother’s fertility or the survival of her offspring. We conclude that specific environmental and economic factors underlay the helpers-at-the-nest phenomenon. (shrink)

Since the late 1970s, disruptions and “failure” of maternal-infant bonding have been causally linked to postpartum depression. Part I of this paper examines the grounds for this connection while tracing the ramifications of bonding theory (Klaus and Kennell 1976) through obstetrics, pediatrics, and psychiatry, as well as in the (mis)representations of it in the popular media. This discussion resolves into a view of maternal attachment as a long-term development progressively established through intensive mother-infant interaction. The forms of this interaction are (...) phylogenetically determined, albeit culturally and personally mediated. Flowing from this premise, Part II of the paper casts postpartum depression as an adaptive response to threat (from whatever cause) to adequate mothering, and develops an argument for the evolutionary role of enacted social ties in the establishment of maternal responsiveness. (shrink)

This study examines child survival and growth in a patrilineal Ethiopian community as a function of father absence and sex. In line with evolutionary predictions for sex-biased parental investment, the absence of a father and associated constraints on household resources is more detrimental for sons’ than daughters’ survival in infancy. Father absence doubles a son’s risk of dying in infancy but has a positive influence on the well-being of female members of the household, improving daughter survival, growth, and maternal nutritional (...) status. Lack of paternal investment may be compensated for by other matrilateral kin through increased reciprocity between mother, daughter, and sister. (shrink)

Aspects of human life history and cognition, such as our long childhoods and extensive use of teaching, theoretically evolved to facilitate the acquisition of complex tasks. The present paper empirically examines the relationship between subsistence task difficulty and age of acquisition, rates of teaching, and rates of oblique transmission among Hadza and BaYaka foragers from Tanzania and the Republic of Congo. We further examine cross-cultural variation in how and from whom learning occurred. Learning patterns and community perceptions of task difficulty (...) were assessed through interviews. We found no relationship between task difficulty, age of acquisition, and oblique transmission, and a weak but positive relationship between task difficulty and rates of teaching. While same-sex transmission was normative in both societies, tasks ranked as more difficult were more likely to be transmitted by men among the BaYaka, but not among the Hadza, potentially reflecting cross-cultural differences in the sexual division of subsistence and teaching labor. Further, the BaYaka were more likely to report learning via teaching, and less likely to report learning via observation, than the Hadza, possibly owing to differences in socialization practices. (shrink)

The evolution of cooperative breeding is complex, and particularly so in humans because many other life history traits likely evolved at the same time. While cooperative childrearing is often presumed ancient, the transition from maternal self-reliance to dependence on allocare leaves no known empirical record. In this paper, an exploratory model is developed that incorporates probable evolutionary changes in birth intervals, juvenile dependence, and dispersal age to predict under what life history conditions mothers are unable to raise children without adult (...) cooperation. The model’s outcome variable (net balance) integrates dependent children’s production and consumption as a function of varying life history parameters to estimate the investment mothers or others have to spend subsidizing children. Results suggest that maternal-juvenile cooperation can support the early transition toward a reduction in birth intervals, a longer period of juvenile dependence, and having overlapping young. The need for adult cooperation is most evident when birth intervals are short and age at net production is late. Findings suggest that the needs of juveniles would not have been an early selective force for adult cooperation. Rather, an age-graded division of labor and the mutual benefits of maternal-juvenile cooperation could be an important, but overlooked step in the evolution of cooperative breeding. (shrink)