Lineal kinship organization in cross-specific perspective

Adoption, Fostering, and Parental Absence in Vanuatu

Alloparenting, wherein people provide care to children who are not their biological offspring, is a key aspect of human child-rearing. In the Pacific, many children are adopted or fostered by custodial alloparents even when both biological parents are still alive. From a behavioral ecology perspective, such behaviors are puzzling: why parent someone else's child at your expense? Furthermore, little is known about how these arrangements are made in Pacific Islander societies today, who provides care, and what kinds of outcomes fostered children experience. A better understanding of these proximate factors may help reveal the ultimate drivers behind custodial alloparenting. Here, we report findings from a survey carried out with the caregivers of 282 children in rural areas of Vanuatu, an island nation in Melanesia. Most fostered and adopted children lived with relatives such as aunts, uncles, and grandparents (87.5%) rather than unrelated caregivers, with a strong preference for maternal kin. The most common reasons for these arrangements were that the parents had separated (16.7%), were engaging in labor migration (27.1%), or a combination of both (27.1%). Results for investment in children's education and their educational outcomes were mixed, although children removed from crisis situations did more poorly than children removed for aspirational reasons. Our findings suggest that custodial alloparenting helps families adapt to socioeconomic transitions and changing marriage practices. Outcomes may depend on a range of factors, such as the reason children were transferred out of the natal home to begin with.

Gender disparities in material and educational resources differ by kinship system

Contemporary inequality exists at an unprecedented scale. Social scientists have emphasized the role played by material wealth in driving its escalation. Evolutionary anthropologists understand the drive to accumulate material wealth as one that is coupled ultimately to increasing reproductive success. Owing to biological caps on reproduction for women, the efficiency of this conversion can differ by gender, with implications for understanding the evolution of gender disparities in resource accumulation. Efficiency also differs according to the type of resources used to support reproductive success. In this paper, we review evolutionary explanations of gender disparities in resources and investigate empirical evidence to support or refute those explanations among matrilineal and patrilineal subpopulations of ethnic Chinese Mosuo, who share an ethnolinguistic identity, but differ strikingly in terms of institutions and norms surrounding kinship and gender. We find that gender differentially predicts income and educational attainment. Men were more likely to report income than women; amounts earned were higher for men overall, but the difference between men and women was minimal under matriliny. Men reported higher levels of educational attainment than women, unexpectedly more so in matrilineal contexts. The results reveal nuances in how biology and cultural institutions affect gender disparities in wealth. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Gender Differences in Social Networks Based on Prevailing Kinship Norms in the Mosuo of China

Although cooperative social networks are considered key to human evolution, emphasis has usually been placed on the functions of men’s cooperative networks. What do women’s networks look like? Do they differ from men’s networks and what does this suggest about evolutionarily inherited gender differences in reproductive and social strategies? In this paper, we test the ‘universal gender differences’ hypothesis positing gender-specific network structures against the ‘gender reversal’ hypothesis that posits that women’s networks look more ‘masculine’ under matriliny. Specifically, we ask whether men’s friendship networks are always larger than women’s networks and we investigate measures of centrality by gender and descent system. To do so, we use tools from social network analysis and data on men’s and women’s friendship ties in matrilineal and patrilineal Mosuo communities. In tentative support of the gender reversal hypothesis, we find that women’s friendship networks in matriliny are relatively large. Measures of centrality and generalized linear models otherwise reveal greater differences between communities than between men and women. The data and analyses we present are primarily descriptive given limitations of sample size and sampling strategy. Nonetheless, our results provide support for the flexible application of social relationships across genders and clearly challenge the predominant narrative of universal gender differences across space and time.

The expendable male hypothesis

Matriliny is a system of kinship in which descent and inheritance are conferred along the female line. The theoretically influential concept of the matrilineal puzzle posits that matriliny poses special problems for understanding men's roles in matrilineal societies. Ethnographic work describes the puzzle as the tension experienced by men between the desire to exert control over their natal kin (i.e. the lineage to which they belong) and over their affinal kin (i.e. their spouses and their biological children). Evolutionary work frames the paradox as one resulting from a man investing in his nieces and nephews at the expense of his own biological offspring. In both cases, the rationale for the puzzle rests on two fundamental assumptions: (i) that men are in positions of authority over women and over resources; and (ii) that men are interested in the outcomes of parenting. In this paper, we posit a novel hypothesis that suggests that certain ecological conditions render men expendable within local kinship configurations, nullifying the above assumptions. This arises when (i) women, without significant assistance from men, are capable of meeting the subsistence needs of their families; and (ii) men have little to gain from parental investment in children. We conclude that the expendable male hypothesis may explain the evolution of matriliny in numerous cases, and by noting that female-centred approaches that call into doubt assumptions inherent to male-centred models of kinship are justified in evolutionary perspective. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

A worldwide view of matriliny: using cross-cultural analyses to shed light on human kinship systems

Although matriliny and matrilocality are relatively rare in contemporary human populations, these female-based descent and residence systems are present in different cultural contexts and across the globe. Previous research has generated numerous hypotheses about which cultural traits are associated with the stability or loss of matrilineal descent. In addition, several studies have examined matrilineal descent with phylogenetic analyses; however, the use of language phylogenies has restricted these analyses to comparisons within a single language family, often confined to a single continent. Cross-cultural comparisons are particularly informative when they account for the relationships between widely distributed populations, as opposed to treating each population as an independent sample or focusing on a single region. Here, we study the evolution of descent systems on a worldwide scale. First, we test for significant associations between matriliny and numerous cultural traits that have been theoretically associated with its stability or loss, such as subsistence strategy, animal domestication, mating system, residence pattern, wealth transfer and property succession. In addition, by combining genetic and linguistic information to build a global supertree that includes 16 matrilineal populations, we also perform phylogenetically controlled analyses to assess the patterns of correlated evolution between descent and other traits: for example, does a change in subsistence strategy generally predict a shift in the rules of descent, or do these transitions happen independently? These analyses enable a worldwide perspective on the pattern and process of the evolution of matriliny and matrilocality. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

Differences between sons and daughters in the intergenerational transmission of wealth

Persistent interest lies in gender inequality, especially with regard to the favouring of sons over daughters. Economists are concerned with how privilege is transmitted across generations, and anthropologists have long studied sex-biased inheritance norms. There has, however, been no focused cross-cultural investigation of how parent-offspring correlations in wealth vary by offspring sex. We estimate these correlations for 38 wealth measures, including somatic and relational wealth, from 15 populations ranging from hunter-gatherers to small-scale farmers. Although small sample sizes limit our statistical power, we find no evidence of ubiquitous male bias, at least as inferred from comparing parent-son and parent-daughter correlations. Rather we find wide variation in signatures of sex bias, with evidence of both son and daughter-biased transmission. Further, we introduce a model that helps pinpoint the conditions under which simple mid-point parent-offspring wealth correlations can reveal information about sex-biased parental investment. Our findings are relevant to the study of female-biased kinship by revealing just how little normative descriptors of kinship systems, such as patrilineal inheritance, capture intergenerational correlations in wealth, and how variable parent-son and parent-daughter correlations can be. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

The evolution of female-biased kinship in humans and other mammals

Female-biased kinship (FBK) arises in numerous species and in diverse human cultures, suggesting deep evolutionary roots to female-oriented social structures. The significance of FBK has been debated for centuries in human studies, where it has often been described as difficult to explain. At the same time, studies of FBK in non-human animals point to its apparent benefits for longevity, social complexity and reproduction. Are female-biased social systems evolutionarily stable and under what circumstances? What are the causes and consequences of FBK? The purpose of this theme issue is to consolidate efforts towards understanding the evolutionary significance and stability of FBK in humans and other mammals. The issue includes broad theoretical and empirical reviews as well as specific case studies addressing the social and ecological correlates of FBK across taxa, time and space. It leverages a comparative approach to test existing hypotheses and presents novel arguments that aim to expand our understanding of how males and females negotiate kinship across diverse contexts in ways that lead to the expression of female biases in kinship behaviour and social structure. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.