Post-marital residence patterns and the timing of reproduction: evidence from a matrilineal society

Humans exhibit a broad range of post-marital residence patterns and there is growing recognition that post-marital residence predicts women's reproductive success; however, the nature of the relationship is probably dependent on whether co-resident kin are cooperators or competitors. Here, we explore this relationship in a Tibetan population, where couples practice a mixture of post-marital residence patterns, co-residing in the same village with the wife's parents, the husband's parents or endogamously with both sets of parents. Using detailed demographic data from 17 villages we find that women who live with only their own parents have an earlier age at first birth (AFB) and age at last birth (ALB) than women who live with only their parents-in-law. Women who co-reside with both sets of parents have the earliest AFB and ALB. However, those with co-resident older siblings postponed reproduction, suggestive of competition-related delay. Shifts to earlier reproductive timing were also observed in relation to the imposition of family planning policies, in line with Fisherian expectations. Our study provides evidence of the costs and benefits to women's direct fitness of co-residing with different kin, against a backdrop of adaptive responses to cultural constraints on completed fertility.

Repercussions of patrilocal residence on mothers' social support networks among Tsimane forager-farmers

While it is commonly thought that patrilocality is associated with worse outcomes for women and their children due to lower social support, few studies have examined whether the structure of female social networks covaries with post-marital residence. Here, we analyse scan sample data collected among Tsimane forager-farmers. We compare the social groups and activity partners of 181 women residing in the same community as their parents, their husband's parents, both or neither. Relative to women living closer to their in-laws, women living closer to their parents are less likely to be alone or solely in the company of their nuclear family (odds ratio (OR): 0.6, 95% CI: 0.3-0.9), and more likely to be observed with others when engaging in food processing and manufacturing of market or household goods, but not other activities. Women are slightly more likely to receive childcare support from outside the nuclear family when they live closer to their parents (OR = 1.8, 95% CI 0.8-3.9). Their social group size and their children's probability of receiving allocare decrease significantly with distance from their parents, but not their in-laws. Our findings highlight the importance of women's proximity to kin, but also indicate that patrilocality per se is not costly to Tsimane women. This article is part of the theme issue 'Cooperation among women: evolutionary and cross-cultural perspectives'.

Lineal kinship organization in cross-specific perspective

I draw on insights from anthropology to outline a framework for the study of kinship systems that applies across animal species with biparental sexual reproduction. In particular, I define lineal kinship organization as a social system that emphasizes interactions among lineally related kin-that is, individuals related through females only, if the emphasis is towards matrilineal kin, and individuals related through males only, if the emphasis is towards patrilineal kin. In a given population, the emphasis may be expressed in one or more social domains, corresponding to pathways for the transmission of different resources across generations (e.g. the allocation of food, the transfer of access to the natal territory or household). A lineal bias in any domain can be viewed as a bias in investment towards a particular set of kin-specifically, towards the offspring of daughters if the bias is matrilineal, and towards the offspring of sons if the bias is patrilineal. Effectively, investment is restricted to the offspring of the females in the population in one case, and to the offspring of the males in the other. This is distinct from a bias in investment towards daughters and towards sons, respectively. Overall, I propose a shift in focus-from viewing matrilineal and patrilineal kinship as unitary phenomena, to consideration of the different aspects of the social system featuring a bias towards lineally related kin. 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'.

Cooperation beyond consanguinity: post-marital residence, delineations of kin and social support among South Indian Tamils

Evolutionary ecologists have shown that relatives are important providers of support across many species. Among humans, cultural reckonings of kinship are more than just relatedness, as they interact with systems of descent, inheritance, marriage and residence. These cultural aspects of kinship may be particularly important when a person is determining which kin, if any, to call upon for help. Here, we explore the relationship between kinship and cooperation by drawing upon social support network data from two villages in South India. While these Tamil villages have a nominally male-biased kinship system (being patrilocal and patrilineal), matrilateral kin play essential social roles and many women reside in their natal villages, letting us tease apart the relative importance of genetic relatedness, kinship and residence in accessing social support. We find that people often name both their consanguineal and affinal kin as providing them with support, and we see some weakening of support with lesser relatedness. Matrilateral and patrilateral relatives are roughly equally likely to be named, and the greatest distinction instead is in their availability, which is highly contingent on post-marital residence patterns. People residing in their natal village have many more consanguineal relatives present than those who have relocated. Still, relocation has only a small effect on an individual's network size, as non-natal residents are more reliant on the few kin that they have present, most of whom are affines. In sum, marriage patterns have an important impact on kin availability, but the flexibility offered by the broadening of the concept of kin helps people develop the cooperative relationships that they rely upon, even in the absence of genetic relatives. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

Kinship as a frequency dependent strategy

Humans divide themselves up into separate cultures, which is a unique and ubiquitous characteristic of our species. Kinship norms are one of the defining features of such societies. Here we show how norms of marital residence can evolve as a frequency-dependent strategy, using real-world cases from southwestern China and an evolutionary game model. The process of kinship change has occurred in the past and is also occurring now in southwestern China. Our data and models show how transitions between residence types can occur both as response to changing costs and benefits of co-residence with kin, and also due to the initial frequency of the strategies adopted by others in the population: patrilocal societies can become matrilocal, and neolocal societies can become duolocal. This illustrates how frequency-dependent selection plays a role both in the maintenance of group-level cultural diversity and in cultural extinction.