Biased parental investment and reproductive success in Gabbra pastoralists

Abandoning female genital mutilation/cutting (FGMC) is an emerging but costly parental investment strategy in rural Ethiopia

Female genital mutilation or cutting (FGMC) has profound consequences for an estimated 200 million women world-wide, yet affected communities often resist efforts to end the practice. Marriage market dynamics have been proposed as key to this resistance, because where FGMC is normative, parents are motivated to cut their daughters to improve their marriage prospects. Some economists have also argued that financial gain, through bride wealth payments, incentivises parents to cut daughter's at time of marriage. Bride wealth, however, does not necessarily equal net economic return, confounding efforts to test this assumption. Here we use detailed data on the financial value of all exchanges at marriage from Ethiopian Arsi Oromo agropastoralists to assess their association with FGMC. We also explore the idea that parents must replace FGMC with other forms of investment (e.g., education) when cutting practices are rejected. Multivariate multilevel Bayesian models were run using data from the first marriages of 358 women to assess the association between FGMC status and education and marriage-related outcomes: bride wealth payments, dowry costs, and age at marriage. Being cut is associated with lower dowry costs and earlier age at marriage but does not predict bride wealth paid by the groom's family. School attendance is associated with higher bride wealth, particularly for women with four or more years of education, and with later age at marriage. These findings indicate that bride wealth payments do not maintain FGMC among the Arsi Oromo. While we find a relative economic loss for parents from FGMC abandonment through higher value dowry gifts, this may be traded-off against the health benefits to uncut daughters. These findings point to the emergence of new norms, whereby Arsi Oromo parents reject cutting for their daughters and prefer their daughters-in-law to be educated.

Why marry early? Parental influence, agency and gendered conflict in Tanzanian marriages

Global health interventions increasingly target the abolishment of 'child marriage' (marriage under 18 years, hereafter referred to as 'early marriage'). Guided by human behavioural ecology theory, and drawing on focus groups and in-depth interviews in an urbanising Tanzanian community where female early marriage is normative, we examine the common assumption that it is driven by the interests and coercive actions of parents and/or men. We find limited support for parent-offspring conflict. Parents often encouraged early marriages, but were motivated by the promise of social and economic security for daughters, rather than bridewealth transfers alone. Moreover, forced marriage appears rare, and adolescent girls and young women (AGYW) were active agents in the transition to marriage, sometimes marrying against parental wishes. Support for gendered conflict was stronger. AGYW were described as being lured into unstable relationships by men misrepresenting their long-term intentions. Community members voiced concerns over these marriages. Overall, early marriage appears rooted in limited options, encouraging strategic, but risky choices on the marriage market. Our results highlight plurality and context dependency in drivers of early marriage, even within a single community. We conclude that engaging with the importance of context is fundamental in forging culturally sensitive policies and programs on early marriage.

Monks relax sibling competition over parental resources in Tibetan populations

Why parents in some societies induce some of their sons to become religious celibates is an evolutionary puzzle. Some have speculated that this might be associated with brother competition for family resources. However, the behavioral ecology of monks and the possible links with competition between brothers remain unexplored. Here, we use demographic data from Amdo Tibetan agropastoralists in western China to evaluate what factors determine the probability of becoming a monk and explore the possible association between wealth and having a monk brother. We found that boys with at least one older brother are more likely to become celibate monks. Patrilocal heads of household, who inherit parental property, are more likely to be first-born sons, whereas men who marry uxorilocally, that is they move to their wife’s household, are generally second- or later-born sons. Moreover, we find that men with at least one monk brother are wealthier than men who only have non-celibate brothers. Together, these results suggest that sending a son to the monastery is a way for parents to decrease competition between brothers over family resources. Harsh and resource-limited environments, like the one we consider, can lead to the emergence of communal households, including polyandrous families, which used to be common in Tibetan areas. Directing one son to become a religious celibate offers a potentially effective solution to brother competition in our population.

Religious celibacy brings inclusive fitness benefits

The influence of inclusive fitness interests on the evolution of human institutions remains unclear. Religious celibacy constitutes an especially puzzling institution, often deemed maladaptive. Here, we present sociodemographic data from an agropastoralist Buddhist population in western China, where parents sometimes sent a son to the monastery. We find that men with a monk brother father more children, and grandparents with a monk son have more grandchildren, suggesting that the practice is adaptive. We develop a model of celibacy to elucidate the inclusive fitness costs and benefits associated with this behaviour. We show that a minority of sons being celibate can be favoured if this increases their brothers' reproductive success, but only if the decision is under parental, rather than individual, control. These conditions apply to monks in our study site. Inclusive fitness considerations appear to play a key role in shaping parental preferences to adopt this cultural practice.

Intention to have a second child, family support and actual fertility behavior in current China: An evolutionary perspective

OBJECTIVES

This study provides an evolutionary perspective to a classic topic in demography, that is, the discrepancy between reproductive intention and subsequent behavior, in the context of China's two-child policy.

METHODS

We conduct an event history analysis of longitudinal data from the 2015 and 2018 waves of the Xi'an Fertility Survey (sample size = 321 followed one-child mothers) to test the hypotheses of how within-family support/conflict affects women's fertility behavior.

RESULTS

Only 50% of positive intentions (i.e., intending to have a second child) led to another (live) birth within the 3-year interval; meanwhile, 15% of uncertain intentions and 5% of negative intentions resulted in a birth. Husband's and the firstborn's emotional support raised the hazard of second childbirth along maternal life course, which cannot be fully mediated by mother's fertility intention and thus, contributed to an intention-behavior gap. Husband's sibship size had dual effects on female childbearing behavior: A positive indirect effect mediated by fertility intention, but a negative direct effect presumably due to sibling competition for intergenerational support. Finally, after controlling for fertility intention, having a firstborn son was still associated significantly with a lower second-childbirth hazard, presumably due to son preference as well as concern over parental investment.

CONCLUSIONS

Our study identifies a discrepancy between maternal fertility intention and realized childbearing, which was partly explained by (lack of) support from other (multiple) stakeholders in family reproduction.

Why the Son-bias in Caregiving? Testing Sex-differences in the Associations Between Paternal Caregiving and Child Outcomes in England

Studies show that fathers across Western populations tend to provide more care to sons than daughters. Following a human behavioral ecological framework, we hypothesize that son-biases in fathering may (at least in part) be due to differences in fitness returns to paternal direct investments by child's sex. In this study, we investigate sex-differences in the associations between paternal caregiving and children's outcomes in stable, two-parent families. Using data from the Avon Longitudinal Study of Parents and Children, we test whether paternal caregiving in early childhood is associated with different effects on children's school test scores and behavioral difficulties by children's sex. Overall, we find that paternal caregiving is associated with higher school test scores and lower behavioral difficulty scores, but the association between paternal caregiving and school test scores was stronger for boys. Our findings highlight possible sex-differences in returns to paternal caregiving for certain domains of child outcomes in England.

Sharing the Load: How Do Coresident Children Influence the Allocation of Work and Schooling in Northwestern Tanzania?

Economic and evolutionary models of parental investment often predict education biases toward earlier-born children, resulting from either household resource dilution or parental preference. Previous research, however, has not always found these predicted biases-perhaps because in societies where children work, older children are more efficient at household tasks and substitute for younger children, whose time can then be allocated to school. The role of labor substitution in determining children's schooling remains uncertain, however, because few studies have simultaneously considered intrahousehold variation in both children's education and work. Here, we investigate the influence of coresident children on education, work, and leisure in northwestern Tanzania, using detailed time use data collected from multiple children per household (n = 1,273). We find that age order (relative age, compared with coresident children) within the household is associated with children's time allocation, but these patterns differ by gender. Relatively young girls do less work, have more leisure time, and have greater odds of school enrollment than older girls. We suggest that this results from labor substitution: older girls are more efficient workers, freeing younger girls' time for education and leisure. Conversely, relatively older boys have the highest odds of school enrollment among coresident boys, possibly reflecting traditional norms regarding household work allocation and age hierarchies. Gender is also important in household work allocation: boys who coreside with more girls do fewer household chores. We conclude that considering children as both producers and consumers is critical to understanding intrahousehold variation in children's schooling and work.

Why men invest in non-biological offspring: paternal care and paternity confidence among Himba pastoralists

Paternal investment is predicted to be a facultative calculation based on expected fitness returns and modulated by a host of social predictors including paternity uncertainty. However, the direct role of paternity confidence on the patterns of paternal investment is relatively unknown, in part due to a lack of research in populations with high levels of paternity uncertainty. Additionally, much of the work on paternity certainty uses cues of paternity confidence rather than direct assessments from fathers. We examine the effect of paternity assertions on the multiple measures of paternal investment in Himba pastoralists. Despite a high degree of paternity uncertainty, Himba have strong norms associated with social fatherhood, with men expected to invest equally in biological and non-biological offspring. Our behavioural data show patterns that largely conform to these norms. For domains of investment that are highly visible to the community, such as brideprice payments, we find no evidence of investment biased by paternity confidence. However, more private investment decisions do show some evidence of sex-specific titration. We discuss these results in light of broader considerations about paternal care and the mating-parenting trade-off.

Fathers favour sons, mothers don't discriminate: Sex-biased parental care in northwestern Tanzania

Variation in parental care by child's sex is evident across cultures. Evolutionary theory provides a functional explanation for this phenomenon, predicting that parents will favour specific children if this results in greater fitness payoffs. Here, we explore evidence for sex-biased parental care in a high-fertility, patriarchal and polygynous population in Tanzania, predicting that both mothers and fathers will favour sons in this cultural setting. Our data come from a cross-sectional study in rural northwestern Tanzania, which included surveys with mothers/guardians of 808 children under age 5. We focus on early childhood, a period with high mortality risk which is fundamental in establishing later-life physical and cognitive development. Examining multiple measures of direct/physical care provision (washing, feeding, playing with, supervising, co-sleeping and caring when sick), we demonstrate that fathers favour sons for washing, feeding and supervising, while maternal care is both more intensive and unrelated to child sex. We find no difference in parental care between girls and boys regarding the allocation of material resources and the duration of breastfeeding; or in terms of parental marital and co-residence status. This bias towards sons may result from higher returns to investment for fathers than mothers, and local gender norms about physical care provision.

Marriage stability in a pastoralist society

We examined how individual investment was associated with the duration of marriage partnerships in a pastoralist society of Amdo Tibetans in China. We collected demographic and socioeconomic data from 420 women and 369 men over five villages to assess which factors predicted partnership length. We found that the payment of dowry and bridewealth from both sides of the family predicted marriage stability. The production of offspring, regardless of their survivorship, also had a positive effect on marriage duration, as did trial marriage, a time period before formal marriage. Finally, we found that if both bride and groom invest resources initially into a partnership—whether wealth or labor—their subsequent partnership is stronger than couples who do not make such investments. This paper adds to our understanding of complex social institutions like marriage from a behavioral ecological perspective.

How can non-human primates inform evolutionary perspectives on female-biased kinship in humans?

The rarity of female-biased kinship organization in human societies raises questions about ancestral hominin family structures. Such questions require grounding in the form and function of kin relationships in our close phylogenetic relatives, the non-human primates. Common features of primate societies, such as low paternity certainty and lack of material wealth, are consistent with features that promote matriliny in humans. In this review, I examine the role of kinship in three primate study systems (socially monogamous species, female-bonded cercopithecines and great apes) that, each for different reasons, offer insights into the evolutionary roots of matriliny. Using these and other examples, I address potential analogues to features of female-biased kinship organization, including residence, descent and inheritance. Social relationships are biased towards matrilineal kin across primates, even where female dispersal limits access to them. In contrast to the strongly intergenerational nature of human kinship, most primate kin relationships function laterally as the basis for cooperative networks and require active reinforcement. There is little evidence that matrilineal kin relationships in primates are functionally equivalent to descent or true inheritance, but further research is needed to understand whether human cultural constructs of kinship produce fundamentally different biological outcomes from their antecedents in primates. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

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'.

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'.

Biological, environmental and socioeconomic determinants of the human birth sex ratio in the Czech Republic

The Trivers-Willard Hypothesis (TWH) states that parents in good conditions bias the sex ratio towards sons and parents in poor conditions bias the sex ratio towards daughters. This study used data from a large nationwide population dataset (N=1,401,851) from the Czech Republic - a modern contemporary society. The study included air pollution and property prices in the TWH estimation, and had a more detailed focus on stillbirths than previous studies. Using official natality microdata from the Czech Statistical Office for years between 1992 and 2010 and data on levels of air pollution in the country over the same period, the study assessed whether the biological and socioeconomic status of mothers and environmental factors affected the sex of children. The results were largely insignificant and not robust across specifications. The presented epidemiological evidence suggests that stillbirths are randomly distributed in the Czech Republic and that the sex ratio is not affected by the socioeconomic status of mothers or by environmental characteristics.

Food Income and the Evolution of Forager Mobility

Forager mobility tends to be high, although ethnographic studies indicate ecological factors such as resource abundance and reliability, population density and effective temperature influence the cost-to-benefit assessment of movement decisions. We investigate the evolution of mobility using an agent-based and spatially explicit cultural evolutionary model that considers the feedback between foragers and their environment. We introduce Outcomes Clustering, an approach to categorizing simulated system states arising from complex stochastic processes shaped by multiple interacting parameters. We find that decreased mobility evolves under conditions of high resource replenishment and low resource depletion, with a concomitant trend of increased population density and, counter-intuitively, decreased food incomes. Conversely, increased mobility co-occurs with lower population densities and higher food incomes. We replicate the well-known relationships between mobility, population density, and resource quality, while predicting reduced food income, and consequently the reduction in health status observed in early sedentary populations without the need to invoke factors such as reduced diet quality or increased pathogen loads.

Sexual conflict and the Trivers-Willard hypothesis: Females prefer daughters and males prefer sons

Because parental care is expected to depend on the fitness returns generated by each unit of investment, it should be sensitive to both offspring condition and parental ability to invest. The Trivers-Willard Hypothesis (TWH) predicts that parents who are in good condition will bias investment towards sons, while parents who are in poor condition will bias investment towards daughters because high-quality sons are expected to out-reproduce high quality daughters, while low-quality daughters are expected to out-reproduce low quality sons. We report results from an online experiment testing the Trivers-Willard effect by measuring implicit and explicit psychological preferences and behaviorally implied preferences for sons or daughters both as a function of their social and economic status and in the aftermath of a priming task designed to make participants feel wealthy or poor. We find only limited support for predictions derived from the TWH and instead find that women have strong preferences for girls and men have preferences for boys.

Parental investment in Tibetan populations does not reflect stated cultural norms

In this paper, we examined both stated norms of sex preference and actual sex-biases in parental investment in a Tibetan pastoralist society. We collected detailed demographic data on infant mortality, infant feeding, the length of interbirth intervals, and a decision when giving gifts, to examine sex-biased parental investment. Our results indicate a mismatch between self-reported son preference and measures of actual parental investment that favor daughters. We interpret this female-biased parental investment as a possible response to daughters generating more economic resources. However, the stated sex preferences of both sexes reflect cultural norms that appear to have remained unchanged over a long period, which may reflect the importance of male roles in the past. Our behavioral measures of parental investment are those most likely to be in the control of women (such as breastfeeding and interbirth interval), so this mismatch between stated and actual investment may be especially true of women.

Kin and birth order effects on male child mortality: three East Asian populations, 1716-1945

Human child survival depends on adult investment, typically from parents. However, in spite of recent research advances on kin influence and birth order effects on human infant and child mortality, studies that directly examine the interaction of kin context and birth order on sibling differences in child mortality are still rare. Our study supplements this literature with new findings from large-scale individual-level panel data for three East Asian historical populations from northeast China (1789-1909), northeast Japan (1716-1870), and north Taiwan (1906-1945), where preference for sons and first-borns is common. We examine and compare male child mortality risks by presence/absence of co-resident parents, grandparents, and other kin, as well as their interaction effects with birth order. We apply discrete-time event-history analysis on over 172,000 observations of 69,125 boys aged 1-9 years old. We find that in all three populations, while the presence of parents is important for child survival, it is more beneficial to first/early-borns than to later-borns. Effects of other co-resident kin are however null or inconsistent between populations. Our findings underscore the importance of birth order in understanding how differential parental investment may produce child survival differentials between siblings.

The evolution of inequality

Understanding how systems of political and economic inequality evolved from relatively egalitarian origins has long been a focus of anthropological inquiry. Many hypotheses have been suggested to link socio-ecological features with the rise and spread of inequality, and empirical tests of these hypotheses in prehistoric and extant societies are increasing. In this review, we synthesize several streams of theory relevant to understanding the evolutionary origins, spread, and adaptive significance of inequality. We argue that while inequality may be produced by a variety of localized processes, its evolution is fundamentally dependent on the economic defensibility and transmissibility of wealth. Furthermore, these properties of wealth could become persistent drivers of inequality only following a shift to a more stable climate in the Holocene. We conclude by noting several key areas for future empirical research, emphasizing the need for more analyses of contemporary shifts toward institutionalized inequality as well as prehistoric cases.

The offspring quantity-quality trade-off and human fertility variation

The idea that trade-offs between offspring quantity and quality shape reproductive behaviour has long been central to economic perspectives on fertility. It also has a parallel and richer theoretical foundation in evolutionary ecology. We review the application of the quantity–quality trade-off concept to human reproduction, emphasizing distinctions between clutch size and lifetime fertility, and the wider set of forces contributing to fertility variation in iteroparous and sexually reproducing species like our own. We then argue that in settings approximating human evolutionary history, several factors limit costly sibling competition. Consequently, while the optimization of quantity–quality trade-offs undoubtedly shaped the evolution of human physiology setting the upper limits of reproduction, we argue it plays a modest role in accounting for socio-ecological and individual variation in fertility. Only upon entering the demographic transition can fertility limitation be clearly interpreted as strategically orientated to advancing offspring quality via increased parental investment per child, with low fertility increasing descendant socio-economic success, although not reproductive success. We conclude that existing economic and evolutionary literature has often overemphasized the centrality of quantity–quality trade-offs to human fertility variation and advocate for the development of more holistic frameworks encompassing alternative life-history trade-offs and the evolved mechanisms guiding their resolution.

Faster reproductive rates trade off against offspring growth in wild chimpanzees

Life history theory predicts a trade-off between offspring quality and quantity. Among large-bodied mammals, prolonged lactation and infant dependence suggest particularly strong potential for a quality-quantity trade-off to exist. Humans are one of the only such species to have been examined, providing mixed evidence under a peculiar set of circumstances, including extensive nutritional provisioning by nonmothers and extrasomatic wealth transmission. Here, we examine trade-offs between reproductive rate and one aspect of offspring quality (body size) in wild chimpanzees (Pan troglodytes schweinfurthii), a species with long periods of infant dependence and little direct provisioning. Juvenile lean body mass, estimated using urinary creatinine excretion, was positively associated with the interval to the next sibling's birth. These effects persisted into adolescence and were not moderated by maternal identity. Maternal depletion could not explain poor offspring growth, as older mothers had larger offspring, and low maternal energy balance during lactation predicted larger, not smaller, juvenile size. Instead, our data suggest that offspring growth suffers when mothers wean early to invest in new reproductive efforts. These findings indicate that chimpanzee mothers with the resources to do so prioritize production of new offspring over prolonged investment in current offspring.

The cultural evolution of fertility decline

Cultural evolutionists have long been interested in the problem of why fertility declines as populations develop. By outlining plausible mechanistic links between individual decision-making, information flow in populations and competition between groups, models of cultural evolution offer a novel and powerful approach for integrating multiple levels of explanation of fertility transitions. However, only a modest number of models have been published. Their assumptions often differ from those in other evolutionary approaches to social behaviour, but their empirical predictions are often similar. Here I offer the first overview of cultural evolutionary research on demographic transition, critically compare it with approaches taken by other evolutionary researchers, identify gaps and overlaps, and highlight parallel debates in demography. I suggest that researchers divide their labour between three distinct phases of fertility decline--the origin, spread and maintenance of low fertility--each of which may be driven by different causal processes, at different scales, requiring different theoretical and empirical tools. A comparative, multi-level and mechanistic framework is essential for elucidating both the evolved aspects of our psychology that govern reproductive decision-making, and the social, ecological and cultural contingencies that precipitate and sustain fertility decline.

Brothers and sisters : How sibling interactions affect optimal parental allocations

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.

Birth order, sibling investment, and fertility among Ju/'Hoansi (!Kung)

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.

Linking dispersal and resources in humans : Life history data from Oakham, Massachusetts (1750-1850)

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.

Kin and Child Survival in Rural Malawi : Are Matrilineal Kin Always Beneficial in a Matrilineal Society?

This paper investigates the impact of kin on child survival in a matrilineal society in Malawi. Women usually live in close proximity to their matrilineal kin in this agricultural community, allowing opportunities for helping behavior between matrilineal relatives. However, there is little evidence that matrilineal kin are beneficial to children. On the contrary, child mortality rates appear to be higher in the presence of maternal grandmothers and maternal aunts. These effects are modified by the sex of child and resource ownership: female children and children in households where women, rather than men, own land suffer higher mortality rates in the presence of maternal kin. These modifiers suggest the detrimental effects of matrilineal kin may result from competition between such kin for resources. There are some positive effects of kin on child survival: the presence of elder siblings of both sexes is correlated with higher survival rates, and there is some weak evidence that paternal grandmothers may be beneficial to a child's survival chances. There is little evidence that any male kin, whether matrilineal or patrilineal, and including fathers, affect child mortality rates. This study highlights the importance of taking social and ecological context into account when investigating relationships between kin.

Kinship, sex, and fitness in a Caribbean community

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.

Does Investment in the Sexes Differ When Fathers Are Absent? : Sex-biased Infant Survival and Child Growth in Rural Ethiopia

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.

Intergenerational and sibling conflict under patrilocality. A model of reproductive skew applied to human kinship

Here we argue that models developed to examine cooperation and conflict in communal breeders, using a "tug-of-war" model of reproductive skew generated by incomplete control, are an appropriate way to model human kinship systems. We apply such models to understand the patterns of effort put into competition between father and son and between brothers in conflict over family resources in a patrilineal kinship system. Co-resident kin do not necessarily emerge with equal shares of the cake in terms of reproductive output. The models show that, depending on the efficiency with which they can gain more control of the resource, on the marriage system, and on the relatedness of the partners in conflict, individuals can do better to help their relatives breed rather than fight each other for the resources needed to reproduce. The models show that when a son's father is still breeding with his mother, sons should not compete for any share of reproduction. However, under polygyny, increased effort is spent on father/son and brother/brother conflict. Fathers will win the majority of reproduction if dominant to sons (in contrast to the finding that daughters-in-law win in conflict over mothers-in-law in patrilocal kinship systems, which has been suggested as explaining the evolution of menopause). Hence who wins in the sharing of reproduction depends not just on which sex disperses but also on the relative competitive ability of all individuals to exploit family resources. Anthropologists have long argued that cultural norms can reduce conflict. These formal evolutionary models help us to quantify the effects of reproductive conflict in families, throwing light on the evolutionary basis not just of patterns of reproductive scheduling, but also human kinship and marriage systems.

Cooperation and conflict between women in the family

Here I review recent research on reproductive conflict between females in families and how it influences their reproductive behaviour. Kin selection can favor cooperation between parent and offspring, siblings, or unrelated co-residents who share interests in other family members such as grand-offspring. However, these are also the individuals most likely to be sharing resources, and so conflict can also emerge. While substantial interest has arisen in evolutionary anthropology, especially over the last two decades, in the possibility of cooperative breeding in humans, less attention has been paid to reproductive conflict among female kin. Communal breeding in animals is generally understood as emerging from competition over the resources needed to breed. Competition for household resources is a problem that also faces human families. Models suggest that in some circumstances, inclusive fitness can be maximized by sharing reproduction rather than harming relatives by fighting with them, even if the shares that emerge are not equal. Thus, competition and cooperation turn out to be strongly related to each other. Reproductive competition within and between families may have underpinned the biological evolution of fertility patterns (such as menopause) and the cultural evolution of marriage, residence, and inheritance norms (such as late male marriage or primogeniture), which can enhance cooperation and minimize the observed incidence of such conflicts.

Reproductive competition between females in the matrilineal Mosuo of southwestern China

The matrilineal Mosuo of southwestern China live in communal households where brothers and sisters of three generations live together (duolocal residence), and men visit their wives, who reside elsewhere, only at night in 'visiting' marriages. Here we show that these communally breeding sisters are in reproductive conflict, in the sense that they share the resources needed to reproduce. We analyse determinants of reproductive success in females and males, and show that co-resident female kin are in competition; the more female kin reside in the household, the more reproductive success is reduced. Male reproductive success, however, is not determined by the kin in his natal household; duolocal males are not in reproductive conflict with their siblings. Competition with female cousins can be worse than that between sisters. We also find that female work on the farm (which is the main communal resource) is not equal. We use a 'tug-of-war' model of reproductive skew generated by incomplete control, to model the patterns of effort put into competition between sisters and cousins. The model predicts that more dominant (older) sisters will put less effort into reproductive conflict than will less dominant (younger) sisters; but younger sisters will also have lower reproductive success because they are less efficient at gaining access to the shared resource. Both predictions are consistent with our data. Younger sisters work less in the fields than do older sisters, which may represent a form of conflict or may be because their average relatedness to the household is lower than that of their more fertile older sisters.

Sibling configuration predicts individual and descendant socioeconomic success in a modern post-industrial society

Growing up with many siblings, at least in the context of modern post-industrial low fertility, low mortality societies, is predictive of relatively poor performance on school tests in childhood, lower levels of educational attainment, and lower income throughout adulthood. Recent studies further indicate these relationships hold across generations, so that the descendants of those who grow up with many siblings are also at an apparent socioeconomic disadvantage. In this paper we add to this literature by considering whether such relationships interact with the sex and relative age of siblings. To do this we utilise a unique Swedish multigenerational birth cohort study that provides sibling configuration data on over 10,000 individuals born in 1915–1929, plus all their direct genetic descendants to the present day. Adjusting for parental and birth characteristics, we find that the ‘socioeconomic cost’ of growing up in a large family is independent of both the sex of siblings and the sex of the individual. However, growing up with several older as opposed to several younger siblings is predictive of relatively poor performance on school tests and a lower likelihood of progression to tertiary education. This later-born disadvantage also holds across generations, with the children of those with many older siblings achieving lower levels of educational attainment. Despite these differences, we find that while individual and descendant income is negatively related to the number of siblings, it is not influenced by the relative age of siblings. Thus, our findings imply that the educational disadvantage of later-born children, demonstrated here and in numerous other studies, does not necessarily translate into reduced earnings in adulthood. We discuss potential explanations for this pattern of results, and consider some important directions for future research into sibling configuration and wellbeing in modern societies.

No evidence for sex biases in milk macronutrients, energy, or breastfeeding frequency in a sample of Filipino mothers

Maternal reproductive investment includes both the energetic costs of gestation and lactation. For most humans, the metabolic costs of lactation will exceed those of gestation. Mothers must balance reproductive investment in any single offspring against future reproductive potential. Among mammals broadly, mothers may differentially invest in offspring based on sex and maternal condition provided such differences investment influence future offspring reproductive success. For humans, there has been considerable debate if there are physiological differences in maternal investment by offspring sex. Two recent studies have suggested that milk composition differs by infant sex, with male infants receiving milk containing higher fat and energy; prior human studies have not reported sex-based differences in milk composition. This study investigates offspring sex-based differences in milk macronutrients, milk energy, and nursing frequency (per 24 h) in a sample of 103 Filipino mothers nursing infants less than 18 months of age. We found no differences in milk composition by infant sex. There were no significant differences in milk composition of mothers nursing first-born versus later-born sons or daughters or between high- and low-income mothers nursing daughters or sons. Nursing frequency also showed no significant differences by offspring sex, sex by birth order, or sex by maternal economic status. In the Cebu sample, there is no support for sex-based differences in reproductive investment during lactation as indexed by milk composition or nursing frequency. Further investigation in other populations is necessary to evaluate the potential for sex-based differences in milk composition among humans.

Why pigs are important in Papua? Wealth, height and reproductive success among the Yali tribe of West Papua

Many studies have investigated how different variables influence the reproductive success (RS) in the populations of natural birth control. Here, we tested hypotheses about positive relationship between wealth, height and several measures of RS in an indigenous, traditional society from West Papua. The study was conducted among the Yali tribe in a few small, isolated mountain villages. In this tribe, a man's wealth is measured by the number of pigs he possesses. We found that wealth was related to fertility and number of living children, but not to child mortality in both men and women. Additionally, child mortality increased with the number of children in a family. Finally, we did not observe any relationship between height and reproductive success measures or wealth. We provide several possible explanations of our results and also put forward hypothetical background for further studies of indigenous populations.

Trees, thickets, or something in between? Recent theoretical and empirical work in cultural phylogeny

Anthropology has always had as one of its goals the explanation of human cultural diversity across space and through time. Over the past several decades, there has been a growing appreciation among anthropologists and other social scientists that the phylogenetic approaches that biologists have developed to reconstruct the evolutionary relationships of species are useful tools for building and explaining patterns of human diversity. Phylogenetic methods offer a means of creating testable propositions of heritable continuity – how one thing is related to another in terms of descent. Such methods have now been applied to a wide range of cultural phenomena, including languages, projectile points, textiles, marital customs, and political organization. Here we discuss several cultural phylogenies and demonstrate how they were used to address long-standing anthropological issues. Even keeping in mind that phylogenetic trees are nothing more than hypotheses about evolutionary relationships, some researchers have argued that when it comes to cultural behaviors and their products, tree building is theoretically unwarranted. We examine the issues that critics raise and find that they in no way sound the death knell for cultural phylogenetic work.

The effect of maternal age and reproductive history on offspring survival and lifetime reproduction in preindustrial humans

Senescence is one of the least understood aspects of organism life history. In part, this stems from the relatively late advent of complete individual-level datasets and appropriate statistical tools. In addition, selection against senescence should depend on the contribution to population growth arising from physiological investment in offspring at given ages, but offspring are rarely tracked over their entire lives. Here, we use a multigenerational dataset of preindustrial (1732-1860) Finns to describe the association of maternal age at offspring birth with offspring survival and lifetime reproduction. We then conduct longitudinal analyses to understand the drivers of this association. At the population level, offspring lifetime reproductive success (LRS) declined by 22% and individual λ, which falls with delays to reproduction, declined by 45% as maternal age at offspring birth increased from 16 to 50 years. These results were mediated by within-mother declines in offspring survival and lifetime reproduction. We also found evidence for modifying effects of offspring sex and maternal socioeconomic status. We suggest that our results emerge from the interaction of physiological with social drivers of offspring LRS, which further weakens selection on late-age reproduction and potentially molds the rate of senescence in humans.

Socioeconomic status determines sex-dependent survival of human offspring

In a polygynous African society, rich men have much more life time offspring than women through the marriage of multiple wives. In line with evolutionary predictions, we found that in rich households more newborn sons were registered, sons had lower mortality and higher body weight. Conscious or unconscious, this maximizes reproductive output.

Background and objectives: In polygynous societies, rich men have many offspring through the marriage of multiple wives. Evolutionary, rich households would therefore benefit more from sons, and according to the Trivers–Willard hypothesis, parents invest more in offspring of the sex that has the best reproductive prospects. We determined the sex differences in number of offspring, sex ratio of offspring, offspring survival and offspring weight in rich and poor households in a polygynous population.

Methodology: We studied a population of 28 994 individuals in Northern Ghana during an 8-year prospective follow-up. We determined the fertility rate for both men and women, sex ratio of 3511 newborn offspring and offspring survival in 16 632 offspring up to reproductive age (≤18 years). Also, we collected 9842 weight measurements of 1470 offspring up to the age of 3 years from growth charts of local clinics.

Results: In rich households, men have a lifetime number of 6.0 offspring, while for women this was 3.1. In line with evolutionary predictions, the male:female sex ratio was higher in rich households (0.52; poor households 0.49), sons had lower mortality in rich households (hazard ratio male versus female 1.06, P = 0.64; poor households: hazard ratio male versus female 1.46, P = 0.01) and sons also had higher weights in rich households (P = 0.008).

Conclusions and implications: In rich households, men have higher reproductive prospects in this polygynous society and, in line with Trivers–Willard, we registered more sons in rich households, sons had lower mortality and higher weights, maximizing the reproductive output in this society.

Are elder siblings helpers or competitors? Antagonistic fitness effects of sibling interactions in humans

Determining the fitness consequences of sibling interactions is pivotal for understanding the evolution of family living, but studies investigating them across lifetime are lacking. We used a large demographic dataset on preindustrial humans from Finland to study the effect of elder siblings on key life-history traits. The presence of elder siblings improved the chances of younger siblings surviving to sexual maturity, suggesting that despite a competition for parental resources, they may help rearing their younger siblings. After reaching sexual maturity however, same-sex elder siblings' presence was associated with reduced reproductive success in the focal individual, indicating the existence of competition among same-sex siblings. Overall, lifetime fitness was reduced by same-sex elder siblings' presence and increased by opposite-sex elder siblings' presence. Our study shows opposite effects of sibling interactions depending on the life-history stage, and highlights the need for using long-term fitness measures to understand the selection pressures acting on sibling interactions.

Alternatives to the grandmother hypothesis: a meta-analysis of the association between grandparental and grandchild survival in patrilineal populations

We conducted a meta-analysis of 17 studies that tested for an association between grandparental survival and grandchild survival in patrilineal populations. Using two different methodologies, we found that the survival of the maternal grandmother and grandfather, but not the paternal grandmother and grandfather, was associated with decreased grandoffspring mortality. These results are consistent with the findings of psychological studies in developed countries (Coall and Hertwig Behavioral and Brain Sciences 33:1-59, 2010). When tested against the predictions of five hypotheses (confidence of paternity; grandmothering, kin proximity, grandparental senescence, and local resource competition), our meta-analysis results are most in line with the local resource competition hypothesis. In patrilineal and predominantly patrilocal societies, the grandparents who are most likely to live with the grandchildren have a less beneficial association than those who do not. We consider the extent to which these results may be influenced by the methodological limitations of the source studies, including the use of retrospective designs and inadequate controls for confounding variables such as wealth.

Evolutionary contributions to solving the "matrilineal puzzle": a test of Holden, Sear, and Mace's model

Matriliny has long been debated by anthropologists positing either its primitive or its puzzling nature. More recently, evolutionary anthropologists have attempted to recast matriliny as an adaptive solution to modern social and ecological environments, tying together much of what was known to be associated with matriliny. This paper briefly reviews the major anthropological currents in studies of matriliny and discusses the contribution of evolutionary anthropology to this body of literature. It discusses the utility of an evolutionary framework in the context of the first independent test of Holden et al.'s 2003 model of matriliny as daughter-biased investment. It finds that historical daughter-biased transmission of land among the Mosuo is consistent with the model, whereas current income transmission is not. In both cases, resources had equivalent impacts on male and female reproduction, a result which predicts daughter-biased resource transmission given any nonzero level of paternity uncertainty. However, whereas land was transmitted traditionally to daughters, income today is invested in both sexes. Possible reasons for this discrepancy are discussed.

Macro-evolutionary studies of cultural diversity: a review of empirical studies of cultural transmission and cultural adaptation

A growing body of theoretical and empirical research has examined cultural transmission and adaptive cultural behaviour at the individual, within-group level. However, relatively few studies have tried to examine proximate transmission or test ultimate adaptive hypotheses about behavioural or cultural diversity at a between-societies macro-level. In both the history of anthropology and in present-day work, a common approach to examining adaptive behaviour at the macro-level has been through correlating various cultural traits with features of ecology. We discuss some difficulties with simple ecological associations, and then review cultural phylogenetic studies that have attempted to go beyond correlations to understand the underlying cultural evolutionary processes. We conclude with an example of a phylogenetically controlled approach to understanding proximate transmission pathways in Austronesian cultural diversity.