Applying evolutionary anthropology

Prestige and gender role ideology: a study of young Tanzanian men

With the objective of informing theoretical accounts of social learning and gendered conflict, we explore the role of prestige in the formation of men's beliefs about gender in a semi-rural but fast urbanizing community in north-western Tanzania. Using focus groups and participant observation, we contrast the extent to which young men view elders and men from the neighbouring city as prestigious, and the beliefs they ascribe to each category. Elders were viewed as prestigious because of their age and position as preservers and teachers of societal norms. Their prestige was culturally mandated, as evidenced by customs bestowing respect. In contrast, only subcategories of city men were deemed prestigious dependent on individual achievement. Prestige was difficult to distinguish from dominance, as both elders and city men can exert penalties on those with differing views. Elders were viewed as mostly, but not always, unsupportive of women's empowerment, whereas city men were viewed as mostly, but not always, supportive of women's empowerment. We conclude that urbanization shifts the distribution of prestige, exposing individuals to novel sources of social influence. However, future studies should be wary not to oversimplify elders as upholders of patriarchal beliefs and city men as universally supportive of women's empowerment.

Little evidence that nonmonogamous family structures are detrimental to children's well-being in Mpimbwe, Tanzania

Nuclear family structures are often thought to be essential for the well-being of children. Divorce, the loss of either biological parent, the presence of step-parents, and the practice of polygynous marriage have all been claimed to negatively impact child well-being. However, empirical research on these topics has been limited by the routine use of cross-regional and cross-sectional databases. Cross-regional data render research vulnerable to the ecological inference fallacy, and cross-sectional data prevent assessment of age-specific impacts of time-varying family-structure variables. When longitudinal data are available, they tend to be drawn from Western/urban contexts. Detailed data on family structure and children's well-being are rarely collected in more marginalized communities. In many rural and traditional communities, nonnuclear family structures are indeed prevalent and viewed as socially permissible-and, as such, may have different impacts on children's well-being than in Western contexts. Here, we draw on a detailed, longitudinal dataset from a 20-y prospective study in rural Tanzania, where polygyny and serial monogamy are common. We analyze survival outcomes for 3,693 children born between 1931 and 2014, growth outcomes for 881 children born between 1976 and 2014, and educational outcomes for 1,370 children born between 1976 and 2014. Our analyses indicate that monogamous marriage is not consistently associated with better outcomes for children-contrary to some popular and public health perspectives on human family structure.

Child and adolescent foraging: New directions in evolutionary research

Young children and adolescents in subsistence societies forage for a wide range of resources. They often target child-specific foods, they can be very successful foragers, and they share their produce widely within and outside of their nuclear family. At the same time, while foraging, they face risky situations and are exposed to diseases that can influence their immune development. However, children's foraging has largely been explained in light of their future (adult) behavior. Here, we reinterpret findings from human behavioral ecology, evolutionary medicine and cultural evolution to center foraging children's contributions to life history evolution, community resilience and immune development. We highlight the need to foreground immediate alongside delayed benefits and costs of foraging, including inclusive fitness benefits, when discussing children's food production from an evolutionary perspective. We conclude by recommending that researchers carefully consider children's social and ecological context, develop cross-cultural perspectives, and incorporate children's foraging into Indigenous sovereignty discourse.

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'.

Gendered conflict in the human family

Sexual conflict is a thriving area of animal behaviour research. Yet parallel research in the evolutionary human sciences remains underdeveloped and has become mired by controversy. In this special collection, we aim to invigorate the study of fitness-relevant conflicts between women and men, advocating for three synergistic research priorities. First, we argue that a commitment to diversity is required to innovate the field, achieve ethical research practice, and foster fruitful dialogue with neighbouring social sciences. Accordingly, we have prioritised issues of diversity as editors, aiming to stimulate new connections and perspectives. Second, we call for greater recognition that human sex/gender roles and accompanying conflict behaviours are both subject to natural selection and culturally determined. This motivates our shift in terminology from sexual to gendered conflict when addressing human behaviour, countering stubborn tendencies to essentialise differences between women and men and directing attention to the role of cultural practices, normative sanctions and social learning in structuring conflict battlegrounds. Finally, we draw attention to contemporary policy concerns, including the wellbeing consequences of marriage practices and the gendered implications of market integration. Focus on these themes, combined with attendance to the dangers of ethnocentrism, promises to inform culturally sensitive interventions promoting gender equality worldwide.

Sexual minority status is associated with earlier recalled age of menarche: Evidence from the 2005-2016 National Health and Nutrition Examination Survey

INTRODUCTION

Menarcheal timing is associated with growth, development, health, wellbeing, and reproduction across the lifespan. Although sexual orientation is a known correlate of health and developmental inequities, relatively little evolutionarily framed research has investigated sexual orientation-based variation in maturational timing. To improve our understanding of menarcheal timing among sexual minority (SM) people, we use a biocultural-evolutionary life history lens that takes into account the stresses of minoritization to examine the relationship between sexual orientation and self-reported age at menarche in a sample of American adults.

METHODS

Using the U.S. National Health and Nutrition Examination Survey (NHANES), a large, nationally representative dataset (n = 9757), we fit multiple logistic regression models and survival curves to evaluate associations between sexual orientation, indicators of somatic and material resources during adolescence (e.g., education, citizenship, upper arm length), and self-reported menarche.

RESULTS

SM respondents were more likely to report earlier (by 4-5 months) ages of menarche (p < .001). Post-hoc tests revealed that these differences were driven by bisexual (p < .001) and same-sex experienced (p < .001) relative to heterosexual and lesbian/gay respondents. Earlier menarcheal timing among SM respondents persisted after adjusting for socio-demographic factors and proxies of developmental conditions.

DISCUSSION

Our findings reveal that SM status is associated with earlier ages of menarche, an important social and reproductive milestone. We argue that uniting life history theory with the minority stress hypothesis better explains differences in menarcheal timing by sexual orientation than previous paradigms. Investigators should attend to sexual orientation-based variation in maturational timing using holistic, inclusive approaches.

Market integration, income inequality, and kinship system among the Mosuo of China

Increased access to defensible material wealth is hypothesised to escalate inequality. Market integration, which creates novel opportunities in cash economies, provides a means of testing this hypothesis. Using demographic data collected from 505 households among the matrilineal and patrilineal Mosuo in 2017, we test whether market integration is associated with increased material wealth, whether increased material wealth is associated with wealth inequality, and whether being in a matrilineal vs. patrilineal kinship system alters the relationship between wealth and inequality. We find evidence that market integration, measured as distance to the nearest source of tourism and primary source of household income, is associated with increased household income and 'modern' asset value. Both village-level market integration and mean asset value were associated negatively, rather than positively, with inequality, contrary to predictions. Finally, income, modern wealth and inequality were higher in matrilineal communities that were located closer to the centre of tourism and where tourism has long provided a relatively stable source of income. However, we also observed exacerbated inequality with increasing farm animal value in patriliny. We conclude that the forces affecting wealth and inequality depend on local context and that the importance of local institutions is obscured by aggregate statistics drawn from modern nation states.

Integrating economic and evolutionary approaches to polygynous marriage

We outline the potential for integrating economic and evolutionary approaches to marriage and the family. Our broad argument is that the approaches share a concern for competition. Evolutionary scholars are concerned with the fitness consequences of competition and economists are centrally concerned with the nature of competition: how the allocation of scarce resources is mediated by potentially complex forms of social interaction and conflicts of interest. We illustrate our argument by focusing on conceptual and empirical approaches to a topic of interest to economists and evolutionary scholars: polygynous marriage. In comparing conceptual approaches, we distinguish between those that emphasise the physical environment and those that emphasise the social environment. We discuss some advantages of analysing marriage through the lens of competitive markets, and outline some of the ways that economists analyse the emergence of rules governing the family. In discussing empirical approaches to polygynous marriage, we describe how a concern for informing contemporary policy leads economists to focus on the consequences of polygyny, and in particular we describe some of the ways in which economists attempt to distinguish causal effects from selection effects.

A Life History Perspective on Athletes with Low Energy Availability

The energy costs of athletic training can be substantial, and deficits arising from costs unmet by adequate energy intake, leading to a state of low energy availability, may adversely impact athlete health and performance. Life history theory is a branch of evolutionary theory that recognizes that the way the body uses energy-and responds to low energy availability-is an evolved trait. Energy is a finite resource that must be distributed throughout the body to simultaneously fuel all biological processes. When energy availability is low, insufficient energy may be available to equally support all processes. As energy used for one function cannot be used for others, energetic "trade-offs" will arise. Biological processes offering the greatest immediate survival value will be protected, even if this results in energy being diverted away from others, potentially leading to their downregulation. Athletes with low energy availability provide a useful model for anthropologists investigating the biological trade-offs that occur when energy is scarce, while the broader conceptual framework provided by life history theory may be useful to sport and exercise researchers who investigate the influence of low energy availability on athlete health and performance. The goals of this review are: (1) to describe the core tenets of life history theory; (2) consider trade-offs that might occur in athletes with low energy availability in the context of four broad biological areas: reproduction, somatic maintenance, growth, and immunity; and (3) use this evolutionary perspective to consider potential directions for future research.

How social learning shapes the efficacy of preventative health behaviors in an outbreak

The global pandemic of COVID-19 revealed the dynamic heterogeneity in how individuals respond to infection risks, government orders, and community-specific social norms. Here we demonstrate how both individual observation and social learning are likely to shape behavioral, and therefore epidemiological, dynamics over time. Efforts to delay and reduce infections can compromise their own success, especially when disease risk and social learning interact within sub-populations, as when people observe others who are (a) infected and/or (b) socially distancing to protect themselves from infection. Simulating socially-learning agents who observe effects of a contagious virus, our modelling results are consistent with with 2020 data on mask-wearing in the U.S. and also concur with general observations of cohort induced differences in reactions to public health recommendations. We show how shifting reliance on types of learning affect the course of an outbreak, and could therefore factor into policy-based interventions incorporating age-based cohort differences in response behavior.

A selection pressure landscape for 870 human polygenic traits

Characterizing the natural selection of complex traits is important for understanding human evolution and both biological and pathological mechanisms. We leveraged genome-wide summary statistics for 870 polygenic traits and attempted to quantify signals of selection on traits of different forms in European ancestry across four periods in human history and evolution. We found that 88% of these traits underwent polygenic change in the past 2,000-3,000 years. Recent selection was associated with ancient selection signals in the same trait. Traits related to pigmentation, body measurement and nutritional intake exhibited strong selection signals across different time scales. Our findings are limited by our use of exclusively European data and the use of genome-wide association study data, which identify associations between genetic variants and phenotypes that may not be causal. In sum, we provide an overview of signals of selection on human polygenic traits and their characteristics across human evolution, based on a European subset of human genetic diversity. These findings could serve as a foundation for further populational and medical genetic studies.

Contributions of evolutionary anthropology to understanding climate-induced human migration

Humans are able to thrive in a multitude of ecological and social environments, including varied environments over an individual lifetime. Migration-leaving one place of residence for another-is a central feature of many people's life histories, and environmental change goes hand-in-hand with migration, both in terms of cause and consequence. Climate change has amplified this connection between environment and migration, with the potential to profoundly impact millions of lives. Although climate-induced migration has been at the forefront of other disciplines in the social sciences, evolutionary anthropologists (EAs) have given it little attention. In this paper we draw upon existing literature and contribute our EA perspective to present a framework for analyzing climate-induced migration that utilizes theoretical approaches from a variety of social science disciplines. We focus on three overlapping dimensions-time, space, and severity-relevant to understanding the impact of climate change on human migration. We apply this framework to case studies from North America of people impacted by climate change and extreme weather events, including hurricanes, droughts, rising sea-levels, and wildfires. We also consider how access to both economic and social resources influence decisions regarding migration. Research focused on climate-induced human migration can benefit equally from the addition of EA perspectives and a more interdisciplinary theoretical approach.

Married Too Young? The Behavioral Ecology of ‘Child Marriage’

For girls and women, marriage under 18 years is commonplace in many low-income nations today and was culturally widespread historically. Global health campaigns refer to marriage below this threshold as ‘child marriage’ and increasingly aim for its universal eradication, citing its apparent negative wellbeing consequences. Here, we outline and evaluate four alternative hypotheses for the persistence of early marriage, despite its associations with poor wellbeing, arising from the theoretical framework of human behavioral ecology. First, early marriage may be adaptive (e.g., it maximizes reproductive success), even if detrimental to wellbeing, when life expectancy is short. Second, parent–offspring conflict may explain early marriage, with parents profiting economically at the expense of their daughter’s best interests. Third, early marriage may be explained by intergenerational conflict, whereby girls marry young to emancipate themselves from continued labor within natal households. Finally, both daughters and parents from relatively disadvantaged backgrounds favor early marriage as a ‘best of a bad job strategy’ when it represents the best option given a lack of feasible alternatives. The explanatory power of each hypothesis is context-dependent, highlighting the complex drivers of life history transitions and reinforcing the need for context-specific policies addressing the vulnerabilities of adolescence worldwide.

He for she? Variation and exaggeration in men's support for women's empowerment in northern Tanzania

Achieving gender equality fundamentally requires a transfer of power from men to women. Yet data on men's support for women's empowerment (WE) remains scant and limited by reliance on self-report methodologies. Here, we examine men's support for WE as a sexual conflict trait, both via direct surveys (n = 590) and indirectly by asking men's wives (n = 317) to speculate on their husband's views. Data come from a semi-urban community in Mwanza, Tanzania. Consistent with reduced resource competition and increased exposure to relatively egalitarian gender norms, higher socioeconomic status predicted greater support for WE. However, potential demographic indicators of sexual conflict (high fertility, polygyny, large spousal age gap) were largely unrelated to men's support for WE. Contrasting self- and wife-reported measures suggests that men frequently exaggerate their support for women in self-reported attitudes. Discrepancies were especially pronounced among men claiming the highest support for WE, but smallest among men who held a professional occupation and whose wife participated in wage labour, indicating that these factors predict genuine support for WE. We discuss the implications of these results for our understanding of both individual variation and patriarchal gender norms, emphasising the benefits of greater exchange between the evolutionary human sciences and global health research on these themes.

How can evolutionary and biological anthropologists engage broader audiences?

OBJECTIVES

With our diverse training, theoretical and empirical toolkits, and rich data, evolutionary and biological anthropologists (EBAs) have much to contribute to research and policy decisions about climate change and other pressing social issues. However, we remain largely absent from these critical, ongoing efforts. Here, we draw on the literature and our own experiences to make recommendations for how EBAs can engage broader audiences, including the communities with whom we collaborate, a more diverse population of students, researchers in other disciplines and the development sector, policymakers, and the general public. These recommendations include: (1) playing to our strength in longitudinal, place-based research, (2) collaborating more broadly, (3) engaging in greater public communication of science, (4) aligning our work with open-science practices to the extent possible, and (5) increasing diversity of our field and teams through intentional action, outreach, training, and mentorship.

CONCLUSIONS

We EBAs need to put ourselves out there: research and engagement are complementary, not opposed to each other. With the resources and workable examples we provide here, we hope to spur more EBAs to action.

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'.

Value of species and the evolution of conservation ethics

The theory of evolution by natural selection can help explain why people care about other species. Building upon recent insights that morality evolves to secure fitness advantages of cooperation, we propose that conservation ethics (moral beliefs, attitudes, intuitions and norms regarding other species) could be adaptations that support cooperation between humans and non-humans. We present eco-evolutionary cost-benefit models of conservation behaviours as interspecific cooperation (altruism towards members of other species). We find that an evolutionary rule identical in structure to Hamilton's rule (which explains altruistic behaviour towards related conspecifics) can explain altruistic behaviour towards members of other species. Natural selection will favour traits for selectively altering the success of members of other species (e.g. conserving them) in ways that maximize inclusive fitness return benefits. Conservation behaviours and the ethics that evolve to reinforce them will be sensitive to local ecological and socio-cultural conditions, so will assume different contours in different places. Difficulties accurately assessing costs and benefits provided by other species, time required to adapt to ecological and socio-cultural change and barriers to collective action could explain the apparent contradiction between the widespread existence of conservation ethics and patterns of biodiversity decline globally.

Modernizing Evolutionary Anthropology : Introduction to the Special Issue

Evolutionary anthropology has traditionally focused on the study of small-scale, largely self-sufficient societies. The increasing rarity of these societies underscores the importance of such research yet also suggests the need to understand the processes by which such societies are being lost-what we call "modernization"-and the effects of these processes on human behavior and biology. In this article, we discuss recent efforts by evolutionary anthropologists to incorporate modernization into their research and the challenges and rewards that follow. Advantages include that these studies allow for explicit testing of hypotheses that explore how behavior and biology change in conjunction with changes in social, economic, and ecological factors. In addition, modernization often provides a source of "natural experiments" since it may proceed in a piecemeal fashion through a population. Challenges arise, however, in association with reduced variability in fitness proxies such as fertility, and with the increasing use of relatively novel methodologies in evolutionary anthropology, such as the analysis of secondary data. Confronting these challenges will require careful consideration but will lead to an improved understanding of humanity. We conclude that the study of modernization offers the prospect of developing a richer evolutionary anthropology, by encompassing ultimate and proximate explanations for behavior expressed across the full range of human societies.

Evaluating reproductive decisions as discrete choices under social influence

Discrete choice, coupled with social influence, plays a significant role in evolutionary studies of human fertility, as investigators explore how and why reproductive decisions are made. We have previously proposed that the relative magnitude of social influence can be compared against the transparency of pay-off, also known as the transparency of a decision, through a heuristic diagram that maps decision-making along two axes. The horizontal axis represents the degree to which an agent makes a decision individually versus one that is socially influenced, and the vertical axis represents the degree to which there is transparency in the pay-offs and risks associated with the decision the agent makes. Having previously parametrized the functions that underlie the diagram, we detail here how our estimation methods can be applied to real-world datasets concerning sexual health and contraception.

Parental investment in child health in sub-Saharan Africa: a cross-national study of health-seeking behaviour

Parents face trade-offs between investing in child health and other fitness enhancing activities. In humans, parental investment theory has mostly been examined through the analysis of differential child outcomes, with less emphasis on the actions parents take to further a particular offspring's condition. Here, we make use of household data on health-seeking for children in a high mortality context where such behaviours are crucial for offspring survival. Using Demographic and Health Survey (DHS) data from 17 sub-Saharan African countries, we examine whether maternal factors (age, health, marital status) and child factors (birth order, health, sex, age) independently influence parental investment in health-seeking behaviours: two preventative behaviours (malaria net use and immunization) and two curative ones (treating fever and diarrhoea). Results indicate that children with lower birth order, older mothers and mothers with better health status have higher odds of investment. The effects of a child's sex and health status and whether the mother is polygynously married vary depending on the type of health-seeking behaviour (preventative versus curative). We discuss how these results square with predictions from parental investment theory pertaining to the state of mothers and children, and reflect on some potential mechanisms and directions for future research.

Obstacles and catalysts of cooperation in humans, bonobos, and chimpanzees: behavioural reaction norms can help explain variation in sex roles, inequality, war and peace

Our closest living relatives, bonobos and chimpanzees, along with small-scale human societies figure prominently in debates about human nature. Here we emphasize and explain behavioural variation within and among these three species. In the logic of behavioural ecology, individuals have been selected to adjust their behaviour along evolved reaction norms that maximize fitness given current socio-ecological conditions. We discuss variation in three behavioural contexts: relationships between the sexes, hierarchy and inequality, and intergroup interactions. In each context, behavioural variation can be related to two broad socio-ecological conditions: (i) the defensibility of contested resources, and (ii) differences in bargaining power. When defensibility of resources and differences in bargaining power are great, interactions are rife with conflict; when they are minimal, interactions are more harmonious. These socio-ecological conditions therefore constitute key catalysts and obstacles of cooperation. We conclude that human nature should be seen as consisting of evolved reaction norms.

Collective behaviour, uncertainty and environmental change

A central aspect of cultural evolutionary theory concerns how human groups respond to environmental change. Although we are painting with a broad brush, it is fair to say that prior to the twenty-first century, adaptation often happened gradually over multiple human generations, through a combination of individual and social learning, cumulative cultural evolution and demographic shifts. The result was a generally resilient and sustainable population. In the twenty-first century, however, considerable change happens within small portions of a human generation, on a vastly larger range of geographical and population scales and involving a greater degree of horizontal learning. As a way of gauging the complexity of societal response to environmental change in a globalized future, we discuss several theoretical tools for understanding how human groups adapt to uncertainty. We use our analysis to estimate the limits of predictability of future societal change, in the belief that knowing when to hedge bets is better than relying on a false sense of predictability.

No evidence that polygynous marriage is a harmful cultural practice in northern Tanzania

Polygyny is cross-culturally common and a topic of considerable academic and policy interest, often deemed a harmful cultural practice serving the interests of men contrary to those of women and children. Supporting this view, large-scale studies of national African demographic surveys consistently demonstrate that poor child health outcomes are concentrated in polygynous households. Negative population-level associations between polygyny and well-being have also been reported, consistent with the hypothesis that modern transitions to socially imposed monogamy are driven by cultural group selection. We challenge the consensus view that polygyny is harmful, drawing on multilevel data from 56 ethnically diverse Tanzanian villages. We first demonstrate the vulnerability of aggregated data to confounding between ecological and individual determinants of health; while across villages polygyny is associated with poor child health and low food security, such relationships are absent or reversed within villages, particularly when children and fathers are coresident. We then provide data indicating that the costs of sharing a husband are offset by greater wealth (land and livestock) of polygynous households. These results are consistent with models of polygyny based on female choice. Finally, we show that village-level negative associations between polygyny prevalence, food security, and child health are fully accounted for by underlying differences in ecological vulnerability (rainfall) and socioeconomic marginalization (access to education). We highlight the need for improved, culturally sensitive measurement tools and appropriate scales of analysis in studies of polygyny and other purportedly harmful practices and discuss the relevance of our results to theoretical accounts of marriage and contemporary population policy.