Choosy but not chaste: multiple mating in human females

Sex ratios and gender norms: why both are needed to understand sexual conflict in humans

Sexual conflict theory has been successfully applied to predict how in non-human animal populations, sex ratios can lead to conflicting reproductive interests of females and males and affect their bargaining positions in resolving such conflicts of interests. Recently this theory has been extended to understand the resolution of sexual conflict in humans, but with mixed success. We argue that an underappreciation of the complex relationship between gender norms and sex ratios has hampered a successful understanding of sexual conflict in humans. In this paper, we review and expand upon existing theory to increase its applicability to humans, where gender norms regulate sex ratio effects on sexual conflict. Gender norms constrain who is on the marriage market and how they are valued, and may affect reproductive decision-making power. Gender norms can also directly affect sex ratios, and we hypothesize that they structure how individuals respond to market value gained or lost through biased sex ratios. Importantly, gender norms are in part a product of women's and men's sometimes conflicting reproductive interests, but these norms are also subject to other evolutionary processes. An integration of sexual conflict theory and cultural evolutionary theory is required to allow for a full understanding of sexual conflict in humans.

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'.

A matter of time: Bateman's principles and mating success as count and duration across social strata in contemporary Finland

Bateman's principles heavily influence the understanding of human reproductive behaviour. Yet, few rigorous studies on Bateman's principles in contemporary industrialized populations exist. Most studies use small samples, exclude non-marital unions, and disregard recent insights on within-population heterogeneity in mating strategies. We assess mating success and reproductive success using population-wide Finnish register data on marital and non-marital cohabitations and fertility. We examine variability across social strata in the Bateman principles and analyse the mate count, the cumulated duration with a mate, and the association with reproductive success. Results support Bateman's first and second principles. Regarding Bateman's third principle, the number of mates is more positively associated with reproductive success for men than women, but this association is driven by ever having a mate. Having more than one mate is on average associated with lower reproductive success. However, for men in the lowest income quartile, having more than one mate positively predicts reproductive success. Longer union duration is associated with higher reproductive success, and more so for men. We note that sex differences in the relationship between mating success and reproductive success differ by social strata, and argue that mate duration may be an important component of mating success alongside mate count.

Only Death Will Separate Us: The Role of Extramarital Partnerships among Himba Pastoralists

Extramarital partnerships are highly stigmatized in many societies and are typically excluded from studies of family dynamics and social support. Nevertheless, in many societies such relationships are common and can have important impacts on resource security and health outcomes. However, current studies of these relationships come mainly from ethnographic studies, with quantitative data extremely rare. Here we present data from a 10-year study of romantic partnerships among a community of Himba pastoralists in Namibia, where concurrency is common. The majority of married men (97%) and women (78%) currently reported having more than one partner (n = 122). Using multilevel models comparing marital and nonmarital relationships, we found that, contrary to conventional wisdom surrounding concurrency, Himba form enduring bonds with extramarital partners that often last decades and are very similar to marital ones in terms of length, emotional affect, reliability, and future prospects. Qualitative interview data showed that extramarital relationships were imbued with a set of rights and obligations that, while distinct from those of spouses, provide an important source of support. Greater inclusion of these relationships in studies of marriage and family would provide a clearer picture of social support and resource transfers in these communities and help to explain variation in the practice and acceptance of concurrency around the world.

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.

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.

Theoretical constraints: Science, caretaking, and the creation of normative ideals

OBJECTIVES

This review explores the dualism in evolutionary anthropology that both acknowledges a broad range of familial caretaking strategies, while also remaining tethered to theories scaffolded around notions of selfish genes that constrain our understanding of who provides adequate kin care. I examine the process of norm creation in the sciences by investigating how theory may limit which data are collected and how those data are interpreted.

METHODS

This paper serves as a literature review and critique of prominent biological, evolutionary, and psychological conceptualizations of parental investment and caretaking in humans, and how these studies shape what is considered normal behavior in scientific literature.

RESULTS

Quantification, assessment, and theory building in evolutionary anthropology, and an oversampling of WEIRD communities in other disciplines, have limited our understanding of what constitutes both evolutionarily adaptive behaviors, and culturally specific human behaviors.

CONCLUSIONS

A synthetic theoretical model of behavioral norms in childrearing must account for an exchange of psycho-social and cultural resources and skills, the transfer of energetic reserves via gestation and lactation, and the indirect benefits of genetic inheritance. The emphasis on tailoring data collection to fit evolutionary theories of the family has limited our ability to understand the diverse proximate mechanisms that humans employ in taking care of kin as biocultural reproducers.

Opportunities and constraints in women's resource security amid climate change: A case study of arid-living Namibian agro-pastoralists

OBJECTIVE

We describe the composition and variation of women's resource strategies in an arid-living Southern African agro-pastoralist society to gain insights into adaptation to climate-change-induced increased aridity.

METHODS

Using cross-sectional data from 210 women collected in 2009 across 28 agro-pastoralist villages in Kaokoveld Namibia, we conducted principal-component (PC) analysis of resource variables and constructed profiles of resource strategies from the major PCs. Next, we explored associations between key resource strategies and demographic measures and fitness proxies.

RESULTS

The first two PCs accounted for 43% of women's overall resource variation. PC1 reflects women's ability to access market resources via livestock trading, while PC2 captured women's direct food access. We found that market strategies were more common among married women and less common among women who have experienced child mortality. Women with higher subsistence security were more likely to be from the OvaHimba tribe and had a higher risk of gonorrhea exposure. We also qualitatively explored drought-induced pressure on women's livestock. Finally, we show that sexual networks were attenuated during drought, indicating strain on social support.

CONCLUSIONS

Our results highlight how agro-pastoralist women manage critical resources in unpredictable environments, and how resource strategies distribute among the women in our study. Goats as a commodity to obtain critical resources suggests that some women have flexibility during drought when gardens fail and cattle die. However, increased aridity and drought may eventually overwhelm husbandry practices in this region.

The Role of Spousal Separation on Norms Related to Gender and Sexuality among Himba Pastoralists

The gender-specific labor demands of arid pastoralism often lead to spousal separation. Men typically respond in one of two ways: engage in mate guarding tactics, or loosen restrictions on female sexuality. Among Himba pastoralists in northwest Namibia, the latter strategy is dominant. Rooted in a history of matriliny, Himba have strong norms promoting female sexual autonomy. We propose that these conditions, combined with a stochastic resource base, have led to women utilizing a combination of formal and informal partnerships to meet their needs and the needs of their children. Aspects of Himba socioecology also increase the costs of mate guarding for men and lower the costs of extra-pair paternity, further bolstering a concurrency strategy. Using a mix of quantitative and qualitative data, we show how spousal separation, female autonomy, and concurrency are linked, and suggest that in this harsh environment having a mix of formal and informal romantic partners may be less costly and more beneficial than a system of monogamous marriage.

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.

Balancing costs and benefits in primates: ecological and palaeoanthropological views

Maintaining the balance between costs and benefits is challenging for species living in complex and dynamic socio-ecological environments, such as primates, but also crucial for shaping life history, reproductive and feeding strategies. Indeed, individuals must decide to invest time and energy to obtain food, services and partners, with little direct feedback on the success of their investments. Whereas decision-making relies heavily upon cognition in humans, the extent to which it also involves cognition in other species, based on their environmental constraints, has remained a challenging question. Building mental representations relating behaviours and their long-term outcome could be critical for other primates, but there are actually very little data relating cognition to real socio-ecological challenges in extant and extinct primates. Here, we review available data illustrating how specific cognitive processes enable(d) modern primates and extinct hominins to manage multiple resources (e.g. food, partners) and to organize their behaviour in space and time, both at the individual and at the group level. We particularly focus on how they overcome fluctuating and competing demands, and select courses of action corresponding to the best possible packages of potential costs and benefits in reproductive and foraging contexts. This article is part of the theme issue 'Existence and prevalence of economic behaviours among non-human primates'.

Beyond the Household: Caribbean Families and Biocultural Models of Alloparenting

Alloparental or extramaternal care is an integral aspect of human childrearing. This behavior has been explored both as an extension of the primary mother–infant dyad that evolved to meet the demands of altricial offspring and as an economic exchange of energy and resources. Much of this research centers on foraging or small-scale communities and positions the household as the central unit through which to explore negotiations of care. In this review, I use evidence from Black Caribbean communities living in industrialized countries to challenge the broad applicability of the analytical model of the bounded household and to question whether our current articulations of theory and empirical assessments of extramaternal care are well suited to investigations of these behaviors in the vast majority of contemporary human populations. Alloparental practices in the Caribbean reflect dynamic responses to maternal migration and the local influence of global labor markets. The children who remain at home experience variability in the care received from their surrogate parents. The dynamic aspect of the care practices enacted by these transnational families reveals the behavioral flexibility that has been integral to human survival.

Resource demands reduce partner discrimination in Himba women

Where autonomy for partner choice is high, partner preferences may be shaped by both social and ecological conditions. In particular, women's access to resources can influence both the type and number of partnerships she engages in. However, most existing data linking resources and partner choice rely on either priming effects or large demographic databases, rather than preferences for specific individuals. Here we leverage a combination of demographic data, food insecurity scores and trait and partner preference ratings to determine whether resource security modulates partner preferences among Himba pastoralists. We find that while food insecurity alone has a weak effect on women's openness to new partners, the interaction of food insecurity and number of dependent children strongly predicts women's openness to potential partners. Further, we show that women who have more dependants have stronger preferences for wealthy and influential men. An alternative hypothesis derived from mating-market dynamics, that female desirability affects female preferences, had no effect. Our data show that women who face greater resource constraints are less discriminating in the number of partners they are open to, and have stronger preferences for resource-related traits. These findings highlight the importance of ecological signals in explaining the plasticity of mate preferences.

Papa Don't Preach? : Using Lies to Expose the Truth about Who Suppresses Female Sexuality

The suppression of sexuality is culturally widespread, and women's sexual promiscuity, activity, and enjoyment are almost always judged and punished more harshly than men's. It remains disputed, however, to what end people suppress sexuality, and who benefits from the suppression of female sexuality. Different theories predict that women in general, men in general, women's intimate partners, or parents benefit most. Here we use the lies women and men tell-or imagine telling-about their sexual histories as an indirect measure of who is most involved in the suppression of sexuality. We asked men and women what they would reply if asked questions by their mother, father, current partner, attractive confederate, and various same- or opposite-sex friends and colleagues about their number of previous sex partners, age at first romantic kiss, age at first consensual sex, and cheating on a previous partner or spouse. By comparing the size and direction of the lies that subjects told, we tested competing predictions of several cultural and evolutionary theories concerning why female sexuality is suppressed and who is driving its suppression. We found that men and women told larger and more frequent lies to their parents, with women telling the largest and most frequent lies of all to their fathers. Additionally, the majority of lies by both men and women were in sexually conservative directions. Our findings suggest that mothers, and especially fathers, restrict female sexuality.

The psychology of breakup sex: Exploring the motivational factors and affective consequences of post-breakup sexual activity

Popular culture has recently publicized a seemingly new postbreakup behavior called breakup sex. While the media expresses the benefits of participating in breakup sex, there is no research to support these claimed benefits. The current research was designed to begin to better understand this postbreakup behavior. In the first study, we examined how past breakup sex experiences made the individuals feel and how people predict they would feel in the future (n = 212). Results suggested that men are more likely than women to have felt better about themselves, while women tend to state they felt better about the relationship after breakup sex. The second study (n = 585) investigated why men and women engage in breakup sex. Results revealed that most breakup sex appears to be motivated by three factors: relationship maintenance, hedonism, and ambivalence. Men tended to support hedonistic and ambivalent reasons for having breakup sex more often than women. The two studies revealed that breakup sex may be differentially motivated (and may have different psychological consequences) for men and women and may not be as beneficial as the media suggests.

High rate of extrapair paternity in a human population demonstrates diversity in human reproductive strategies

Among nonhuman species, social monogamy is rarely accompanied by complete fidelity. Evolutionary theory predicts that the rate of extrapair paternity (EPP) should vary according to socioecological conditions. In humans, however, geneticists contend that EPP is negligible and relatively invariable. This conclusion is based on a limited set of studies, almost all of which describe European-descent groups. Using a novel, double-blind method designed in collaboration with a community of Himba pastoralists, we find that the rate of EPP in this population is 48%, with 70% of couples having at least one EPP child. Both men and women were very accurate at detecting cases of EPP. These data suggest that the range of variation in EPP across human populations is substantially greater than previously thought. We further show that a high rate of EPP can be accompanied by high paternity confidence, which highlights the importance of disaggregating EPP from the notion of "cuckoldry."

The ethics and logistics of field-based genetic paternity studies

The rapidly decreasing costs of generating genetic data sequencing and the ease of new DNA collection technologies have opened up new opportunities for anthropologists to conduct field-based genetic studies. An exciting aspect of this work comes from linking genetic data with the kinds of individual-level traits evolutionary anthropologists often rely on, such as those collected in long-term demographic and ethnographic studies. However, combining these two types of data raises a host of ethical questions related to the collection, analysis and reporting of such data. Here we address this conundrum by examining one particular case, the collection and analysis of paternity data. We are particularly interested in the logistics and ethics involved in genetic paternity testing in the localized settings where anthropologists often work. We discuss the particular issues related to paternity testing in these settings, including consent and disclosure, consideration of local identity and beliefs and developing a process of continued community engagement. We then present a case study of our own research in Namibia, where we developed a multi-tiered strategy for consent and community engagement, built around a double-blind procedure for data collection, analysis and reporting.

Paternity testing in anthropology raises ethical and methodological issues. We summarize these and describe a novel double-blind method used in our work.

Family Matters: Rethinking the Psychology of Human Social Motivation

What motives do people prioritize in their social lives? Historically, social psychologists, especially those adopting an evolutionary perspective, have devoted a great deal of research attention to sexual attraction and romantic-partner choice (mate seeking). Research on long-term familial bonds (mate retention and kin care) has been less thoroughly connected to relevant comparative and evolutionary work on other species, and in the case of kin care, these bonds have been less well researched. Examining varied sources of data from 27 societies around the world, we found that people generally view familial motives as primary in importance and mate-seeking motives as relatively low in importance. Compared with other groups, college students, single people, and men place relatively higher emphasis on mate seeking, but even those samples rated kin-care motives as more important. Furthermore, motives linked to long-term familial bonds are positively associated with psychological well-being, but mate-seeking motives are associated with anxiety and depression. We address theoretical and empirical reasons why there has been extensive research on mate seeking and why people prioritize goals related to long-term familial bonds over mating goals. Reallocating relatively greater research effort toward long-term familial relationships would likely yield many interesting new findings relevant to everyday people’s highest social priorities.

Unpacking mating success and testing Bateman's principles in a human population

Human marriage systems, characterized by long-term partnerships and extended windows of parental care, differ from the mating systems of pulsed or seasonally breeding non-human animals in which Bateman’s principles were originally tested. These features, paradigmatic of but not unique to humans, complicate the accurate measurement of mating success in evaluating Bateman’s three principles. Here, we unpack the concept of mating success into distinct components: number of partners, number of years partnered, the timing of partnerships, and the quality of partners. Drawing on longitudinal records of marriage and reproduction collected in a natural-fertility East African population over a 20-year period, we test and compare various models of the relationship between mating success and reproductive success (RS), and show that an accurate assessment of male and female reproductive behaviour requires consideration of all major components of mating success. Furthermore, we demonstrate that while Bateman’s third principle holds when mating success is defined in terms of years married, women’s fitness increases whereas men’s fitness decreases from an increase in the number of marriage partners, holding constant the total effective duration of marriages. We discuss these findings in terms of the distinct, sex-specific pathways through which RS can be optimized, and comment on the contribution of this approach to the broader study of sexual selection.

Patterns of paternal investment predict cross-cultural variation in jealous response

Long-lasting, romantic partnerships are a universal feature of human societies, but almost as ubiquitous is the risk of instability when one partner strays. Jealous response to the threat of infidelity is well studied, but most empirical work on the topic has focused on a proposed sex difference in the type of jealousy (sexual or emotional) that men and women find most upsetting, rather than on how jealous response varies^1,2. This stems in part from the predominance of studies using student samples from industrialized populations, which represent a relatively homogenous group in terms of age, life history stage and social norms^3,4. To better understand variation in jealous response, we conducted a 2-part study in 11 populations (1,048 individuals). In line with previous work, we find a robust sex difference in the classic forced-choice jealousy task. However, we also show substantial variation in jealous response across populations. Using parental investment theory, we derived several predictions about what might trigger such variation. We find that greater paternal investment and lower frequency of extramarital sex are associated with more severe jealous response. Thus, partner jealousy appears to be a facultative response, reflective of the variable risks and costs of men's investment across societies.

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'.

Variation among populations in the immune protein composition of mother's milk reflects subsistence pattern

LAY SUMMARY

Adaptive immune proteins in mothers' milk are more variable than innate immune proteins across populations and subsistence strategies. These results suggest that the immune defenses in milk are shaped by a mother's environment throughout her life.

BACKGROUND AND OBJECTIVES

Mother's milk contains immune proteins that play critical roles in protecting the infant from infection and priming the infant's developing immune system during early life. The composition of these molecules in milk, particularly the acquired immune proteins, is thought to reflect a mother's immunological exposures throughout her life. In this study, we examine the composition of innate and acquired immune proteins in milk across seven populations with diverse disease and cultural ecologies.

METHODOLOGY

Milk samples (n = 164) were collected in Argentina, Bolivia, Nepal, Namibia, Philippines, Poland and the USA. Populations were classified as having one of four subsistence patterns: urban-industrialism, rural-shop, horticulturalist-forager or agro-pastoralism. Milk innate (lactalbumin, lactoferrin and lysozyme) and acquired (Secretory IgA, IgG and IgM) protein concentrations were determined using triple-quadrupole mass spectrometry.

RESULTS

Both innate and acquired immune protein composition in milk varied among populations, though the acquired immune protein composition of milk differed more among populations. Populations living in closer geographic proximity or having similar subsistence strategies (e.g. agro-pastoralists from Nepal and Namibia) had more similar milk immune protein compositions. Agro-pastoralists had different milk innate immune protein composition from horticulturalist-foragers and urban-industrialists. Acquired immune protein composition differed among all subsistence strategies except horticulturist-foragers and rural-shop.

CONCLUSIONS AND IMPLICATIONS

Our results reveal fundamental variation in milk composition that has not been previously explored in human milk research. Further study is needed to understand what specific aspects of the local environment influence milk composition and the effects this variation may have on infant health outcomes.

Remoteness influences access to sexual partners and drives patterns of viral sexually transmitted infection prevalence among nomadic pastoralists

Sexually transmitted infections (STIs) comprise a significant portion of the infectious-disease burden among rural people in the Global South. Particular characteristics of ruralness-low-density settlements and poor infrastructure-make healthcare provision difficult, and remoteness, typically a characteristic of ruralness, often compounds the difficultly. Remoteness may also accelerate STI transmission, particularly that of viral STIs, through formation of small, highly connected sexual networks through which pathogens can spread rapidly, especially when partner concurrency is broadly accepted. Herein, we explored the effect of remoteness on herpes simplex virus type-2 (HSV-2) epidemiology among semi-nomadic pastoralists in northwestern (Kaokoveld) Namibia, where, in 2009 we collected HSV-2-specific antibody status, demographic, sexual network, and travel data from 446 subjects (women = 213, men = 233) in a cross-sectional study design. HSV-2 prevalence was high overall in Kaokoveld (>35%), but was heterogeneously distributed across locally defined residential regions: some regions had significantly higher HSV-2 prevalence (39-48%) than others (21-33%). Using log-linear models, we asked the following questions: 1) Are sexual contacts among people in high HSV-2-prevalence regions more likely to be homophilous (i.e., from the same region) than those among people from low-prevalence regions? 2) Are high-prevalence regions more "functionally" remote, in that people from those regions are more likely to travel within their own region than outside, compared to people from other regions? We found that high-prevalence regions were more sexually homophilous than low-prevalence regions and that those regions also had higher rates of within-region travel than the other regions. These findings indicate that remoteness can create contact structures for accelerated STI transmission among people who are already disproportionately vulnerable to consequences of untreated STIs.

Nonpaternity and Half-Siblingships as Objective Measures of Extramarital Sex: Mathematical Modeling and Simulations

Background

Understanding the epidemiology of HIV and other sexually transmitted infections (STIs) requires knowledge of sexual behavior, but self-reported behavior has limitations. We explored the reliability and validity of nonpaternity and half-siblings ratios as biomarkers of current and past extramarital sex.

Methods

An individual-based Monte Carlo simulation model was constructed to describe partnering and conception in human populations with a focus on Sub-Saharan Africa (SSA). The model was parameterized with representative biological, behavioral, and demographic data.

Results

Nonpaternity and half-siblings ratios were strongly correlated with extramarital sex, with Pearson correlation coefficients (PCC) of 0.79 (95% CI: 0.71–0.86) and 0.77 (0.68–0.84), respectively. Age-specific nonpaternity ratios correlated with past extramarital sex at time of conception for different scenarios: for example, PCC, after smoothing by moving averages, was 0.75 (0.52–0.89) in a scenario of steadily decreasing nonmarital sex and 0.39 (0.01–0.73) in a scenario of transient drops in nonmarital sex. Simulations assuming self-reported levels of extramarital sex from Kenya yielded nonpaternity levels lower than global nonpaternity data, suggesting sizable underreporting of extramarital sex.

Conclusions

Nonpaternity and half-siblings ratios are useful objective measures of extramarital sex that avoid limitations in self-reported sexual behavior.

Concentrations of trace elements in human milk: Comparisons among women in Argentina, Namibia, Poland, and the United States

Human milk contains essential micronutrients for growth and development during early life. Environmental pollutants, such as potentially toxic metals, can also be transferred to the infant through human milk. These elements have been well-studied, but changing diets and environments and advances in laboratory technology require re-examining these elements in a variety of settings. The aim of this study was to characterize the concentrations of essential and toxic metals in human milk from four diverse populations. Human milk samples (n = 70) were collected in Argentina (n = 21), Namibia (n = 6), Poland (n = 23), and the United States (n = 20) using a standardized mid-feed collection procedure. Milk concentrations of calcium, zinc, iron, copper, manganese, lead, arsenic, and cadmium were determined using inductively coupled plasma mass spectrometry (ICP-MS). We used standard multiple linear regression models to evaluate differences among populations, while including infant age, infant sex, and maternal parity status (multiparous or primiparous) as covariates. Concentrations of all elements, except zinc, varied across populations after controlling for infant age, infant sex, and maternal parity. Calcium and magnesium showed more differences across populations than iron or copper. There were no significant differences among population in zinc concentrations. Mean concentrations of lead, but not arsenic, were low compared to recently published values from other populations. The concentrations of trace elements in human milk are variable among populations. Limitations due to small sample sizes and environmental contamination of some samples prevent us from drawing robust conclusions about the causes of these differences.

Marital Dissolution and Child Educational Outcomes in San Borja, Bolivia

Serial monogamy is likely an adaptive mating strategy for women when the expected future fitness gains with a different partner are greater than expected future fitness with one's current partner. Using interview data from more than 400 women in San Borja, Bolivia, discrete-time event history analyses and random effects regression analyses were conducted to examine predictors of marital dissolution, separated by remarriage status, and child educational outcomes. Male income was found to be inversely associated with women's risk of "divorce and remarriage," whereas female income is positively associated with women's risk of "divorce, but not remarriage." Children of women who divorce and remarry tend to have significantly lower educational outcomes than children of married parents, but women with higher incomes are able to buffer their children from the negative educational outcomes of divorce and remarriage. Counter to predictions, there is no evidence that women with kin in the community have a significant difference in likelihood of divorce or a buffering effect of child outcomes. In conclusion, predictors of divorce differ depending on whether the woman goes on to remarry, suggesting that male income may be a better predictor of a serial monogamy strategy whereas female income predicts marital dissolution only. Thus, women who are relatively autonomous because of greater income may not benefit from remarriage.

Female Genital Cutting Restricts Sociosexuality Among the Igbo People of Southeast Nigeria

Female genital cutting (FGC) involves partial or total removal of the external female genitalia and causes detrimental effects on woman’s physical and psychological health. Estimates suggest that 130 million women and girls have experienced FGC worldwide. A frequently cited reason for performing this procedure is to restrict female sexuality. To test this idea, we examined women’s willingness to engage in uncommitted sexual relations (sociosexuality) among the traditional Igbo community in Southeastern Nigeria, a region in which FGC is prevalent. Women with FGC reported more restricted sociosexuality in all three domains (attitude, behavior, and desire) compared to women without FGC. Our results suggest that FGC significantly restricts female extra-pair behavior. We provide evidence that this practice is partially attributable to sexual conflict over reproduction by decreasing paternity uncertainty and increasing the reproductive costs to women.

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.

Pair-bonding, romantic love, and evolution: the curious case of Homo sapiens

This article evaluates a thesis containing three interconnected propositions. First, romantic love is a "commitment device" for motivating pair-bonding in humans. Second, pair-bonding facilitated the idiosyncratic life history of hominins, helping to provide the massive investment required to rear children. Third, managing long-term pair bonds (along with family relationships) facilitated the evolution of social intelligence and cooperative skills. We evaluate this thesis by integrating evidence from a broad range of scientific disciplines. First, consistent with the claim that romantic love is an evolved commitment device, our review suggests that it is universal; suppresses mate-search mechanisms; has specific behavioral, hormonal, and neuropsychological signatures; and is linked to better health and survival. Second, we consider challenges to this thesis posed by the existence of arranged marriage, polygyny, divorce, and infidelity. Third, we show how the intimate relationship mind seems to be built to regulate and monitor relationships. Fourth, we review comparative evidence concerning links among mating systems, reproductive biology, and brain size. Finally, we discuss evidence regarding the evolutionary timing of shifts to pair-bonding in hominins. We conclude there is interdisciplinary support for the claim that romantic love and pair-bonding, along with alloparenting, played critical roles in the evolution of Homo sapiens.