Women's subsistence strategies predict fertility across cultures, but context matters

While it is commonly assumed that farmers have higher, and foragers lower, fertility compared to populations practicing other forms of subsistence, robust supportive evidence is lacking. We tested whether subsistence activities-incorporating market integration-are associated with fertility in 10,250 women from 27 small-scale societies and found considerable variation in fertility. This variation did not align with group-level subsistence typologies. Societies labeled as "farmers" did not have higher fertility than others, while "foragers" did not have lower fertility. However, at the individual level, we found strong evidence that fertility was positively associated with farming and moderate evidence of a negative relationship between foraging and fertility. Markers of market integration were strongly negatively correlated with fertility. Despite strong cross-cultural evidence, these relationships were not consistent in all populations, highlighting the importance of the socioecological context, which likely influences the diverse mechanisms driving the relationship between fertility and subsistence.

The cuckoldry conundrum

Concerns about cuckoldry are a dominant theme in evolutionary studies of mating, frequently used to explain sex differences in reproductive strategies. However, studies in nonhuman species have shown that cuckoldry can be associated with important benefits. These insights have not been well integrated with the human literature, which continues to focus on anticuckoldry tactics and negative repercussions for men. I evaluate two key assumptions central to human models of cuckoldry: (1) men are being tricked into investing in nonbiological offspring and (2) investment in nonbiological offspring is wasted. The ethnographic data on fatherhood shows that the concepts of pater and genitor are complex and locally constructed ideas that often include explicit knowledge of extra-pair paternity, countering the idea that nonpaternity results from trickery. Furthermore, rather than being a "waste," paternity loss can be associated with important gains for men, helping to explain why men invest in nonbiological offspring.

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.

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.

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.

Adoption of outgroup norms provides evidence for social transmission in perinatal care practices among rural Namibian women

Background and objectives

How do new ideas spread in social groups? We apply the framework of cultural evolution theory to examine what drives change in perinatal care norms among Himba women in the Kunene region of Namibia. Access to formal medical care is on the rise in this region, and medical workers regularly visit communities to promote WHO-recommended perinatal care practices. This study investigates how various forms of social transmission affect women's uptake of medical recommendations concerning perinatal care.

Methodology

Based on interviews with one hundred Himba mothers, we used Bayesian multi-level logistical regression models to examine how perceptions of group preferences, prestige ascribed to outgroup conformers, interaction with the outgroup and access to resources affect norm adoption.

Results

Women who perceive medical recommendations as common in their group prefer, plan and practice these recommendations more often themselves. We observed a shift toward medical recommendations regarding birth location and contraception use that was in line with conformity bias predictions. Practices that serve as cultural identity markers persist in the population.

Conclusions and implications

Norm changes, and the cultural evolutionary processes that can lead to them, are not uniform, either in process or pace. Empirical studies like this one provide important examples of how these changes reflect local culture and circumstance and are critical for better understanding the models that currently predominate in cultural evolution work. These cases can also help bridge the gap between evolutionary anthropology and public health by demonstrating where promotion and prevention campaigns might be most effective.

Lay Summary

The recent promotion of WHO-recommended perinatal care practices in Namibia provides an opportunity to empirically study norm change using a cultural evolution framework. We found women adopt medical recommendations when they believe these are common in their social group. Local norms that were not discouraged persisted in the study group.

Crucial Contributions : A Biocultural Study of Grandmothering During the Perinatal Period

Maternal grandmothers play a key role in allomaternal care, directly caring for and provisioning their grandchildren as well as helping their daughters with household chores and productive labor. Previous studies have investigated these contributions across a broad time period, from infancy through toddlerhood. Here, we extend and refine the grandmothering literature to investigate the perinatal period as a critical window for grandmaternal contributions. We propose that mother-daughter co-residence during this period affords targeted grandmaternal effort during a period of heightened vulnerability and appreciable impact. We conducted two focus groups and 37 semi-structured interviews with Himba women. Interviews focused on experiences from their first and, if applicable, their most recent birth and included information on social support, domains of teaching and learning, and infant feeding practices. Our qualitative findings reveal three domains in which grandmothers contribute: learning to mother, breastfeeding support, and postnatal health and well-being. We show that informational, emotional, and instrumental support provided to new mothers and their neonates during the perinatal period can aid in the establishment of the mother-infant bond, buffer maternal energy balance, and improve nutritional outcomes for infants. These findings demonstrate that the role of grandmother can be crucial, even when alloparenting is common and breastfeeding is frequent and highly visible. Situated within the broader anthropological and clinical literature, these findings substantiate the claim that humans have evolved in an adaptive sociocultural perinatal complex in which grandmothers provide significant contributions to the health and well-being of their reproductive-age daughters and grandchildren.

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.

The disequilibrium of double descent: changing inheritance norms among Himba pastoralists

Matrilineal systems in sub-Saharan Africa tend to co-occur with horticulture and are rare among pastoralists, with the causal arrow pointing from the introduction of cattle to the loss of matriliny. However, most work on this topic stems from either phylogenetic analyses or historical data. To better understand the shift from matrilineal to patrilineal inheritance that occurred among Bantu populations after the adoption of pastoralism, data from societies that are currently in transition are needed. Himba pastoralists, who practice 'double descent', may represent one such society. Using multi-generational ethnography and structured survey data, we describe current norms and preferences about inheritance, as well as associated norms related to female autonomy. We find that preferences for patrilineal inheritance are strong, despite the current practice of matrilineal cattle inheritance. We also find that a preference for patriliny predicts greater acceptance of norm violating behaviour favouring sons over nephews. Finally, we show that there are important generational differences in how men view women's autonomy, which are probably attributable to both changing norms about inheritance and exposure to majority-culture views on women's roles. Our data shed light on how systemic change like the shifts in descent reckoning that occurred during the Bantu expansion can occur. This article is part of the theme issue 'The evolution of female-biased kinship in humans and other mammals'.

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.

Differences between sons and daughters in the intergenerational transmission of wealth

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

The influence of age- and sex-specific labor demands on sleep in Namibian agropastoralists

OBJECTIVE

To understand the basics of sleep quality in a pastoralist population and to explore predictors of this variation.

DESIGN

Cross-sectional.

SETTING

Northern Namibia, dry seasons of 2016 and 2017.

PARTICIPANTS

The Himba, a nonindustrial seminomadic agropastoralist population without access to the electrical grid.

MEASUREMENTS

Using Actiwatch-2 devices, 75 participants completed 721 days of wear. We calculated sleep duration, efficiency, and activity counts before and after sunset/sunrise and onset/offset. Participants were also interviewed about sleeping arrangements and nighttime disruptions.

RESULTS

Himba show lower sleep duration and efficiency than other populations studied, and men had substantially lower duration and efficiency than women. Sex differences were not attenuated when napping was included with total sleep time. Age predicted longer sleep duration and lower evening and morning activity levels. Number of adult co-sleepers predicted increased sleep duration and efficiency in women. Livestock disturbance was not a commonly reported cause of nighttime waking.

CONCLUSIONS

These findings support predictions that pastoralist groups may have lower sleep quality than other subsistence populations, but this does not appear to be a consequence of noise from livestock. Instead, lower sleep quality appears to be the result of subsistence and social activities, particularly for men and young adults overall.

Child fosterage and sex-biased nutritional outcomes among Namibian pastoralists

OBJECTIVES

Across cultures, fosterage has been shown to impact child health. Contextual factors, such as the reason for fosterage and the relationship between foster parent and child, are known to magnify variance in nutritional outcomes for foster children. Another important, but less studied, factor is the role of gender. Sex-biases in physiology and cultural norms are both known to affect child nutrition, and we posit these effects might be magnified in the presence of fosterage. In this study, we investigate how sex interacts with fosterage to affect nutritional outcomes among Namibian pastoralists.

METHODS

Anthropometrics for children and adults were collected using standard procedures, and linear models were used to predict the effects of age, sex, and fosterage on height, weight, and body mass index Z-scores. Semi-structured interviews with adults provided context for understanding sex specific reasons for fosterage and biases in investment.

RESULTS

Boys in this population have lower nutritional scores than girls, and fostered boys have lower weight and BMI Z-scores than nonfostered boys. Fostered girls have lower height Z-scores and are more likely to be stunted and underweight than nonfostered girls. These effects extend into adulthood, with fostered women being shorter than their nonfostered counterparts.

CONCLUSIONS

Sex plays a role in the nutritional impact of fosterage among Himba children. These differences could be related to differential child labor demands, investment patterns, and the divergent reasons girls and boys are placed into fosterage. Future studies should consider how fosterage can magnify existing biases, like sex, when studying its impact on child health.

Fosterage as a system of dispersed cooperative breeding: evidence from the Himba

Humans are obligate cooperative breeders, relying heavily on support from kin to raise children. To date, most studies of cooperative breeding have focused on help that supplements rather than replaces parental care. Here we propose that fosterage can act as a form of dispersed cooperative breeding, one that enhances women's fitness by allowing them to disinvest in some children and reallocate effort to others. We test this hypothesis through a series of predictions about the costs and benefits of fosterage for mothers, foster parents, and foster children using data from the Himba, a group of Namibian agro-pastoralists. We show that fostering out children enhances mothers' fitness, and we provide evidence for a causal link from fosterage to enhanced fitness by showing that fosterage of early-born children is associated with greater maternal reproductive success. Foster parents minimize the costs of fosterage by skewing their care toward their postreproductive years, and by mainly fostering close kin. However, the system is associated with some detrimental effects on foster children, who are more likely to be stunted and underweight than their non-fostered counterparts.

Perceptions of polygyny: the effects of offspring and other kin on co-wife satisfaction

The costs and benefits of polygyny have been widely debated and vary according to local sociopolitical context, the level of female autonomy, and economic considerations such as the mode of production. This study aims to understand perceptions of polygyny as a function of household demography, particularly the number of female kin present in the household who can provide labor that is largely substitutable to that of a co-wife. The presence of these helpers is proposed to shift the cost-benefit structure of polygyny, in which having more female kin available is associated with a more negative view of the practice. Interview and census data from 106 Himba women, who are traditional, seminomadic pastoralists, were used to test this prediction. Among married women who reside patrilocally, the presence of more elder daughters was associated with a more negative view of polygyny. Among unmarried women, who reside in their natal homes, it is the total number of adult female kin that predicts perception of polygyny. In addition, unmarried women are significantly more likely to report fights over resources as a source of co-wife conflicts when they have more dependent children, but no such association was found among married women.

Choosy but not chaste: multiple mating in human females

When Charles Darwin set out to relate his theory of evolution by natural selection to humans he discovered that a complementary explanation was needed to properly understand the great variation seen in human behavior. The resulting work, The Descent of Man and Selection in Relation to Sex, laid out the defining principles and evidence of sexual selection. In brief, this work is best known for illuminating the typically male strategy of intrasexual competition and the typically female response of intersexual choice. While these sexual stereotypes were first laid out by Darwin, they grew in importance when, years later, A. J. Bateman, in a careful study of Drosophila mating strategies, noted that multiple mating appeared to provide great benefit to male reproductive success, but to have no such effect on females. As a result, female choice soon became synonymous with being coy, and only males were thought to gain from promiscuous behavior. However, the last thirty years of research have served to question much of the traditional wisdom about sex differences proposed by Darwin and Bateman, illuminating the many ways that women (and females more generally) can and do engage in multiple mating.

Ontogeny of prosocial behavior across diverse societies

Humans are an exceptionally cooperative species, but there is substantial variation in the extent of cooperation across societies. Understanding the sources of this variability may provide insights about the forces that sustain cooperation. We examined the ontogeny of prosocial behavior by studying 326 children 3-14 y of age and 120 adults from six societies (age distributions varied across societies). These six societies span a wide range of extant human variation in culture, geography, and subsistence strategies, including foragers, herders, horticulturalists, and urban dwellers across the Americas, Oceania, and Africa. When delivering benefits to others was personally costly, rates of prosocial behavior dropped across all six societies as children approached middle childhood and then rates of prosociality diverged as children tracked toward the behavior of adults in their own societies. When prosocial acts did not require personal sacrifice, prosocial responses increased steadily as children matured with little variation in behavior across societies. Our results are consistent with theories emphasizing the importance of acquired cultural norms in shaping costly forms of cooperation and creating cross-cultural diversity.

The place of proximity: social support in mother-adult daughter relationships

The mother-adult daughter relationship has been highlighted in both the social sciences and the public health literature as an important facet of social support networks, particularly as they pertain to maternal and child health. Evolutionary anthropologists also have shown positive associations between support from maternal grandmothers and various outcomes related to reproductive success; however, many of these studies rely on proximity as a surrogate measure of support. Here I present data from the Puerto Rican Maternal and Infant Health Survey (PRMIHS) comparing geographic proximity of mother and daughter with a self-reported measure of mother-to-daughter support. These two measures were used to predict infant health outcomes as well as various measures of instrumental and emotional aid provided during pregnancy and after birth. Primary support was shown to have a positive effect across the analyses, whereas geographic proximity was associated with an increased risk of infant mortality and low birth weight as well as reduced odds of receiving support. This paradox was then examined using a combination variable that teased out the interactions of maternal support and proximity. Women who were geographically close to their mothers but who did not consider them a primary source of support had increased odds of infant death and low birth weight, and were less likely to receive either tangible or intangible forms of aid, while women whose mothers were both close and primary showed uniformly positive outcomes. These results place the role of propinquity within the larger context of social support and highlight the need for more detailed studies of social support within evolutionary anthropology.

Food scarcity, not economic constraint limits consumption in a rural Aboriginal community

OBJECTIVE

To determine whether food scarcity or economic constraint is more strongly associated with purchasing patterns in a rural Aboriginal community.

DESIGN

Store receipts were collected to determine money spent in the community shop across a four-month period from January to April 2006. Variability in expenditures is then studied using measures of food scarcity (days since shop loading) and economic constraint (days since payday).

SETTING

Parnngurr Outstation in the Western Desert of Australia.

MAIN OUTCOME MEASURES

Daily gross and per-household gross expenditures at the community shop.

RESULTS

  There is a significant association between food scarcity and money spent in the shop (-42.89, confidence interval -62.62 to -23.18, P < 0.001). There is no association between economic constraint and money spent in the shop.

CONCLUSION

In rural and remote communities, reliable access to food is critical to food security. Circumstantial constraints such as seasonal flooding and political insecurity can augment food access problems and put Aboriginal people at risk of malnutrition and other dietary harms.

Female choice and extra-pair paternity in a traditional human population

Seeking out extra-pair paternity (EPP) is a viable reproductive strategy for females in many pair-bonded species. Across human societies, women commonly engage in extra-marital affairs, suggesting this strategy may also be an important part of women's reproductive decision-making. Here, I show that among the Himba 17 per cent of all recorded marital births are attributed by women to EPP, and EPP is associated with significant increases in women's reproductive success. In contrast, there are no cases of EPP among children born into 'love match' marriages. This rate of EPP is higher than has been recorded in any other small-scale society. These results illustrate the importance of seeking EPP as a mechanism of female choice in humans, while simultaneously showing it to be highly variable and context-dependent.

The grandmaternal niche: Critical caretaking among Martu Aborigines

This paper expands upon the existing literature on the evolutionary importance of grandmothers by examining how direct care by grandmothers differs from care provided by other helpers within a population of Martu Aborigines. Behavioral observations were collected on ten babies who ranged from 3 months to 3 years of age. The results show that Martu grandmothers were in contact with their grandchildren more than any person other than the mother, and they were also more likely than any other category of caregiver to perform high-demand tasks, such as bathing or feeding. These results suggest that Martu grandmothers are specializing in the type of care they provide and posits that high-quality allocare is an important pathway to increased health and survival of grandchildren.