Human preferences for sexually dimorphic faces may be evolutionarily novel.

A large literature proposes that preferences for exaggerated sex typicality in human faces (masculinity/femininity) reflect a long evolutionary history of sexual and social selection. This proposal implies that dimorphism was important to judgments of attractiveness and personality in ancestral environments. It is difficult to evaluate, however, because most available data come from large-scale, industrialized, urban populations. Here, we report the results for 12 populations with very diverse levels of economic development. Surprisingly, preferences for exaggerated sex-specific traits are only found in the novel, highly developed environments. Similarly, perceptions that masculine males look aggressive increase strongly with development and, specifically, urbanization. These data challenge the hypothesis that facial dimorphism was an important ancestral signal of heritable mate value. One possibility is that highly developed environments provide novel opportunities to discern relationships between facial traits and behavior by exposing individuals to large numbers of unfamiliar faces, revealing patterns too subtle to detect with smaller samples.

Applying evolutionary anthropology.

Evolutionary anthropology provides a powerful theoretical framework for understanding how both current environments and legacies of past selection shape human behavioral diversity. This integrative and pluralistic field, combining ethnographic, demographic, and sociological methods, has provided new insights into the ultimate forces and proximate pathways that guide human adaptation and variation. Here, we present the argument that evolutionary anthropological studies of human behavior also hold great, largely untapped, potential to guide the design, implementation, and evaluation of social and public health policy. Focusing on the key anthropological themes of reproduction, production, and distribution we highlight classic and recent research demonstrating the value of an evolutionary perspective to improving human well-being. The challenge now comes in transforming relevance into action and, for that, evolutionary behavioral anthropologists will need to forge deeper connections with other applied social scientists and policy-makers. We are hopeful that these developments are underway and that, with the current tide of enthusiasm for evidence-based approaches to policy, evolutionary anthropology is well positioned to make a strong contribution.

Land inheritance establishes sibling competition for marriage and reproduction in rural Ethiopia.

Intergenerational transfer of wealth has been proposed as playing a pivotal role in the evolution of human sibling relationships. Sibling rivalry is assumed to be more marked when offspring compete for limited heritable resources, which are crucial for reproductive success (e.g., land and livestock); whereas in the absence of heritable wealth, related siblings may cooperate. To date, comparative studies undertaken to support this evolutionary assumption have been confounded by other socioecological factors, which vary across populations, e.g., food sharing and intergroup conflict. In this article we explore effects of sibling competition and cooperation for agricultural resources, marriage, and reproduction in one contemporary Ethiopian agropastoralist society. Here recent changes in land tenure policy, altering transfers of land from parents to offspring, present a unique framework to test the importance of intergenerational transfers of wealth in driving sibling competition, while controlling for socioeconomic biases. In households where land is inherited, the number of elder brothers reduces a man's agricultural productivity, marriage, and reproductive success, as resources diminish and competition increases with each additional sibling. Where land is not inherited (for males receiving land directly from the government and all females) older siblings do not have a competitive effect and in some instances may be beneficial. This study has wider implications for the evolution of human family sizes. Recent changes in wealth transfers, which have driven sibling competition, may be contributing to an increased desire for smaller family sizes.

Testing evolutionary conflict theories for sexual and physical intimate partner violence in Sub-Saharan Africa.

Intimate partner violence (IPV) refers to physical, sexual and psychological violence. Here an evolutionary approach is used to compare risk factors for male-to-female IPV perpetration, analysing physical and sexual IPV separately. Two hypotheses based on sexual conflict theory have been applied to IPV perpetration, but they remain largely untested using empirical data: (a) men perpetrate IPV in response to a perceived threat to their paternity certainty; and (b) IPV is caused by men pursuing a higher fertility optima than their partners, either within marriage (reproductive coercion) or outside marriage (paternal disinvestment). Demographic Health Survey data from couples in 12 sub-Saharan African countries (n = 25,577) were used to test these evolutionary hypotheses, using multilevel models and controlling for potential social and environmental confounds. The results show that evolutionary theory provides important insight into different risk factors by IPV type. Indicators of paternity concern are associated with an increased risk of both physical and sexual IPV, indicators of paternal disinvestment are associated with an increased risk of physical IPV only, while reproductive coercion is not associated with either IPV type. The risk factors identified here correspond with proximate-level explanations for IPV perpetration, but an evolutionary interpretation explains why these particular factors may motivate IPV in certain contexts.

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

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

The life-history trade-off between fertility and child survival.

Evolutionary models of human reproduction argue that variation in fertility can be understood as the local optimization of a life-history trade-off between offspring quantity and 'quality'. Child survival is a fundamental dimension of quality in these models as early-life mortality represents a crucial selective bottleneck in human evolution. This perspective is well-rehearsed, but current literature presents mixed evidence for a trade-off between fertility and child survival, and little empirical ground to evaluate how socioecological and individual characteristics influence the benefits of fertility limitation. By compiling demographic survey data, we demonstrate robust negative relationships between fertility and child survival across 27 sub-Saharan African countries. Our analyses suggest this relationship is primarily accounted for by offspring competition for parental investment, rather than by reverse causal mechanisms. We also find that the trade-off increases in relative magnitude as national mortality declines and maternal somatic (height) and extrasomatic (education) capital increase. This supports the idea that socioeconomic development, and associated reductions in extrinsic child mortality, favour reduced fertility by increasing the relative returns to parental investment. Observed fertility, however, falls considerably short of predicted optima for maximizing total offspring survivorship, strongly suggesting that additional unmeasured costs of reproduction ultimately constrain the evolution of human family size.

No relationship found between female genital cutting and intimate partner violence across six sub-Saharan African countries.

Female genital cutting (FGC) and intimate partner violence (IPV) are highly prevalent forms of violence against women, and the UN has encouraged strengthening policy linkages between FGC and IPV programme work (UN Women, Female genital mutilation/cutting and violence against women and girls. Strengthening the policy linkages between different forms of violence. UN Women, 2017a). However few studies have examined the relationship between these behaviours. This study addresses two research gaps identified by the UN; (a) whether women with FGC are more likely to experience physical or sexual IPV, and (b) whether women's IPV experiences are associated with their support for FGC. Multilevel multivariate logistic regression models were run using Demographic and Health Survey data from 6 sub-Saharan African countries (Burkina Faso, Ethiopia, The Gambia, Ivory Coast, Kenya and Nigeria, n33,170). Our results show no association between FGC and IPV. Women with FGC are not at greater risk of experiencing IPV, and women who experienced IPV in the preceding year are not more likely to support FGC. Further, the importance of individual and community factors differ; IPV experience is influenced by individual factors, whereas FGC support is more influenced by ethnic group characteristics. Although the proposition that FGC and IPV could be tackled collaboratively is attractive for policy makers, these results are important because they indicate that eradication programmes will be more effective when targeted at specific behaviours.

Measuring Hidden Support for Physical Intimate Partner Violence: A List Randomization Experiment in South-Central Ethiopia.

Understanding how and why physical intimate partner violence (IPV) persists in high-risk communities has proven difficult. As IPV is both sensitive and illegal, people may be inclined to misreport their views and experiences. By embedding a list randomization experiment (LRE), which increases respondent privacy, in a survey of 809 adult Arsi Oromo men and women in rural south-central Ethiopia, we test the reliability of direct questioning survey methods (e.g., used in the Demographic and Health Surveys) for measuring attitudes that underpin the acceptability of IPV. Participants were randomly assigned versions of the survey in which they were asked either directly or indirectly about the acceptability of wife-beating. By comparing responses across these surveys, we identify the extent to which views are being misreported using direct questioning methods, as well as identifying the "true" predictors of continued support for wife-beating. Indirect questioning reveals that almost one third of the sample believe that wife-beating is acceptable. Adults (particularly men) who are less educated (<3 years schooling) or living in households where women do not participate in economic decision making are among those most likely to identify wife-beating as justifiable (>50% endorsement). These individuals, however, are also more inclined to hide their approval when asked directly by an interviewer. That we find high but underreported support for wife-beating among some members of the community demonstrates a clear need to encourage a more open dialogue, to prevent violence toward women remaining undetected and thus unchanged. This finding also raises questions about the accuracy of traditional direct questioning for capturing information on IPV attitudes and norms. Of relevance to policy, we find that wife-beating is entirely absent only among adults with higher levels of education, living in households where decision making is shared between couples.

Does Kin-Selection Theory Help to Explain Support Networks among Farmers in South-Central Ethiopia?

Social support networks play a key role in human livelihood security, especially in vulnerable communities. Here we explore how evolutionary ideas of kin selection and intrahousehold resource competition can explain individual variation in daily support network size and composition in a south-central Ethiopian agricultural community. We consider both domestic and agricultural help across two generations with different wealth-transfer norms that yield different contexts for sibling competition. For farmers who inherited land rights from family, firstborns were more likely to report daily support from parents and to have larger nonparental kin networks (n = 180). Compared with other farmers, firstborns were also more likely to reciprocate their parents' support, and to help nonparental kin without reciprocity. For farmers who received land rights from the government (n = 151), middle-born farmers reported more nonparental kin in their support networks compared with other farmers; nonreciprocal interactions were particularly common in both directions. This suggests a diversification of adult support networks to nonparental kin, possibly in response to a long-term parental investment disadvantage of being middle-born sons. In all instances, regardless of inheritance, lastborn farmers were the most disadvantaged in terms of kin support. Overall, we found that nonreciprocal interactions among farmers followed kin selection predictions. Direct reciprocity explained a substantial part of the support received from kin, suggesting the importance of the combined effects of kin selection and reciprocity for investment from kin.

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

This study examines child survival and growth in a patrilineal Ethiopian community as a function of father absence and sex. In line with evolutionary predictions for sex-biased parental investment, the absence of a father and associated constraints on household resources is more detrimental for sons' than daughters' survival in infancy. Father absence doubles a son's risk of dying in infancy but has a positive influence on the well-being of female members of the household, improving daughter survival, growth, and maternal nutritional status. Lack of paternal investment may be compensated for by other matrilateral kin through increased reciprocity between mother, daughter, and sister.

Indirect questioning method reveals hidden support for female genital cutting in South Central Ethiopia.

Female genital cutting (FGC) has major implications for women's physical, sexual and psychological health, and eliminating the practice is a key target for public health policy-makers. To date one of the main barriers to achieving this has been an inability to infer privately-held views on FGC within communities where it is prevalent. As a sensitive (and often illegal) topic, people are anticipated to hide their true support for the practice when questioned directly. Here we use an indirect questioning method (unmatched count technique) to identify hidden support for FGC in a rural South Central Ethiopian community where the practice is common, but thought to be in decline. Employing a socio-demographic household survey of 1620 Arsi Oromo adults, which incorporated both direct and indirect direct response (unmatched count) techniques we compare directly-stated versus privately-held views in support of FGC, and individual variation in responses by age, gender and education and target female (daughters versus daughters-in-law). Both genders express low support for FGC when questioned directly, while indirect methods reveal substantially higher acceptance (of cutting both daughters and daughters-in-law). Educated adults (those who have attended school) are privately more supportive of the practice than they are prepared to admit openly to an interviewer, indicating that education may heighten secrecy rather than decrease support for FGC. Older individuals hold the strongest views in favour of FGC (particularly educated older males), but they are also more inclined to conceal their support for FGC when questioned directly. As these elders represent the most influential members of society, their hidden support for FGC may constitute a pivotal barrier to eliminating the practice in this community. Our results demonstrate the great potential for indirect questioning methods to advance knowledge and inform policy on culturally-sensitive topics like FGC; providing more reliable data and improving understanding of the "true" drivers of FGC.

Frequency-dependent female genital cutting behaviour confers evolutionary fitness benefits.

Female genital cutting (FGC) has immediate and long-term negative health consequences that are well-documented, and its elimination is a priority for policymakers. The persistence of this widespread practice also presents a puzzle for evolutionary anthropologists due to its potentially detrimental impact on survival and reproductive fitness. Using multilevel modelling on demographic health survey datasets from five West African countries, here we show that FGC behaviour is frequency-dependent; the probability that girls are cut varies in proportion to the FGC frequency found in their ethnic group. We also show that this frequency-dependent behaviour is adaptive in evolutionary fitness terms; in ethnic groups with high FGC frequency, women with FGC have significantly more surviving offspring than their uncut peers, and the reverse is found in ethnic groups with low FGC frequency. Our results demonstrate how evolutionary and cultural forces can drive the persistence of harmful behaviours.

An energy-saving development initiative increases birth rate and childhood malnutrition in rural Ethiopia.

BACKGROUND: Evolutionary life history theory predicts that, in the absence of contraception, any enhancement of maternal condition can increase human fertility. Energetic trade-offs are likely to be resolved in favour of maximizing reproductive success rather than health or longevity. Here we find support for the hypothesis that development initiatives designed to improve maternal and child welfare may also incur costs associated with increased family sizes if they do not include a family planning component. METHODS AND FINDINGS: Demographic and anthropometric data were collected in a rural Ethiopian community benefiting from a recent labour-saving development technology that reduces women's energetic expenditure (n = 1,976 households). Using logistic hazards models and general linear modelling techniques, we found that whilst infant mortality has declined, the birth rate has increased, causing greater scarcity of resources within households. CONCLUSIONS: This study is, to our knowledge, the first to demonstrate a link between a technological development intervention and an increase in both birth rate and childhood malnutrition. Women's nutritional status was not improved by the energy-saving technology, because energy was diverted into higher birth rates. We argue that the contribution of biological processes to increased birth rates in areas of the developing world without access to modern contraception has been overlooked. This highlights the continued need for development programmes to be multisectoral, including access to and promotion of contraception.

Strong mothers bear more sons in rural Ethiopia.

In humans, there is evidence that the physiological cost to the mother of bearing sons is greater than of bearing daughters. Parents should manipulate the sex of offspring born in response to resource availability to maximize their reproductive success. Here, we demonstrate that, within a rural food-stressed community in southern Ethiopia, there is a strong association between the sex of the most recent birth and maternal nutritional status, measured either by body mass index or mid-upper arm muscle area (AMA) (measures of fat and muscle mass). The effect of muscle mass is very marked: those women in the upper 25th percentile of AMA were more than twice as likely to have had a recent male birth than those in the lowest 25th percentile.

Beyond bushmeat: animal contact, injury, and zoonotic disease risk in Western Uganda.

Zoonotic pathogens cause an estimated 70% of emerging and re-emerging infectious diseases in humans. In sub-Saharan Africa, bushmeat hunting and butchering is considered the primary risk factor for human-wildlife contact and zoonotic disease transmission, particularly for the transmission of simian retroviruses. However, hunting is only one of many activities in sub-Saharan Africa that bring people and wildlife into contact. Here, we examine human-animal interaction in western Uganda, identifying patterns of injuries from animals and contact with nonhuman primates. Additionally, we identify individual-level risk factors associated with contact. Nearly 20% (246/1,240) of participants reported either being injured by an animal or having contact with a primate over their lifetimes. The majority (51.7%) of injuries were dog bites that healed with no long-term medical consequences. The majority (76.8%) of 125 total primate contacts involved touching a carcass; however, butchering (20%), hunting (10%), and touching a live primate (10%) were also reported. Red colobus (Piliocolobus rufomitratus tephrosceles) accounted for most primate contact events. Multivariate logistic regression indicated that men who live adjacent to forest fragments are at elevated risk of animal contact and specifically primate contact. Our results provide a useful comparison to West and Central Africa where "bushmeat hunting" is the predominant paradigm for human-wildlife contact and zoonotic disease transmission.

Rural to urban migration is an unforeseen impact of development intervention in Ethiopia.

Rural development initiatives across the developing world are designed to improve community well-being and livelihoods. However they may also have unforeseen consequences, in some cases placing further demands on stretched public services. In this paper we use data from a longitudinal study of five Ethiopian villages to investigate the impact of a recent rural development initiative, installing village-level water taps, on rural to urban migration of young adults. Our previous research has identified that tap stands dramatically reduced child mortality, but were also associated with increased fertility. We demonstrate that the installation of taps is associated with increased rural-urban migration of young adults (15-30 years) over a 15 year period (15.5% migrate out, n = 1912 from 1280 rural households). Young adults with access to this rural development intervention had three times the relative risk of migrating to urban centres compared to those without the development. We also identify that family dynamics, specifically sibling competition for limited household resources (e.g. food, heritable land and marriage opportunities), are key to understanding the timing of out-migration. Birth of a younger sibling doubled the odds of out-migration and starting married life reduced it. Rural out-migration appears to be a response to increasing rural resource scarcity, principally competition for agricultural land. Strategies for livelihood diversification include education and off-farm casual wage-labour. However, jobs and services are limited in urban centres, few migrants send large cash remittances back to their families, and most return to their villages within one year without advanced qualifications. One benefit for returning migrants may be through enhanced social prestige and mate-acquisition on return to rural areas. These findings have wide implications for current understanding of the processes which initiate rural-to-urban migration and transitions to low fertility, as well as for the design and implementation of development intervention across the rural and urban developing world.

Social transmission and the spread of modern contraception in rural Ethiopia.

Socio-economic development has proven to be insufficient to explain the time and pace of the human demographic transition. Shifts to low fertility norms have thus been thought to result from social diffusion, yet to date, micro-level studies are limited and are often unable to disentangle the effect of social transmission from that of extrinsic factors. We used data which included the first ever use of modern contraception among a population of over 900 women in four villages in rural Ethiopia, where contraceptive prevalence is still low (<20%). We investigated whether the time of adoption of modern contraception is predicted by (i) the proportion of ever-users/non ever-users within both women and their husbands' friendships networks and (ii) the geographic distance to contraceptive ever-users. Using a model comparison approach, we found that individual socio-demographic characteristics (e.g. parity, education) and a religious norm are the most likely explanatory factors of temporal and spatial patterns of contraceptive uptake, while the role of person-to-person contact through either friendship or spatial networks remains marginal. Our study has broad implications for understanding the processes that initiate transitions to low fertility and the uptake of birth control technologies in the developing world.

Polygyny, reproductive success and child health in rural Ethiopia: why marry a married man?

This study examines the reproductive success of men and women in rural Ethiopia as a function of their marital status, specifically by comparing polygamously and monogamously married individuals. In line with predictions from evolutionary theory, polygamy is beneficial to male reproductive success (i.e. producing larger numbers of surviving offspring). The success of polygamously married females depends on wife rank: the first wives of polygamous husbands do better than monogamously married women and much better than second or third wives. These effects are mirrored in child nutritional status: the children of second and third wives have lower weight for height. Due to potential, largely unmeasurable differences in marriageability (quality) between individuals, it was not possible to support a model of either resource-holding polygyny combined with female choice or female coercion into unwanted marriages. First wives of polygamously married men marry at a younger age and attract a higher brideprice, suggesting that both the males and females in the marriage are likely to be of higher quality (due to wealth, family status or some other factor such as beauty). Unions that end up monogamous are likely to be between slightly lower quality individuals; and second and third wives, who marry at the oldest ages and attract the lowest brideprice, may be 'making the best of a bad job'. The relatively long gap between first and second marriages may mean that first wives of highly marriageable males can enjoy considerable reproductive success before their husbands marry again.