Reproductive inequality in humans and other mammals.

To address claims of human exceptionalism, we determine where humans fit within the greater mammalian distribution of reproductive inequality. We show that humans exhibit lower reproductive skew (i.e., inequality in the number of surviving offspring) among males and smaller sex differences in reproductive skew than most other mammals, while nevertheless falling within the mammalian range. Additionally, female reproductive skew is higher in polygynous human populations than in polygynous nonhumans mammals on average. This patterning of skew can be attributed in part to the prevalence of monogamy in humans compared to the predominance of polygyny in nonhuman mammals, to the limited degree of polygyny in the human societies that practice it, and to the importance of unequally held rival resources to women's fitness. The muted reproductive inequality observed in humans appears to be linked to several unusual characteristics of our species-including high levels of cooperation among males, high dependence on unequally held rival resources, complementarities between maternal and paternal investment, as well as social and legal institutions that enforce monogamous norms.

Niche Diversity Predicts Personality Structure Across 115 Nations.

The niche-diversity hypothesis proposes that personality structure arises from the affordances of unique trait combinations within a society. It predicts that personality traits will be both more variable and differentiated in populations with more distinct social and ecological niches. Prior tests of this hypothesis in 55 nations suffered from potential confounds associated with differences in the measurement properties of personality scales across groups. Using psychometric methods for the approximation of cross-national measurement invariance, we tested the niche-diversity hypothesis in a sample of 115 nations (N = 685,089). We found that an index of niche diversity was robustly associated with lower intertrait covariance and greater personality dimensionality across nations but was not consistently related to trait variances. These findings generally bolster the core of the niche-diversity hypothesis, demonstrating the contingency of human personality structure on socioecological contexts.

Mother's social status is associated with child health in a horticulturalist population.

High social status is often associated with greater mating opportunities and fertility for men, but do women also obtain fitness benefits of high status? Greater resource access and child survivorship may be principal pathways through which social status increases women's fitness. Here, we examine whether peer-rankings of women's social status (indicated by political influence, project leadership, and respect) positively covaries with child nutritional status and health in a community of Amazonian horticulturalists. We find that maternal political influence is associated with improved child health outcomes in models adjusting for maternal age, parental height and weight, level of schooling, household income, family size, and number of kin in the community. Children of politically influential women have higher weight-for-age (B = 0.33; 95% CI = 0.12-0.54), height-for-age (B = 0.32; 95% CI = 0.10-0.54), and weight-for-height (B = 0.24; 95% CI = 0.04-0.44), and they are less likely to be diagnosed with common illnesses (OR = 0.48; 95% CI = 0.31-0.76). These results are consistent with women leveraging their social status to enhance reproductive success through improvements in child health. We discuss these results in light of parental investment theory and the implications for the evolution of female social status in humans.

The dynamics of men's cooperation and social status in a small-scale society.

We propose that networks of cooperation and allocation of social status co-emerge in human groups. We substantiate this hypothesis with one of the first longitudinal studies of cooperation in a preindustrial society, spanning 8 years. Using longitudinal social network analysis of cooperation among men, we find large effects of kinship, reciprocity and transitivity in the nomination of cooperation partners over time. Independent of these effects, we show that (i) higher-status individuals gain more cooperation partners, and (ii) individuals gain status by cooperating with individuals of higher status than themselves. We posit that human hierarchies are more egalitarian relative to other primates species, owing in part to greater interdependence between cooperation and status hierarchy.

Men's status and reproductive success in 33 nonindustrial societies: Effects of subsistence, marriage system, and reproductive strategy.

Social status motivates much of human behavior. However, status may have been a relatively weak target of selection for much of human evolution if ancestral foragers tended to be more egalitarian. We test the "egalitarianism hypothesis" that status has a significantly smaller effect on reproductive success (RS) in foragers compared with nonforagers. We also test between alternative male reproductive strategies, in particular whether reproductive benefits of status are due to lower offspring mortality (parental investment) or increased fertility (mating effort). We performed a phylogenetic multilevel metaanalysis of 288 statistical associations between measures of male status (physical formidability, hunting ability, material wealth, political influence) and RS (mating success, wife quality, fertility, offspring mortality, and number of surviving offspring) from 46 studies in 33 nonindustrial societies. We found a significant overall effect of status on RS (r = 0.19), though this effect was significantly lower than for nonhuman primates (r = 0.80). There was substantial variation due to marriage system and measure of RS, in particular status associated with offspring mortality only in polygynous societies (r = -0.08), and with wife quality only in monogamous societies (r = 0.15). However, the effects of status on RS did not differ significantly by status measure or subsistence type: foraging, horticulture, pastoralism, and agriculture. These results suggest that traits that facilitate status acquisition were not subject to substantially greater selection with domestication of plants and animals, and are part of reproductive strategies that enhance fertility more than offspring well-being.

How Universal Is the General Factor of Personality? An Analysis of the Big Five in Forager Farmers of the Bolivian Amazon.

In various personality models, such as the Big Five, a consistent higher order general factor of personality (GFP) can be identified. One view in the literature is that the GFP reflects general social effectiveness. Most GFP studies, however, have been conducted in Western, educated, industrialized, and rich democracies (WEIRD). Therefore, to address the question of the universality of the GFP, we test whether the GFP can also be identified in a preliterate indigenous sample of Tsimane by using self-reports, spouse reports, and interviewer ratings. In the Tsimane, a viable GFP could be identified and the intercorrelations between personality traits were significantly stronger than in samples from industrial countries. The GFP correlated with the ratings of social engagement. In addition, self and spouse ratings of the GFP overlapped. Overall, the findings are in line with the notion that the GFP is a human universal and a substantive personality factor reflecting social effectiveness.

Growth references for Tsimane forager-horticulturalists of the Bolivian Amazon.

OBJECTIVES: Growth standards and references currently used to assess population and individual health are derived primarily from urban populations, including few individuals from indigenous or subsistence groups. Given environmental and genetic differences, growth may vary in these populations. Thus, there is a need to assess whether international standards are appropriate for all populations, and to produce population specific references if growth differs. Here we present and assess growth references for the Tsimane, an indigenous population of Bolivian forager-horticulturalists. METHODS: Mixed cross-sectional/longitudinal anthropometrics (9,614 individuals; 30,118 observations; ages 0-29 years) were used to generate centile curves and Lambda-Mu-Sigma (LMS) tables for height-for-age, weight-for-age, body mass index (BMI)-for-age, and weight-for-height (WFH) using Generalized Additive Models for Location Shape and Scale (GAMLSS). Velocity curves were generated using SuperImposition by Translation and Rotation (SITAR). Tsimane ≤5 years were compared to World Health Organization (WHO) standards while those >5 years were compared to WHO school age references. All ages were compared to published references for Shuar forager-horticulturalists of the Ecuadorian Amazon. RESULTS: Tsimane growth differs from WHO values in height and weight, but is similar for BMI and WFH. Tsimane growth is characterized by slow height velocity in childhood and early adolescent peak height velocity at 11.3 and 13.2 years for girls and boys. Tsimane growth patterns are similar to Shuar, suggesting shared features of growth among indigenous South Americans. CONCLUSIONS: International references for BMI-for-age and WFH are likely appropriate for Tsimane, but differences in height-for-age and weight-for-age suggest Tsimane specific references may be useful for these measures.

Moral parochialism and contextual contingency across seven societies.

Human moral judgement may have evolved to maximize the individual's welfare given parochial culturally constructed moral systems. If so, then moral condemnation should be more severe when transgressions are recent and local, and should be sensitive to the pronouncements of authority figures (who are often arbiters of moral norms), as the fitness pay-offs of moral disapproval will primarily derive from the ramifications of condemning actions that occur within the immediate social arena. Correspondingly, moral transgressions should be viewed as less objectionable if they occur in other places or times, or if local authorities deem them acceptable. These predictions contrast markedly with those derived from prevailing non-evolutionary perspectives on moral judgement. Both classes of theories predict purportedly species-typical patterns, yet to our knowledge, no study to date has investigated moral judgement across a diverse set of societies, including a range of small-scale communities that differ substantially from large highly urbanized nations. We tested these predictions in five small-scale societies and two large-scale societies, finding substantial evidence of moral parochialism and contextual contingency in adults' moral judgements. Results reveal an overarching pattern in which moral condemnation reflects a concern with immediate local considerations, a pattern consistent with a variety of evolutionary accounts of moral judgement.

Repercussions of patrilocal residence on mothers' social support networks among Tsimane forager-farmers.

While it is commonly thought that patrilocality is associated with worse outcomes for women and their children due to lower social support, few studies have examined whether the structure of female social networks covaries with post-marital residence. Here, we analyse scan sample data collected among Tsimane forager-farmers. We compare the social groups and activity partners of 181 women residing in the same community as their parents, their husband's parents, both or neither. Relative to women living closer to their in-laws, women living closer to their parents are less likely to be alone or solely in the company of their nuclear family (odds ratio (OR): 0.6, 95% CI: 0.3-0.9), and more likely to be observed with others when engaging in food processing and manufacturing of market or household goods, but not other activities. Women are slightly more likely to receive childcare support from outside the nuclear family when they live closer to their parents (OR = 1.8, 95% CI 0.8-3.9). Their social group size and their children's probability of receiving allocare decrease significantly with distance from their parents, but not their in-laws. Our findings highlight the importance of women's proximity to kin, but also indicate that patrilocality per se is not costly to Tsimane women. This article is part of the theme issue 'Cooperation among women: evolutionary and cross-cultural perspectives'.

Physical competition increases testosterone among Amazonian forager-horticulturalists: a test of the 'challenge hypothesis'.

The challenge hypothesis posits that acute increases in testosterone (T) during male-male competition enhance performance and survivability while limiting the physiological costs of consistently high T. Human challenge hypothesis research focuses on young men in industrial populations, who have higher baseline T levels than men in subsistence populations. We tested whether the Tsimane, pathogenically stressed forager-horticulturalists of the Bolivian Amazon, would express acute T increases in response to physical competition. Saliva was collected from 88 Tsimane men (aged 16-59 years) before and after a competitive soccer match. Tsimane men had significantly lower baseline levels of T (ß = -0.41, p < 0.001) when compared with age-matched United States (US) males. Linear mixed-effects models were used to establish that T increased significantly immediately following competition (ß = 0.23, p < 0.001), remaining high 1 h later (ß = 0.09, p = 0.007); equivalent to 30.1 and 15.5 per cent increases in T, respectively. We did not find larger increases in T among winners (p = 0.412), although T increases were positively associated with self-rated performance (ß = 9.07, p = 0.004). These results suggest that despite lower levels of T than US males, Tsimane males exhibit acute increases in T at the same relative magnitude reported by studies in industrialized settings, with larger increases in T for those who report better individual performance.

When the strong punish: why net costs of punishment are often negligible.

In small-scale societies, punishment of adults is infrequent and employed when the anticipated cost-to-benefit ratio is low, such as when punishment is collectively justified and administered. In addition, benefits may exceed costs when punishers have relatively greater physical and social capital and gain more from cooperation. We provide examples from the Tsimane horticulturalists of Bolivia to support our claims.

Why do men seek status? Fitness payoffs to dominance and prestige.

In many human societies, high male social status associates with higher fertility, but the means by which status increases lifetime fitness have not been systematically investigated. We analyse the pathways by which male status begets reproductive success in a small-scale, Amerindian society. Men who are more likely to win a dyadic physical confrontation, i.e. dominant men, have higher intra-marital fertility for their age, and men with more community-wide influence, i.e. prestigious men, exhibit both higher intra-marital fertility and lower offspring mortality. Both forms of status elicit support from allies and deference from competitors, but high status men are not provisioned more than their peers. Prestigious but not dominant men marry wives who first give birth at earlier ages, which multivariate analysis suggests is the strongest pathway between status and fitness in this population. Furthermore, men are motivated to pursue status because of fitness gains both within and outside of marital unions: dominant and prestigious men have more in-pair surviving offspring as well as more extra-marital affairs.

Coalitions and conflict: A longitudinal analysis of men's politics.

To negotiate conflict and navigate status hierarchy, individuals in many species form coalitions. We describe inter-personal conflicts and assess theories of coalition formation in a small-scale human society. Based on longitudinal and cross-sectional social network analysis of men in two communities of Tsimane forager-horticulturalists, we find evidence of reciprocity in coalitional support, as well as evidence of transitivity: an ally of my ally is likely to become my ally. We find mixed support for coalition formation between individuals who share a common adversary. Coalition formation was also predicted by food- and labour-sharing and especially by kinship. Physically formidable men and men higher in informal status were more likely to provide coalitional support over time; evidence was mixed that they receive more coalitional support. The highest status men are hubs of a dense coalitional support network that indirectly link all men in the community. These findings suggest that male coalition formation is multiply motivated, and in general reveals the political dynamics that structure men's lives in small, relatively egalitarian communities.

Do wealth and inequality associate with health in a small-scale subsistence society?

In high-income countries, one's relative socio-economic position and economic inequality may affect health and well-being, arguably via psychosocial stress. We tested this in a small-scale subsistence society, the Tsimane, by associating relative household wealth (n = 871) and community-level wealth inequality (n = 40, Gini = 0.15-0.53) with a range of psychological variables, stressors, and health outcomes (depressive symptoms [n = 670], social conflicts [n = 401], non-social problems [n = 398], social support [n = 399], cortisol [n = 811], body mass index [n = 9,926], blood pressure [n = 3,195], self-rated health [n = 2523], morbidities [n = 1542]) controlling for community-average wealth, age, sex, household size, community size, and distance to markets. Wealthier people largely had better outcomes while inequality associated with more respiratory disease, a leading cause of mortality. Greater inequality and lower wealth were associated with higher blood pressure. Psychosocial factors did not mediate wealth-health associations. Thus, relative socio-economic position and inequality may affect health across diverse societies, though this is likely exacerbated in high-income countries.

Poverty is bad for health. People living in poverty are more likely to struggle to afford nutritious food, lack access to health care, or be overworked or stressed. This may make them susceptible to chronic diseases, contribute to faster aging, and shorten their lifespans. In high-income countries, there is growing evidence to suggest that a person's 'rank' in society also impacts their health. For example, individuals who have a lower position in the social hierarchy report worse health outcomes, regardless of their incomes. But it is unclear why living in an unequal society or having a lower social status contributes to poorer health. One possibility is that inequalities in society are creating a stressful environment that leads to worse physical and mental outcomes. It is thought that this stress largely comes from how humans evolved to prioritize reaching a higher social status over having a long and healthy life. If this is the case, this would mean that the link between social status and health would also be present in non-industrialized communities where social hierarchies tend to be less pronounced. To test this, Jaeggi, Blackwell et al. studied the Indigenous Tsimane population in Bolivia who live in small communities and forage and farm their own food. The income and relative wealth of 870 households from 40 Tsimane communities were compared against various outcomes, including symptoms associated with depression, stress hormone levels, blood pressure, self-rated health and several diseases. Jaeggi, Blackwell et al. found poverty and inequality did not negatively impact all of the health outcomes measured as has been previously reported for industrialized societies. However, blood pressure was higher among people with lower incomes or those who lived in more unequal communities. But because the Tsimane people generally have low blood pressure, the differences were too small to have much effect on their health. People who lived in more unequal communities were also three times more likely to have respiratory infections, but the reason for this was unclear. This shows that social determinants such as a person's wealth or inequality can affect health, even in communities with less rigid social hierarchies. In industrial societies the effect may be worse in part because they are compounded by lifestyle factors, such as diets rich in fat and sugar, and physical inactivity which can also increase blood pressure. This information may help policy makers reduce health disparities by addressing some of the social determinants of health and the lifestyle factors that cause them.

Adaptations in humans for assessing physical strength from the voice.

Recent research has shown that humans, like many other animals, have a specialization for assessing fighting ability from visual cues. Because it is probable that the voice contains cues of strength and formidability that are not available visually, we predicted that selection has also equipped humans with the ability to estimate physical strength from the voice. We found that subjects accurately assessed upper-body strength in voices taken from eight samples across four distinct populations and language groups: the Tsimane of Bolivia, Andean herder-horticulturalists and United States and Romanian college students. Regardless of whether raters were told to assess height, weight, strength or fighting ability, they produced similar ratings that tracked upper-body strength independent of height and weight. Male voices were more accurately assessed than female voices, which is consistent with ethnographic data showing a greater tendency among males to engage in violent aggression. Raters extracted information about strength from the voice that was not supplied from visual cues, and were accurate with both familiar and unfamiliar languages. These results provide, to our knowledge, the first direct evidence that both men and women can accurately assess men's physical strength from the voice, and suggest that estimates of strength are used to assess fighting ability.

Human adaptations for the visual assessment of strength and fighting ability from the body and face.

Selection in species with aggressive social interactions favours the evolution of cognitive mechanisms for assessing physical formidability (fighting ability or resource-holding potential). The ability to accurately assess formidability in conspecifics has been documented in a number of non-human species, but has not been demonstrated in humans. Here, we report tests supporting the hypothesis that the human cognitive architecture includes mechanisms that assess fighting ability-mechanisms that focus on correlates of upper-body strength. Across diverse samples of targets that included US college students, Bolivian horticulturalists and Andean pastoralists, subjects in the US were able to accurately estimate the physical strength of male targets from photos of their bodies and faces. Hierarchical linear modelling shows that subjects were extracting cues of strength that were largely independent of height, weight and age, and that corresponded most strongly to objective measures of upper-body strength-even when the face was all that was available for inspection. Estimates of women's strength were less accurate, but still significant. These studies are the first empirical demonstration that, for humans, judgements of strength and judgements of fighting ability not only track each other, but accurately track actual upper-body strength.

A bioeconomic approach to marriage and the sexual division of labor.

Children may be viewed as public goods whereby both parents receive equal genetic benefits yet one parent often invests more heavily than the other. We introduce a microeconomic framework for understanding household investment decisions to address questions concerning conflicts of interest over types and amount of work effort among married men and women. Although gains and costs of marriage may not be spread equally among marriage partners, marriage is still a favorable, efficient outcome under a wide range of conditions. This bioeconomic framework subsumes both cooperative and conflictive views on the sexual division of labor. We test hypotheses concerning marriage markets, assortative mating, and men's labor motivations among Tsimane forager-horticulturalists of Bolivia and find that: (1) men and women both value work effort in marital partners, (2) marital labor contributions are complementary, (3) work effort is correlated between spouses, (4) total production is correlated with total reproduction, and (5) better hunters have higher fitness gains within marital unions.

Niche diversity can explain cross-cultural differences in personality structure.

The covariance structure of personality traits derived from statistical models (for example, Big Five) is often assumed to be a human universal. Cross-cultural studies have challenged this view, finding that less-complex societies exhibit stronger covariation among behavioural characteristics, resulting in fewer derived personality factors. To explain these results, we propose the niche diversity hypothesis, in which a greater diversity of social and ecological niches elicits a broader range of multivariate behavioural profiles and, hence, lower trait covariance in a population. We formalize this as a computational model, which reproduces empirical results from recent cross-cultural studies and also yields an additional prediction for which we find empirical support. This work provides a general explanation for population differences in personality structure in both humans and other animals and suggests a substantial reimagining of personality research: instead of reifying statistical descriptions of manifest personality structures, research should focus more on modelling their underlying causes.