Homo medicus: The transition to meat eating increased pathogen pressure and the use of pharmacological plants in Homo

The human lineage transitioned to a more carnivorous niche 2.6 mya and evolved a large body size and slower life history, which likely increased zoonotic pathogen pressure. Evidence for this increase includes increased zoonotic infections in modern hunter-gatherers and bushmeat hunters, exceptionally low stomach pH compared to other primates, and divergence in immune-related genes. These all point to change, and probably intensification, in the infectious disease environment of Homo compared to earlier hominins and other apes. At the same time, the brain, an organ in which immune responses are constrained, began to triple in size. We propose that the combination of increased zoonotic pathogen pressure and the challenges of defending a large brain and body from pathogens in a long-lived mammal, selected for intensification of the plant-based self-medication strategies already in place in apes and other primates. In support, there is evidence of medicinal plant use by hominins in the middle Paleolithic, and all cultures today have sophisticated, plant-based medical systems, add spices to food, and regularly consume psychoactive plant substances that are harmful to helminths and other pathogens. We propose that the computational challenges of discovering effective plant-based treatments, the consequent ability to consume more energy-rich animal foods, and the reduced reliance on energetically-costly immune responses helped select for increased cognitive abilities and unique exchange relationships in Homo. In the story of human evolution, which has long emphasized hunting skills, medical skills had an equal role to play.

All Models Are Wrong, and Some Are Religious: Supernatural Explanations as Abstract and Useful Falsehoods about Complex Realities

Many cognitive and evolutionary theories of religion argue that supernatural explanations are byproducts of our cognitive adaptations. An influential argument states that our supernatural explanations result from a tendency to generate anthropomorphic explanations, and that this tendency is a byproduct of an error management strategy because agents tend to be associated with especially high fitness costs. We propose instead that anthropomorphic and other supernatural explanations result as features of a broader toolkit of well-designed cognitive adaptations, which are designed for explaining the abstract and causal structure of complex, unobservable, and uncertain phenomena that have substantial impacts on fitness. Specifically, we argue that (1) mental representations about the abstract vs. the supernatural are largely overlapping, if not identical, and (2) when the data-generating processes for scarce and ambiguous observations are complex and opaque, a naive observer can improve a bias-variance trade-off by starting with a simple, underspecified explanation that Western observers readily interpret as "supernatural." We then argue that (3) in many cases, knowledge specialists across cultures offer pragmatic services that involve apparently supernatural explanations, and their clients are frequently willing to pay them in a market for useful and effective services. We propose that at least some ethnographic descriptions of religion might actually reflect ordinary and adaptive responses to novel problems such as illnesses and natural disasters, where knowledge specialists possess and apply the best available explanations about phenomena that would otherwise be completely mysterious and unpredictable.

The Biological Roots of Music and Dance : Extending the Credible Signaling Hypothesis to Predator Deterrence

After they diverged from panins, hominins evolved an increasingly committed terrestrial lifestyle in open habitats that exposed them to increased predation pressure from Africa's formidable predator guild. In the Pleistocene, Homo transitioned to a more carnivorous lifestyle that would have further increased predation pressure. An effective defense against predators would have required a high degree of cooperation by the smaller and slower hominins. It is in the interest of predator and potential prey to avoid encounters that will be costly for both. A wide variety of species, including carnivores and apes and other primates, have therefore evolved visual and auditory signals that deter predators by credibly signaling detection and/or the ability to effectively defend themselves. In some cooperative species, these predator deterrent signals involve highly synchronized visual and auditory displays among group members. Hagen and Bryant (Human Nature, 14(1), 21-51, 2003) proposed that synchronized visual and auditory displays credibly signal coalition quality. Here, this hypothesis is extended to include credible signals to predators that they have been detected and would be met with a highly coordinated defensive response, thereby deterring an attack. Within-group signaling functions are also proposed. The evolved cognitive abilities underlying these behaviors were foundations for the evolution of fully human music and dance.

Strength is negatively associated with depression and accounts for some of the sex difference

Background

Depression occurs about twice as often in women as in men, a disparity that remains poorly understood. In a previous publication, Hagen and Rosenström predicted and found that grip strength, a highly sexually dimorphic index of physical formidability, mediated much of the effect of sex on depression. Striking results like this are more likely to be published than null results, potentially biasing the scientific record. It is therefore critical to replicate and extend them.

Methodology

Using new data from the 2013–14 cycle of the National Health and Nutrition Examination Survey, a nationally representative sample of US households (n = 3650), we replicated models of the effect of sex and grip strength on depression reported in Hagen and Rosenström, along with additional potential confounds and a new detailed symptom-level exploration.

Results

Overall, the effects from the original paper were reproduced although with smaller effect sizes. Grip strength mediated 38% of the effect of sex on depression, compared to 63% in Hagen and Rosenström. These results were extended with findings that grip strength had a stronger association with some depression symptoms, like suicidality, low interest and low mood than with other symptoms, like appetite changes.

Conclusions

Grip strength is negatively associated with depression, especially its cognitive–affective symptoms, controlling for numerous possible confounds. Although many factors influence depression, few of these reliably occur cross-culturally in a sex-stratified manner and so are unlikely to explain the well-established, cross-cultural sex difference in depression. The sex difference in upper body strength occurs in all populations and is therefore a candidate evolutionary explanation for some of the sex difference in depression.

Lay summary: Why are women at twice the risk of developing depression as men? Depression typically occurs during social conflicts, such as physical or sexual abuse. Physically strong individuals can often single-handedly resolve conflicts in their favor, whereas physically weaker individuals often need help from others. We argue that depression is a credible cry for help. Because men generally have greater strength than women, we argue that men may be more likely to resolve conflicts using physical formidability and women to signal others for help. We find that higher grip strength is associated with lower depression, particularly symptoms like feeling down or thoughts of suicide and that strength accounts for part of the sex difference in rates of depression.

Competitive gossip: the impact of domain, resource value, resource scarcity and coalitions

Those with better reputations often obtain more resources than those with poorer reputations. Consequently, gossip might be an evolved strategy to compete for valuable and scarce material and social resources. Influenced by models of non-human primate competition, we test the hypotheses that gossip: (i) targets aspects of reputation relevant to the domain in which the competition is occurring, (ii) increases when contested resources are more valuable, and (iii) increases when resources are scarcer. We then test hypotheses derived from informational warfare theory, which proposes that coalitions strategically collect, analyse and disseminate gossip. Specifically, we test whether: (iv) coalitions deter negative gossip, and (v) whether they increase expectations of reputational harm to competitors. Using experimental methods in a Mechanical Turk sample (n = 600), and survey and ego network analysis methods in a sample of California sorority women (n = 74), we found that gossip content is specific to the context of the competition; that more valuable and scarcer resources cause gossip, particularly negative gossip, to intensify; and that allies deter negative gossip and increase expectations of reputational harm to an adversary. These results support social competition theories of gossip. This article is part of the theme issue 'The language of cooperation: reputation and honest signalling'.

Toward a productive evolutionary understanding of music

We discuss approaches to the study of the evolution of music (sect. R1); challenges to each of the two theories of the origins of music presented in the companion target articles (sect. R2); future directions for testing them (sect. R3); and priorities for better understanding the nature of music (sect. R4).

Ethnoscientific expertise and knowledge specialisation in 55 traditional cultures

People everywhere acquire high levels of conceptual knowledge about their social and natural worlds, which we refer to as ethnoscientific expertise. Evolutionary explanations for expertise are still widely debated. We analysed ethnographic text records (N = 547) describing ethnoscientific expertise among 55 cultures in the Human Relations Area Files to investigate the mutually compatible roles of collaboration, proprietary knowledge, cultural transmission, honest signalling, and mate provisioning. We found relatively high levels of evidence for collaboration, proprietary knowledge, and cultural transmission, and lower levels of evidence for honest signalling and mate provisioning. In our exploratory analyses, we found that whether expertise involved proprietary vs. transmitted knowledge depended on the domain of expertise. Specifically, medicinal knowledge was positively associated with secretive and specialised knowledge for resolving uncommon and serious problems, i.e. proprietary knowledge. Motor skill-related expertise, such as subsistence and technological skills, was positively associated with broadly competent and generous teachers, i.e. cultural transmission. We also found that collaborative expertise was central to both of these models, and was generally important across different knowledge and skill domains.

Acculturation and market integration are associated with greater trust among Tanzanian Maasai pastoralists

Acting on socially learned information involves risk, especially when the consequences imply certain costs with uncertain benefits. Current evolutionary theories argue that decision-makers evaluate and respond to this information based on context cues, such as prestige (the prestige bias model) and/or incentives (the risk and incentives model). We tested the roles of each in explaining trust using a preregistered vignette-based study involving advice about livestock among Maasai pastoralists. In exploratory analyses, we also investigated how the relevance of each might be influenced by recent cultural and economic changes, such as market integration and shifting cultural values. Our confirmatory analysis failed to support the prestige bias model, and partially supported the risk and incentives model. Exploratory analyses suggested that regional acculturation varied strongly between northern vs. southern areas, divided by a small mountain. Consistent with the idea that trust varies with socially transmitted values and regional differences in market integration, people living near densely populated towns in the southern region were more likely to trust socially learned information about livestock. Higher trust among market-integrated participants might reflect a coordination solution in a region where traditional pastoralism is beset with novel conflicts of interest.

Origins of music in credible signaling

Music comprises a diverse category of cognitive phenomena that likely represent both the effects of psychological adaptations that are specific to music (e.g., rhythmic entrainment) and the effects of adaptations for non-musical functions (e.g., auditory scene analysis). How did music evolve? Here, we show that prevailing views on the evolution of music - that music is a byproduct of other evolved faculties, evolved for social bonding, or evolved to signal mate quality - are incomplete or wrong. We argue instead that music evolved as a credible signal in at least two contexts: coalitional interactions and infant care. Specifically, we propose that (1) the production and reception of coordinated, entrained rhythmic displays is a co-evolved system for credibly signaling coalition strength, size, and coordination ability; and (2) the production and reception of infant-directed song is a co-evolved system for credibly signaling parental attention to secondarily altricial infants. These proposals, supported by interdisciplinary evidence, suggest that basic features of music, such as melody and rhythm, result from adaptations in the proper domain of human music. The adaptations provide a foundation for the cultural evolution of music in its actual domain, yielding the diversity of musical forms and musical behaviors found worldwide.

0850 Self-report And Polysomnography Sleep And Mortality In Adults: A Machine Learning Replication Analysis

Introduction

Individual sleep health characteristics (e.g. efficiency, timing, duration, architecture) and signs and symptoms of sleep disorders (e.g., difficulty falling and staying asleep, apnea hypopnea index, measures of oxygen desaturation) predict mortality in adults using traditional regression methods. However, it is important to examine and compare their predictive abilities in context of other established non-sleep predictors using high-dimensional methods that better reflect the complexity of the data. Therefore, we applied a novel random forest machine learning (RFML) hypothesis-testing framework to data from the Sleep Heart Health Study (SHHS) and the Wisconsin Sleep Cohort (WSC) to determine which risk factor domains (sleep, physical health, sociodemographic factors, medications, health behaviors, mental health) and sleep subdomains (self-report and polysomnography sleep health characteristics and signs and symptoms of sleep disorders) predict time to mortality in adults.

Methods

We harmonized 82 predictors across SHHS and WSC (32 sleep, 24 physical health, 8 sociodemographic, 9 medications, 4 mental health, 5 health behaviors) and fit sociodemographic-adjusted and fully-adjusted RFML models in each cohort to test the overall predictive importance of each domain and sleep subdomain. Permutation-based p-values and unbiased variable importance metrics (change in Harrell’s C *100, ΔC) were computed and summarized with medians across 20 independent subsampled testing sets in each cohort.

Results

In the fully-adjusted SHHS and WSC models, the most predictive domains were physical health (SHHS p<0.001, ΔC=1.48; WSC p=0.002, ΔC=2.68) and sleep (SHHS p=0.008, ΔC=0.71; WSC p=0.044, ΔC=1.65). Sleep subdomains were not significant in the fully adjusted model. However, the sociodemographic-adjusted models indicated that the predictive importance of sleep may be driven by polysomnography sleep health characteristics in SHHS (p=0.026, ΔC =0.77) and polysomnography signs of sleep apnea in WSC (p<0.001, ΔC=3.20).

Conclusion

Sleep is a strong predictor of mortality in adults that should be considered among other more routinely used predictors. Future research should examine differences and similarities between SHHS and WSC that may explain the finding that different aspects of sleep were important in each cohort.

Support

NIA grant AG056331, NHLBI grant HL114473, NHLBI grant R01HL62252, NIA grant R01AG036838, NIA grant R01AG058680.

Mental health is biological health: Why tackling "diseases of the mind" is an imperative for biological anthropology in the 21st century

The germ theory of disease and the attendant public health initiatives, including sanitation, vaccination, and antibiotic treatment, led to dramatic increases in global life expectancy. As the prevalence of infectious disease declines, mental disorders are emerging as major contributors to the global burden of disease. Scientists understand little about the etiology of mental disorders, however, and many of the most popular psychopharmacological treatments, such as antidepressants and antipsychotics, have only moderate-to-weak efficacy in treating symptoms and fail to target biological systems that correspond to discrete psychiatric syndromes. Consequently, despite dramatic increases in the treatment of some mental disorders, there has been no decrease in the prevalence of most mental disorders since accurate record keeping began. Many researchers and theorists are therefore endeavoring to rethink psychiatry from the ground-up. Anthropology, especially biological anthropology, can offer critical theoretical and empirical insights to combat mental illness globally. Biological anthropologists are unique in that we take a panhuman approach to human health and behavior and are trained to address each of Tinbergen's four levels of analysis as well as culture. The field is thus exceptionally well-situated to help resolve the mysteries of mental illness by integrating biological, evolutionary, and sociocultural perspectives.

Evolutionary Models of Leadership : Tests and Synthesis

This study tested four theoretical models of leadership with data from the ethnographic record. The first was a game-theoretical model of leadership in collective actions, in which followers prefer and reward a leader who monitors and sanctions free-riders as group size increases. The second was the dominance model, in which dominant leaders threaten followers with physical or social harm. The third, the prestige model, suggests leaders with valued skills and expertise are chosen by followers who strive to emulate them. The fourth proposes that in small-scale, kin-based societies, men with high neural capital are best able to achieve and maintain positions of social influence (e.g., as headmen) and thereby often become polygynous and have more offspring than other men, which positively selects for greater neural capital. Using multiple search strategies we identified more than 1000 texts relevant to leadership in the Probability Sample of 60 cultures from the Human Relations Area Files (HRAF). We operationalized the model with variables and then coded all retrieved text records on the presence or absence of evidence for each of these 24 variables. We found mixed support for the collective action model, broad support for components of the prestige leadership style and the importance of neural capital and polygyny among leaders, but more limited support for the dominance leadership style. We found little evidence, however, of emulation of, or prestige-biased learning toward, leaders. We found that improving collective actions, having expertise, providing counsel, and being respected, having high neural capital, and being polygynous are common properties of leaders, which warrants a synthesis of the collective action, prestige, and neural capital and reproductive skew models. We sketch one such synthesis involving high-quality decision-making and other computational services.

Household composition and the infant fecal microbiome: The INSPIRE study

OBJECTIVES

Establishment and development of the infant gastrointestinal microbiome (GIM) varies cross-culturally and is thought to be influenced by factors such as gestational age, birth mode, diet, and antibiotic exposure. However, there is little data as to how the composition of infants' households may play a role, particularly from a cross-cultural perspective. Here, we examined relationships between infant fecal microbiome (IFM) diversity/composition and infants' household size, number of siblings, and number of other household members.

MATERIALS AND METHODS

We analyzed 377 fecal samples from healthy, breastfeeding infants across 11 sites in eight different countries (Ethiopia, The Gambia, Ghana, Kenya, Peru, Spain, Sweden, and the United States). Fecal microbial community structure was determined by amplifying, sequencing, and classifying (to the genus level) the V1-V3 region of the bacterial 16S rRNA gene. Surveys administered to infants' mothers identified household members and composition.

RESULTS

Our results indicated that household composition (represented by the number of cohabitating siblings and other household members) did not have a measurable impact on the bacterial diversity, evenness, or richness of the IFM. However, we observed that variation in household composition categories did correspond to differential relative abundances of specific taxa, namely: Lactobacillus, Clostridium, Enterobacter, and Klebsiella.

DISCUSSION

This study, to our knowledge, is the largest cross-cultural study to date examining the association between household composition and the IFM. Our results indicate that the social environment of infants (represented here by the proxy of household composition) may influence the bacterial composition of the infant GIM, although the mechanism is unknown. A higher number and diversity of cohabitants and potential caregivers may facilitate social transmission of beneficial bacteria to the infant gastrointestinal tract, by way of shared environment or through direct physical and social contact between the maternal-infant dyad and other household members. These findings contribute to the discussion concerning ways by which infants are influenced by their social environments and add further dimensionality to the ongoing exploration of social transmission of gut microbiota and the "old friends" hypothesis.

Social networks, cooperative breeding, and the human milk microbiome

OBJECTIVES

We present the first available data on the human milk microbiome (HMM) from small-scale societies (hunter-gatherers and horticulturalists in the Central African Republic [CAR]) and explore relationships among subsistence type and seasonality on HMM diversity and composition. Additionally, as humans are cooperative breeders and, throughout our evolutionary history and today, we rear offspring within social networks, we examine associations between the social environment and the HMM. Childrearing and breastfeeding exist in a biosocial nexus, which we hypothesize influences the HMM.

METHODS

Milk samples from hunter-gatherer and horticultural mothers (n = 41) collected over two seasons, were analyzed for their microbial composition. A subsample of these women's infants (n = 33) also participated in detailed naturalistic behavioral observations which identified the breadth of infants' social and caregiving networks and the frequency of contact they had with caregivers.

RESULTS

Analyses of milk produced by CAR women indicated that HMM diversity and community composition were related to the size of the mother-infant dyad's social network and frequency of care that infants receive. The abundance of some microbial taxa also varied significantly across populations and seasons. Alpha diversity, however, was not related to subsistence type or seasonality.

CONCLUSION

While the origins of the HMM are not fully understood, our results provide evidence regarding possible feedback loops among the infant, the mother, and the mother's social network that might influence HMM composition.

DHEAS patterning across childhood in three sub-Saharan populations: Associations with age, sex, ethnicity, and cortisol

OBJECTIVES

Hormones have many roles in human ontogeny, including the timing of life history 'switch points' across development. Limited hormonal data exist from non-Western children, leaving a significant gap in our understanding of the diversity of life history patterning. This cross-sectional study examines dehydroepiandrosterone sulfate (DHEAS) production in relation to age, sex, ethnicity, and cortisol concentrations, as well as average age of adrenarche, among Aka and Ngandu children of the Central African Republic and Sidama children of Ethiopia.

METHODS

Hair was collected from 480 children (160 per population) aged 3-18 years old. These samples were analyzed for DHEAS and cortisol concentrations using ELISAs. A generalized additive model was used to examine DHEAS patterning in relation to age, sex, cortisol, and ethnicity. The derivative of DHEAS as a function of age was used to identify average age of adrenarche in each population.

RESULTS

DHEAS patterning in these three populations is distinct from Euro-American patterns of production. In all three groups, the population-level age at adrenarche onset occurs slightly later than Euro-American averages, with both Central African populations experiencing a later onset than the Ethiopian population.

CONCLUSIONS

DHEAS patterns and age at adrenarche vary across cultures, perhaps indicating adaptive life history responses in diverse eco-cultural environments. Delayed involution of the fetal zone and DHEAS patterning may offer both cognitive protection and immune defense in high-risk, nutritionally-poor environments. Additional research in the majority world is essential to improving our understanding of the diversity of hormonal development and timing of 'switch points' in life history trajectories.

Radical framing effects in the ultimatum game: the impact of explicit culturally transmitted frames on economic decision-making

Many studies have documented framing effects in economic games. These studies, however, have tended to use minimal framing cues (e.g. a single sentence labelling the frame), and the frames did not involve unambiguous offer expectations. Results often did not differ substantially from those in the unframed games. Here we test the hypothesis that, in contrast to the modal offer in the unframed ultimatum game (UG) (e.g. 60% to the proposer and 40% to the responder), offers in a UG explicitly framed either as a currency exchange or a windfall will closely conform to expectations for the frame and diverge substantially from the modal offer. Participants recruited from MTurk were randomized into one of two conditions. In the control condition, participants played a standard UG. In the treatment conditions, players were provided a vignette explicitly describing the frame with their roles: some were customers and bankers in a currency exchange, and others were in a windfall scenario. We predicted (i) that modal offers in the currency exchange would involve an asymmetric split where greater than 80% went to customers and less than 20% went to bankers, and (ii) that variation in windfall offers would converge onto a 50-50 split with significantly less variation than the control condition. Our first prediction was confirmed with substantial effect sizes (d = 1.09 and d = -2.04), whereas we found no evidence for our second prediction. The first result provides further evidence that it is difficult to draw firm conclusions about economic decision-making from decontextualized games.

Innate food aversions and culturally transmitted food taboos in pregnant women in rural southwest India: separate systems to protect the fetus?

Pregnancy increases women's nutritional requirements, yet causes aversions to nutritious foods. Most societies further restrict pregnant women's diet with food taboos. Pregnancy food aversions are theorized to protect mothers and fetuses from teratogens and pathogens or increase dietary diversity in response to resource scarcity. Tests of these hypotheses have had mixed results, perhaps because many studies are in Westernized populations with reliable access to food and low exposure to pathogens. If pregnancy food aversions are adaptations, however, then they likely evolved in environments with uncertain access to food and high exposure to pathogens. Pregnancy food taboos, on the other hand, have been theorized to limit resource consumption, mark social identity, or also protect mothers and fetuses from dangerous foods. There have been few tests of evolutionary theories of culturally transmitted food taboos. We investigated these and other theories of psychophysiological food aversions and culturally transmitted food taboos among two non-Western populations of pregnant women in Mysore, India, that vary in food insecurity and exposure to infectious disease. The first was a mixed caste rural farming population (N = 72), and the second was the Jenu Kurubas, a resettled population of former hunter-gatherers (N = 30). Women rated their aversions to photos of 31 foods and completed structured interviews that assessed aversions and socially learned avoidances of foods, pathogen exposure, food insecurity, sources of culturally acquired dietary advice, and basic sociodemographic information. Aversions to spicy foods were associated with early trimester and nausea and vomiting, supporting a protective role against plant teratogens. Variation in exposure to pathogens did not explain variation in meat aversions or avoidances, however, raising some doubts about the importance of pathogen avoidance. Aversions to staple foods were common, but were not associated with resource stress, providing mixed support for the role of dietary diversification. Avoided foods outnumbered aversive foods, were believed to be abortifacients or otherwise harmful to the fetus, influenced diet throughout pregnancy, and were largely distinct from aversive foods. These results suggest that aversions target foods with cues of toxicity early in pregnancy, and taboos target suspected abortifacients throughout pregnancy.

Adaptive and non-adaptive models of depression: A comparison using register data on antidepressant medication during divorce

Divorce is associated with an increased probability of a depressive episode, but the causation of events remains unclear. Adaptive models of depression propose that depression is a social strategy in part, whereas non-adaptive models tend to propose a diathesis-stress mechanism. We compare an adaptive evolutionary model of depression to three alternative non-adaptive models with respect to their ability to explain the temporal pattern of depression around the time of divorce. Register-based data (304,112 individuals drawn from a random sample of 11% of Finnish people) on antidepressant purchases is used as a proxy for depression. This proxy affords an unprecedented temporal resolution (a 3-monthly prevalence estimates over 10 years) without any bias from non-compliance, and it can be linked with underlying episodes via a statistical model. The evolutionary-adaptation model (all time periods with risk of divorce are depressogenic) was the best quantitative description of the data. The non-adaptive stress-relief model (period before divorce is depressogenic and period afterwards is not) provided the second best quantitative description of the data. The peak-stress model (periods before and after divorce can be depressogenic) fit the data less well, and the stress-induction model (period following divorce is depressogenic and the preceding period is not) did not fit the data at all. The evolutionary model was the most detailed mechanistic description of the divorce-depression link among the models, and the best fit in terms of predicted curvature; thus, it offers most rigorous hypotheses for further study. The stress-relief model also fit very well and was the best model in a sensitivity analysis, encouraging development of more mechanistic models for that hypothesis.

Fetal Protection : The Roles of Social Learning and Innate Food Aversions in South India

Pregnancy involves puzzling aversions to nutritious foods. Although studies generally support the hypotheses that such aversions are evolved mechanisms to protect the fetus from toxins and/or pathogens, other factors, such as resource scarcity and psychological distress, have not been investigated as often. In addition, many studies have focused on populations with high-quality diets and low infectious disease burden, conditions that diverge from the putative evolutionary environment favoring fetal protection mechanisms. This study tests the fetal protection, resource scarcity, and psychological distress hypotheses of food aversions in a resource-constrained population with high infectious disease burden. The role of culture is also explored. In the first of two studies in Tamil Nadu, India, we investigated cultural explanations of pregnancy diet among non-pregnant women (N = 54). In the second study, we conducted structured interviews with pregnant women (N = 94) to determine their cravings and aversions, resource scarcity, indices of pathogen exposure, immune activation, psychological distress, and emic causes of aversions. Study 1 found that fruits were the most commonly reported food that pregnant women should avoid because of their harmful effects on infants. Study 2 found modest support for the fetal protection hypothesis for food aversions. It also found that pregnant women most commonly avoided fruits as well as "black" and "hot" foods. Aversions were primarily acquired through learning and focused on protecting the infant from harm. Our findings provide modest support for the fetal protection hypothesis and surprisingly strong support for the influence of cultural norms and learning on dietary aversions in pregnancy.

The low prevalence of female smoking in the developing world: gender inequality or maternal adaptations for fetal protection?

BACKGROUND

Female smoking prevalence is dramatically lower in developing countries (3.1%) than developed countries (17.2%), whereas male smoking is similar (32% vs 30.1%). Low female smoking has been linked to high gender inequality. Alternatively, to protect their offspring from teratogenic substances, pregnant and lactating women appear to have evolved aversions to toxic plant substances like nicotine, which are reinforced by cultural proscriptions. Higher total fertility rates (TFRs) in developing countries could therefore explain their lower prevalence of female smoking.

OBJECTIVE

To compare the associations of TFR and gender inequality with national prevalence rates of female and male smoking.

METHODS

Data from a previous study of smoking prevalence vs gender inequality in 74 countries were reanalysed with a regression model that also included TFR. We replicated this analysis with three additional measures of gender equality and 2012 smoking data from 173 countries.

RESULTS

A 1 SD increase in TFR predicted a decrease in female smoking prevalence by factors of 0.58-0.77, adjusting for covariates. TFR had a smaller and unexpected negative association with male smoking prevalence. Increased gender equality was associated with increased female smoking prevalence, and, unexpectedly, with decreased male smoking prevalence. TFR was also associated with an increase in smoking prevalence among postmenopausal women.

CONCLUSIONS

High TFR and gender inequality both predict reduced prevalence of female smoking across nations. In countries with high TFR, adaptations and cultural norms that protect fetuses from plant toxins might suppress smoking among frequently pregnant and lactating women.

Explaining the sex difference in depression with a unified bargaining model of anger and depression

BACKGROUND

Women are twice as likely as men to be depressed, a bias that is poorly understood. One evolutionary model proposes that depression is a bargaining strategy to compel reluctant social partners to provide more help in the wake of adversity. An evolutionary model of anger proposes that high upper body strength predisposes individuals to angrily threaten social partners who offer too few benefits or impose too many costs. Here, we propose that when social partners provide too few benefits or impose too many costs, the physically strong become overtly angry and the physically weak become depressed. The sexual dimorphism in upper body strength means that men will be more likely to bargain with anger and physical threats and women with depression.

METHODOLOGY

We tested this idea using the 2011-12 National Health and Nutrition Examination Survey (NHANES), a large nationally representative sample of US households that included measures of depression and upper body strength.

RESULTS

A 2 SD increase in grip strength decreased the odds of depression by more than half ([Formula: see text],[Formula: see text]), which did not appear to be a consequence of confounds with anthropometric, hormonal or socioeconomic variables, but was partially explained by a confound with physical disability. Nevertheless, upper body strength mediated 63% of the effect of sex on depression, but the mediation effect was unexpectedly moderated by age.

CONCLUSIONS

Low upper body strength is a risk factor for depression, especially in older adults, and the sex difference in body strength appears to explain much of the perplexing sex difference in depression.

Music and dance as a coalition signaling system

Evidence suggests that humans might have neurological specializations for music processing, but a compelling adaptationist account of music and dance is lacking. The sexual selection hypothesis cannot easily account for the widespread performance of music and dance in groups (especially synchronized performances), and the social bonding hypothesis has severe theoretical difficulties. Humans are unique among the primates in their ability to form cooperative alliances between groups in the absence of consanguineal ties. We propose that this unique form of social organization is predicated on music and dance. Music and dance may have evolved as a coalition signaling system that could, among other things, credibly communicate coalition quality, thus permitting meaningful cooperative relationships between groups. This capability may have evolved from coordinated territorial defense signals that are common in many social species, including chimpanzees. We present a study in which manipulation of music synchrony significantly altered subjects' perceptions of music quality, and in which subjects' perceptions of music quality were correlated with their perceptions of coalition quality, supporting our hypothesis. Our hypothesis also has implications for the evolution of psychological mechanisms underlying cultural production in other domains such as food preparation, clothing and body decoration, storytelling and ritual, and tools and other artifacts.

Psychological adaptations for assessing gossip veracity

Evolutionary models of human cooperation are increasingly emphasizing the role of reputation and the requisite truthful "gossiping" about reputation-relevant behavior. If resources were allocated among individuals according to their reputations, competition for resources via competition for "good" reputations would have created incentives for exaggerated or deceptive gossip about oneself and one's competitors in ancestral societies. Correspondingly, humans should have psychological adaptations to assess gossip veracity. Using social psychological methods, we explored cues of gossip veracity in four experiments. We found that simple reiteration increased gossip veracity, but only for those who found the gossip relatively uninteresting. Multiple sources of gossip increased its veracity, as did the independence of those sources. Information that suggested alternative, benign interpretations of gossip decreased its veracity. Competition between a gossiper and her target decreased gossip veracity. These results provide preliminary evidence for psychological adaptations for assessing gossip veracity, mechanisms that might be used to assess veracity in other domains involving social exchange of information.

High prevalence of cannabis use among Aka foragers of the Congo Basin and its possible relationship to helminthiasis

OBJECTIVES

Little is known about cannabis use in hunter-gatherers. Therefore, we investigated cannabis use in the Aka, a population of foragers of the Congo Basin. Because cannabis contains anthelminthic compounds, and the Aka have a high prevalence of helminthiasis, we also tested the hypothesis that cannabis use might be an unconscious form of self-medication against helminths.

METHODS

We collected self- and peer-reports of cannabis use from all adult Aka in the Lobaye district of the Central African Republic (n = 379). Because female cannabis use was low, we restricted sample collection to men. Using an immunoassay for Δ9-tetrahydrocannabinol-11-oic acid (THCA), a urinary biomarker of recent cannabis consumption, we validated cannabis use in men currently residing in camps near a logging road (n = 62). We also collected stool samples to assay worm burden. A longitudinal reinfection study was conducted among a subsample of the male participants (n = 23) who had been treated with a commercial anthelmintic 1 year ago.

RESULTS

The prevalence of self- and peer-reported cannabis use was 70.9% among men and 6.1% among women, for a total prevalence of 38.6%. Using a 50 ng/ml threshold for THCA, 67.7% of men used cannabis. Cannabis users were significantly younger and had less material wealth than the non-cannabis users. There were significant negative associations between THCA levels and worm burden, and reinfection with helminths 1 year after treatment with a commercial anthelmintic.

CONCLUSIONS

The prevalence of cannabis use among adult Aka men was high when compared to most global populations. THCA levels were negatively correlated with parasite infection and reinfection, supporting the self-medication hypothesis.

Explaining human recreational use of 'pesticides': The neurotoxin regulation model of substance use vs. the hijack model and implications for age and sex differences in drug consumption

Most globally popular drugs are plant neurotoxins or their close chemical analogs. These compounds evolved to deter, not reward or reinforce, consumption. Moreover, they reliably activate virtually all toxin defense mechanisms, and are thus correctly identified by human neurophysiology as toxins. Acute drug toxicity must therefore play a more central role in drug use theory. We accordingly challenge the popular idea that the rewarding and reinforcing properties of drugs "hijack" the brain, and propose instead that the brain evolved to carefully regulate neurotoxin consumption to minimize fitness costs and maximize fitness benefits. This perspective provides a compelling explanation for the dramatic changes in substance use that occur during the transition from childhood to adulthood, and for pervasive sex differences in substance use: because nicotine and many other plant neurotoxins are teratogenic, children, and to a lesser extent women of childbearing age, evolved to avoid ingesting them. However, during the course of human evolution many adolescents and adults reaped net benefits from regulated intake of plant neurotoxins.

A test of three hypotheses of pica and amylophagy among pregnant women in Tamil Nadu, India

OBJECTIVES

Pica has been studied in India and elsewhere for more than 100 years, yet no compelling and empirically well-supported explanation for it has emerged. Amylophagy, sometimes considered a type of pica and sometimes studied separately, is less frequently investigated and also lacks a convincing explanation. This study used a biocultural approach to test three hypotheses of pica and amylophagy: protection, hunger/nutrition, and psychological distress.

METHODS

The research took place in Tamil Nadu, India. In study 1, a cultural investigation was carried out among nonpregnant, adult women (n = 54) to determine nonfood substances that are consumed in this region and perceptions of health consequences. Next, using the substances identified in Study 1, three hypotheses of pica and amylophagy were tested in a cross-sectional study of pregnant women (Study 2, n = 95). Logistic regression analysis was used to analyze the presence or absence of engaging in pica and amylophagy. A series of bivariate analyses were used to examine the variation in amount and frequency of consumption.

RESULTS

Study 1 revealed that cultural attitudes strongly shape the selection of nonfood substances. In Study 2, the presence or absence of pica was not predicted by any of the variables included in the study, whereas the frequency and amount of consumption of pica substances were primarily explained by the psychological distress and hunger/nutrition hypotheses. Both the presence or absence of amylophagy as well as the frequency and amount of consumption were best explained by the protection hypothesis.

CONCLUSIONS

This research provided partial support for the protection and hunger/nutrition hypotheses for amylophagy, and also provided some evidence for the role of psychological distress and hunger or nutrition in pica.

Evolutionary theories of depression: a critical review

We critically review evolutionary theories of major depressive disorder (MDD). Because most instances of MDD appear to be caused by adversity, evolutionary theories of MDD generally propose that sadness and low mood evolved as beneficial responses to adversity, and that MDD is dysfunctional sadness and low mood. If so, MDD research should focus much more heavily on understanding the healthy functions of sadness and low mood to better understand how they dysfunction. A debate about the boundary between healthy sadness and MDD is then reviewed. In part, this debate turns on whether MDD's costliest symptoms could provide unknown benefits. Therefore, the review concludes by discussing 2 theories that explore possible benefits of prolonged anhedonia and suicidality.

Revealing the paradox of drug reward in human evolution

Neurobiological models of drug abuse propose that drug use is initiated and maintained by rewarding feedback mechanisms. However, the most commonly used drugs are plant neurotoxins that evolved to punish, not reward, consumption by animal herbivores. Reward models therefore implicitly assume an evolutionary mismatch between recent drug-profligate environments and a relatively drug-free past in which a reward centre, incidentally vulnerable to neurotoxins, could evolve. By contrast, emerging insights from plant evolutionary ecology and the genetics of hepatic enzymes, particularly cytochrome P450, indicate that animal and hominid taxa have been exposed to plant toxins throughout their evolution. Specifically, evidence of conserved function, stabilizing selection, and population-specific selection of human cytochrome P450 genes indicate recent evolutionary exposure to plant toxins, including those that affect animal nervous systems. Thus, the human propensity to seek out and consume plant neurotoxins is a paradox with far-reaching implications for current drug-reward theory. We sketch some potential resolutions of the paradox, including the possibility that humans may have evolved to counter-exploit plant neurotoxins. Resolving the paradox of drug reward will require a synthesis of ecological and neurobiological perspectives of drug seeking and use.

Perinatal Sadness among Shuar Women: Support for an Evolutionary Theory of Psychic Pain

Psychiatry faces an internal contradiction in that it regards mild sadness and low mood as normal emotions, yet when these emotions are directed toward a new infant, it regards them as abnormal. We apply parental investment theory, a widely used framework from evolutionary biology, to maternal perinatal emotions, arguing that negative emotions directed toward a new infant could serve an important evolved function. If so, then under some definitions of psychiatric disorder, these emotions are not disorders. We investigate the applicability of parental investment theory to maternal postpartum emotions among Shuar mothers. Shuar mothers' conceptions of perinatal sadness closely match predictions of parental investment theory.

Do human parents face a quantity-quality tradeoff?: Evidence from a Shuar community

A number of evolutionary theories of human life history assume a quantity-quality tradeoff for offspring production: parents with fewer offspring can have higher biological fitness than those with more. Direct evidence for such a tradeoff, however, is mixed. We tested this assumption in a community of Ecuadorian Shuar hunter-horticulturalists, using child anthropometry as a proxy for fitness. We measured the impact of household consumer/producer (CP) ratio on height, weight, skinfold thicknesses, and arm and calf circumferences of 85 children and young adults. To control for possible "phenotypic" correlates that might mask the effect of CP ratio on anthropometry, we also measured household garden productivity, wealth, and social status. Regression models of the age-standardized variables indicated a significant negative impact of CP ratio on child growth and nutrition. The age-standardized height and weight of children in households with the largest CP ratio (10) were 1.38 and 1.44 standard deviations, respectively, below those of children in households with the smallest CP ratio (2). Surprisingly, garden productivity, wealth, and status had little to no effect on the fitness proxies. There was, however, an interesting and unexpected interaction between status and sex: for females, but not males, higher father status correlated significantly with higher values on the proxies.

Game theory and human evolution: A critique of some recent interpretations of experimental games

Economists and psychologists have been testing Nash equilibrium predictions of game theory models of human behavior. In many instances, humans do not conform to the predictions. These results are of great interest to biologists because they also raise questions about well-known ESS models of cooperation. Cooperation in certain one-shot, anonymous interactions, and a willingness to punish others at a net cost to oneself are some of the most intriguing deviations from standard theory. One proposed explanation for these results that is receiving increasing attention invokes the cultural group selection of 'other regarding' social norms. We critically review this explanation. We conclude that experimental results reveal limits in two implicit models of cognitive structure commonly employed by economists and evolutionary biologists.

The second wave of evolutionary economics in biology

Several core issues in economics and biology overlap substantially. At the theoretical level, these include analogies and differences among rational choice, learning, genetic evolution and cultural evolution. At the empirical level, they include the structure of decision making, its neural basis and, more generally, human nature. We illustrate here the increasingly important collaboration between economics and biology with several characteristic examples, including signaling, markets, statistical reasoning, cooperation, punishment, reputation and social norms. In contrast to the mutual borrowing of ideas during the 1970s and 1980s, we now see the joint exploration of empirical and theoretical issues by biologists and economists that constitutes a second wave of interactions between the two disciplines.

Psychotropic substance-seeking: evolutionary pathology or adaptation?

According to a conventional evolutionary perspective, the human propensity for substance use is the product of a 'mismatch' between emotional mechanisms that evolved in a past without pure drugs or direct routes of drug administration, and the occurrence of these phenomena in the contemporary environment. The primary purpose of this review is to assert that, contrary to the conventional view, humans have shared a coevolutionary relationship with psychotropic plant substances that is millions of years old. We argue that this 'deep time' relationship is self-evident both in the extant chemical-ecological adaptations that have evolved in mammals to metabolize psychotropic plant substances and in the structure of plant defensive chemicals that have evolved to mimic the structure, and interfere with the function, of mammalian neurotransmitters. Given this evidence, we question how emotional mechanisms easily triggered by plant toxins can have evolved. Our argument is also supported with archeological and historical evidence of substance use in antiquity suggesting that, for people in the past, psychotropic plant substances were as much a mundane everyday item as they are for many people today. Our second, and more speculative objective is to suggest provisional hypotheses of human substance-using phenomena that can incorporate the evolutionary implications of a deep time relationship between psychotropic substances and people. We discuss hypotheses of selective benefits of substance use, including the idea that neurotransmitter-analog plant chemicals were exploited as substitutes for costly, nutritionally constrained endogenous neurotransmitters. However, even if substance seeking was adaptive in the environment of our hominid ancestors, it may not still be so in the contemporary environment. Thus, the implications of our argument are not that the mismatch concept does not apply to human substance-using phenomena, but that it must be reconsidered and extended to incorporate the implications of a substance-rich, rather than substance-free, evolutionary past.

Parental investment and child health in a Yanomamö village suffering short-term food stress

The 1998 El Niño significantly reduced garden productivity in the Upper Orinoco region in Venezuela. Consequently, parents were forced to allocate food carefully to their children. Nutrition data collected from village children combined with genealogical data allowed the determination of which children suffered most, and whether the patterns of food distribution accorded with predictions from parental investment theory. For boys, three social variables accounted for over 70% of the variance in subcutaneous fat after controlling for age: number of siblings, age of the mother's youngest child, and whether the mother was the senior or junior co-wife, or was married monogamously. These results accord well with parental investment theory. Parents experiencing food stress faced a trade-off between quantity and quality, and between investing in younger versus older offspring. In addition, boys with access to more paternal investment (i.e. no stepmother) were better nourished. These variables did not account for any of the variance in female nutrition. Girls' nutrition was associated with the size of their patrilineage and the number of non-relatives in the village, suggesting that lineage politics may have played a role. An apparent lack of relationship between orphan status and nutrition is also interesting, given that orphans suffered high rates of skin flea infections. The large number of orphans being cared for by only two grandparents suggests that grooming time may have been the resource in short supply.