Child and adolescent foraging: New directions in evolutionary research.

Young children and adolescents in subsistence societies forage for a wide range of resources. They often target child-specific foods, they can be very successful foragers, and they share their produce widely within and outside of their nuclear family. At the same time, while foraging, they face risky situations and are exposed to diseases that can influence their immune development. However, children's foraging has largely been explained in light of their future (adult) behavior. Here, we reinterpret findings from human behavioral ecology, evolutionary medicine and cultural evolution to center foraging children's contributions to life history evolution, community resilience and immune development. We highlight the need to foreground immediate alongside delayed benefits and costs of foraging, including inclusive fitness benefits, when discussing children's food production from an evolutionary perspective. We conclude by recommending that researchers carefully consider children's social and ecological context, develop cross-cultural perspectives, and incorporate children's foraging into Indigenous sovereignty discourse.

What We Can Learn About Emotion by Talking With the Hadza.

Emotions are often thought of as internal mental states centering on individuals' subjective feelings and evaluations. This understanding is consistent with studies of emotion narratives, or the descriptions people give for experienced events that they regard as emotions. Yet these studies, and contemporary psychology more generally, often rely on observations of educated Europeans and European Americans, constraining psychological theory and methods. In this article, we present observations from an inductive, qualitative analysis of interviews conducted with the Hadza, a community of small-scale hunter-gatherers in Tanzania, and juxtapose them with a set of interviews conducted with Americans from North Carolina. Although North Carolina event descriptions largely conformed to the assumptions of eurocentric psychological theory, Hadza descriptions foregrounded action and bodily sensations, the physical environment, immediate needs, and the experiences of social others. These observations suggest that subjective feelings and internal mental states may not be the organizing principle of emotion the world around. Qualitative analysis of emotion narratives from outside of a U.S. (and western) cultural context has the potential to uncover additional diversity in meaning-making, offering a descriptive foundation on which to build a more robust and inclusive science of emotion.

Evidence for an emotional adaptive function of dreams: a cross-cultural study.

The function of dreams is a longstanding scientific research question. Simulation theories of dream function, which are based on the premise that dreams represent evolutionary past selective pressures and fitness improvement through modified states of consciousness, have yet to be tested in cross-cultural populations that include small-scale forager societies. Here, we analyze dream content with cross-cultural comparisons between the BaYaka (Rep. of Congo) and Hadza (Tanzania) foraging groups and Global North populations, to test the hypothesis that dreams in forager groups serve a more effective emotion regulation function due to their strong social norms and high interpersonal support. Using a linear mixed effects model we analyzed 896 dreams from 234 individuals across these populations, recorded using dream diaries. Dream texts were processed into four psychosocial constructs using the Linguistic Inquiry and Word Count (LIWC-22) dictionary. The BaYaka displayed greater community-oriented dream content. Both the BaYaka and Hadza exhibited heightened threat dream content, while, at the same time, the Hadza demonstrated low negative emotions in their dreams. The Global North Nightmare Disorder group had increased negative emotion content, and the Canadian student sample during the COVID-19 pandemic displayed the highest anxiety dream content. In conclusion, this study supports the notion that dreams in non-clinical populations can effectively regulate emotions by linking potential threats with non-fearful contexts, reducing anxiety and negative emotions through emotional release or catharsis. Overall, this work contributes to our understanding of the evolutionary significance of this altered state of consciousness.

Prevalence and Predictors of Postpartum Cosleeping Practices after Midwife-Led Births in the US.

OBJECTIVE: To evaluate patterns of mother-infant sleeping behaviors among US-based mothers who received care from midwives and breastfed their infants the majority of time at 6 weeks postpartum. STUDY DESIGN: Infant sleep locations were reported for 24 915 mother-infant dyads followed through 6 weeks postpartum, following midwife-led singleton births. Using data derived from medical records, we used multinomial logistic regression to identify predictors of sleep location. RESULTS: The median maternal age was 31 years (IQR, 27-34 years). The majority were White (84.5%), reported having a partner or spouse (95%), had a community birth (87%), and reported bedsharing with their infant for part (13.2%) or most of the night (43.8%). In the adjusted analysis, positive predictors of always bedsharing included increasing maternal age (OR, 1.17; 95% CI, 1.13-1.21; per 5 years), cesarean birth (OR, 1.49; 95% CI, 1.18-1.86), Medicaid eligibility (OR, 1.76; 95% CI, 1.62-1.91), and maternal race/ethnicity (Black OR, 1.40 [95% CI, 1.09-1.79]; Latinx OR, 1.53 [95% CI, 1.35-1.74]; multiracial OR, 1.69 [95% CI, 1.39-2.07]). Negative predictors of bedsharing included having a partner/spouse (OR, 0.66; 95% CI, 0.56-0.77) and birth location in hospitals (OR, 0.56; 95% CI, 0.49-0.64) or birthing centers (OR, 0.48; 95% CI, 0.44-0.51). Partial breastfeeding dyads were less likely to bedshare than those who were exclusively breastfeeding (always bedsharing OR, 0.48 [95% CI, 0.41-0.56]; sometimes bedsharing OR 0.69 [95% CI, 0.56-0.83]). CONCLUSIONS: These data suggest that cosleeping is common among US families who choose community births, most of whom exclusively breastfeed through at least 6 weeks.

Socioecology shapes child and adolescent time allocation in twelve hunter-gatherer and mixed-subsistence forager societies.

A key issue distinguishing prominent evolutionary models of human life history is whether prolonged childhood evolved to facilitate learning in a skill- and strength-intensive foraging niche requiring high levels of cooperation. Considering the diversity of environments humans inhabit, children's activities should also reflect local social and ecological opportunities and constraints. To better understand our species' developmental plasticity, the present paper compiled a time allocation dataset for children and adolescents from twelve hunter-gatherer and mixed-subsistence forager societies (n = 690; 3-18 years; 52% girls). We investigated how environmental factors, local ecological risk, and men and women's relative energetic contributions were associated with cross-cultural variation in child and adolescent time allocation to childcare, food production, domestic work, and play. Annual precipitation, annual mean temperature, and net primary productivity were not strongly associated with child and adolescent activity budgets. Increased risk of encounters with dangerous animals and dehydration negatively predicted time allocation to childcare and domestic work, but not food production. Gender differences in child and adolescent activity budgets were stronger in societies where men made greater direct contributions to food production than women. We interpret these findings as suggesting that children and their caregivers adjust their activities to facilitate the early acquisition of knowledge which helps children safely cooperate with adults in a range of social and ecological environments. These findings compel us to consider how childhood may have also evolved to facilitate flexible participation in productive activities in early life.

Ethical microbiome research with Indigenous communities.

Human-microbiome interactions have been associated with evolutionary, cultural and environmental processes. With clinical applications of microbiome research now feasible, it is crucial that the science conducted, particularly among Indigenous communities, adheres to principles of inclusion. This necessitates a transdisciplinary dialogue to decide how biological samples are collected and who benefits from the research and any derived products. As a group of scholars working at the interface of biological and social science, we offer a candid discussion of the lessons learned from our own research and introduce one approach to carry out ethical microbiome research with Indigenous communities.

The energetics of uniquely human subsistence strategies.

The suite of derived human traits, including enlarged brains, elevated fertility rates, and long developmental periods and life spans, imposes extraordinarily high energetic costs relative to other great apes. How do human subsistence strategies accommodate our expanded energy budgets? We found that relative to other great apes, human hunter-gatherers and subsistence farmers spend more energy but less time on subsistence, acquire substantially more energy per hour, and achieve similar energy efficiencies. These findings revise our understanding of human energetic evolution by indicating that humans afford expanded energy budgets primarily by increasing rates of energy acquisition, not through energy-saving adaptations such as economical bipedalism or sophisticated tool use that decrease subsistence costs and improve the energetic efficiency of subsistence. We argue that the time saved by human subsistence strategies provides more leisure time for social interaction and social learning in central-place locations and would have been critical for cumulative cultural evolution.

An evolutionary theory of music needs to care about developmental timing.

Both target papers cite evidence from infancy and early childhood to support the notion of human musicality as a somewhat static suite of capacities; however, in our view they do not adequately acknowledge the critical role of developmental timing, the acquisition process, or the dynamics of social learning, especially during later periods of development such as middle childhood.

Harm Avoidance and Mobility During Middle Childhood and Adolescence among Hadza Foragers.

Cross-cultural sex differences in mobility and harm avoidance have been widely reported, often emphasizing fitness benefits of long-distance travel for males and high costs for females. Data emerging from adults in small-scale societies, however, are challenging the assumption that female mobility is restricted during reproduction. Such findings warrant further exploration of the ontogeny of mobility. Here, using a combination of machine-learning, mixed-effects linear regression, and GIS mapping, we analyze range size, daily distance traveled, and harm avoidance among Hadza foragers during middle childhood and adolescence. Distance traveled increased with age and, while male adolescents had the longest daily ranges, average daily distance traveled by each sex was similar. We found few age- or sex-related patterns in harm-avoidant responses and a high degree of individual variation. When queried on the same issues, children and their parents were often in alignment as to expectations pertaining to harm avoidance, and siblings tended to behave in similar ways. To the extent that sex differences in mobility did emerge, they were associated with ecological differences in physical threats associated with sex-specific foraging behaviors. Further, we found no strong association between harm avoidance and mobility. Young Hadza foragers of both sexes are highly mobile, regardless of how harm avoidant they are. Taken together, our findings indicate that the causal arrows between harm avoidance and mobility must be evaluated in ecologically specific frameworks where cultural expectations of juvenile mobility can be contextualized.

The Life History of Learning Subsistence Skills among Hadza and BaYaka Foragers from Tanzania and the Republic of Congo.

Aspects of human life history and cognition, such as our long childhoods and extensive use of teaching, theoretically evolved to facilitate the acquisition of complex tasks. The present paper empirically examines the relationship between subsistence task difficulty and age of acquisition, rates of teaching, and rates of oblique transmission among Hadza and BaYaka foragers from Tanzania and the Republic of Congo. We further examine cross-cultural variation in how and from whom learning occurred. Learning patterns and community perceptions of task difficulty were assessed through interviews. We found no relationship between task difficulty, age of acquisition, and oblique transmission, and a weak but positive relationship between task difficulty and rates of teaching. While same-sex transmission was normative in both societies, tasks ranked as more difficult were more likely to be transmitted by men among the BaYaka, but not among the Hadza, potentially reflecting cross-cultural differences in the sexual division of subsistence and teaching labor. Further, the BaYaka were more likely to report learning via teaching, and less likely to report learning via observation, than the Hadza, possibly owing to differences in socialization practices.

RéSUMé: Certains aspects de l'histoire de la vie humaine et de la cognition, comme la longue enfance et le recours intensif à l'enseignement, ont théoriquement évolué pour faciliter l'acquisition de tâches complexes. Le présent article examine empiriquement la relation entre la difficulté des tâches de subsistance et l'âge d'acquisition, les taux d'enseignement et les taux de transmission oblique chez les chasseurs-cueilleurs Hadza et BaYaka de Tanzanie et de la République du Congo. Nous avons également examiné les variations interculturelles sur la façon dont l'apprentissage se fait et auprès de qui. Les modèles d'apprentissage et les perceptions de la communauté concernant la difficulté des tâches ont été évalués par le biais d'entretiens. Nous n'avons trouvé aucune relation entre la difficulté de la tâche, l'âge d'acquisition et la transmission oblique, et une relation faible mais positive entre la difficulté de la tâche et les taux d'enseignement. Alors que la transmission entre personnes de même sexe était normative dans les deux sociétés, les tâches classées comme plus difficiles étaient plus susceptibles d'être transmises par les hommes chez les BaYaka, mais pas chez les Hadza, ce qui reflète potentiellement les différences interculturelles dans la division sexuelle touchant le travail impliqué dans la subsistance et l'enseignement. En outre, les BaYaka étaient plus susceptibles que les Hadza de déclarer qu'ils apprenaient au moyen de l'enseignement et moins susceptibles d'apprendre par observation, peut-être en raison de différences dans les pratiques de socialisation.

Ecological and Developmental Perspectives on Social Learning : Introduction to the Special Issue.

In this special issue of Human Nature we explore the possible adaptive links between teaching and learning during childhood, and we aim to expand the dialogue on the ways in which the social sciences, and in particular current anthropological research, may better inform our shifting understanding of how these processes vary in different social and ecological environments. Despite the cross-disciplinary trend toward incorporating more behavioral and cognitive data outside of postindustrial state societies, much of the published cross-cultural data is presented as stand-alone population-level studies, making it challenging to extrapolate trends or incorporate both ecological and developmental perspectives. Here, contributors explore how human life history, ecological experience, cumulative culture, and ethnolinguistics impact social learning and child development in foraging and transitioning societies around the world. Using historical ethnographic data and qualitative and quantitative data from studies with contemporary populations, authors interrogate the array of factors that likely interact with cognitive development and learning. They provide contributions that explore the unique environmental, social, and cultural conditions that characterize such populations, offering key insights into processes of social learning, adaptive learning responses, and culture change. This series of articles demonstrates that children are taught culturally and environmentally salient skills in myriad ways, ranging from institutionalized instruction to brief, nuanced, and indirect instruction. Our hope is that this collection stimulates more research on the evolutionary and developmental implications associated with teaching and learning among humans.

Gendered movement ecology and landscape use in Hadza hunter-gatherers.

Understanding how gendered economic roles structure space use is critical to evolutionary models of foraging behaviour, social organization and cognition. Here, we examine hunter-gatherer spatial behaviour on a very large scale, using GPS devices worn by Hadza foragers to record 2,078 person-days of movement. Theory in movement ecology suggests that the density and mobility of targeted foods should predict spatial behaviour and that strong gender differences should arise in a hunter-gatherer context. As predicted, we find that men walked further per day, explored more land, followed more sinuous paths and were more likely to be alone. These data are consistent with the ecology of male- and female-targeted foods and suggest that male landscape use is more navigationally challenging in this hunter-gatherer context. Comparisons of Hadza space use with space use data available for non-human primates suggest that the sexual division of labour likely co-evolved with increased sex differences in spatial behaviour and landscape use.

Effects of a mixed-subsistence diet on the growth of Hadza children.

INTRODUCTION: We investigated the preliminary effects of dietary changes on the anthropometric measurements of child and adolescent Hadza foragers. METHODS: We conducted a cross-sectional study comparing height and weight of participants (aged 0-17 years) at two time points, 2005 (n = 195) and 2017 (n = 52), from two locations: semi-nomadic "bush camps" and sedentary "village camps". World Health Organization (WHO) calculators were used to generate standardized z-scores for weight-for-height (WHZ), weight-for-age (WAZ), height-for-age (HAZ), and BMI-for-age (BMIFAZ). Cross tabulations were constructed for each measurement variable as a function of z-score categories and the variables year, location, and sex. RESULTS: Residency in a village, and associated mixed-subsistence diet, was associated with favorable growth, including greater WAZ (P < .001), HAZ (P < .001), and BMIFAZ (P = .004), but not WHZ (P = .717). Regardless of residency location, participants showed an improved WAZ (P = .021) and HAZ (P < .001) in the 2017 study year. We found no sex differences. DISCUSSION AND CONCLUSION: These preliminary findings suggest that a mixed-subsistence diet may confer advantages over an exclusive wild food diet, a trend also reported among other transitioning foragers.

Navigating cross-cultural research: methodological and ethical considerations.

The intensifying pace of research based on cross-cultural studies in the social sciences necessitates a discussion of the unique challenges of multi-sited research. Given an increasing demand for social scientists to expand their data collection beyond WEIRD (Western, educated, industrialized, rich and democratic) populations, there is an urgent need for transdisciplinary conversations on the logistical, scientific and ethical considerations inherent to this type of scholarship. As a group of social scientists engaged in cross-cultural research in psychology and anthropology, we hope to guide prospective cross-cultural researchers through some of the complex scientific and ethical challenges involved in such work: (a) study site selection, (b) community involvement and (c) culturally appropriate research methods. We aim to shed light on some of the difficult ethical quandaries of this type of research. Our recommendation emphasizes a community-centred approach, in which the desires of the community regarding research approach and methodology, community involvement, results communication and distribution, and data sharing are held in the highest regard by the researchers. We argue that such considerations are central to scientific rigour and the foundation of the study of human behaviour.

Changes in Juvenile Foraging Behavior among the Hadza of Tanzania during Early Transition to a Mixed-Subsistence Economy.

The Hadza foragers of Tanzania are currently experiencing a nutritional shift that includes the intensification of domesticated cultigens in the diet. Despite these changes, no study, to date, has examined the possible effects of this transition on the food collection behavior of young foragers. Here we present a cross-sectional study on foraging behavior taken from two time points, 2005 and 2017. We compare the number of days foraged and the type and amount of food collected for young foragers, aged 5-14 years, in age- and season-matched samples. Compared with 2005, in 2017 fewer subadults left camp to forage, and overall, they targeted a smaller variety of wild foods, with the noticeable absence of wild honey, figs, and tubers. In addition, participants in 2017 were significantly more likely to have attended school. Despite the increased presence of domesticated plant foods in the diet and increased attendance at school, some young foragers continue to be highly productive in collecting wild, undomesticated foods. Despite the preliminary nature of our results, our findings suggest that the range of wild foods targeted by subadults is decreasing as the amount of domesticated cultigens in the diet increases. These data underscore the importance of studying diet composition and foraging decisions across temporal, nutritional, and ecological landscapes.

Emotion Perception in Hadza Hunter-Gatherers.

It has long been claimed that certain configurations of facial movements are universally recognized as emotional expressions because they evolved to signal emotional information in situations that posed fitness challenges for our hunting and gathering hominin ancestors. Experiments from the last decade have called this particular evolutionary hypothesis into doubt by studying emotion perception in a wider sample of small-scale societies with discovery-based research methods. We replicate these newer findings in the Hadza of Northern Tanzania; the Hadza are semi-nomadic hunters and gatherers who live in tight-knit social units and collect wild foods for a large portion of their diet, making them a particularly relevant population for testing evolutionary hypotheses about emotion. Across two studies, we found little evidence of universal emotion perception. Rather, our findings are consistent with the hypothesis that people infer emotional meaning in facial movements using emotion knowledge embrained by cultural learning.

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

Long-lasting, romantic partnerships are a universal feature of human societies, but almost as ubiquitous is the risk of instability when one partner strays. Jealous response to the threat of infidelity is well studied, but most empirical work on the topic has focused on a proposed sex difference in the type of jealousy (sexual or emotional) that men and women find most upsetting, rather than on how jealous response varies1,2. This stems in part from the predominance of studies using student samples from industrialized populations, which represent a relatively homogenous group in terms of age, life history stage and social norms3,4. To better understand variation in jealous response, we conducted a 2-part study in 11 populations (1,048 individuals). In line with previous work, we find a robust sex difference in the classic forced-choice jealousy task. However, we also show substantial variation in jealous response across populations. Using parental investment theory, we derived several predictions about what might trigger such variation. We find that greater paternal investment and lower frequency of extramarital sex are associated with more severe jealous response. Thus, partner jealousy appears to be a facultative response, reflective of the variable risks and costs of men's investment across societies.

Universal norm psychology leads to societal diversity in prosocial behaviour and development.

Recent studies have proposed that social norms play a key role in motivating human cooperation and in explaining the unique scale and cultural diversity of our prosociality. However, there have been few studies that directly link social norms to the form, development and variation in prosocial behaviour across societies. In a cross-cultural study of eight diverse societies, we provide evidence that (1) the prosocial behaviour of adults is predicted by what other members of their society judge to be the correct social norm, (2) the responsiveness of children to novel social norms develops similarly across societies and (3) societally variable prosocial behaviour develops concurrently with the responsiveness of children to norms in middle childhood. These data support the view that the development of prosocial behaviour is shaped by a psychology for responding to normative information, which itself develops universally across societies.

Postpartum Maternal Mood Among Hadza Foragers of Tanzania: A Mixed Methods Approach.

Infant and maternal mortality rates are among the highest in the world in low and middle-income countries where postpartum depression impacts at least one in five women. Currently, there is a dearth of data on maternal mood and infant health outcomes in small-scale non-industrial populations from such countries, particularly during the postnatal period. Here, we present the first investigation of postpartum maternal mood among a foraging population, the Hadza of Tanzania. We administered the Edinburgh Postnatal Depression Scale (EPDS) to twenty-three women, all with infants under the age of 12 months. Semi-structured interviews on happiness and unhappiness during the post-partum period served as a validity cross-check for the EPDS. The combined results of the EPDS surveys and the interview responses suggest that a high proportion of Hadza women experience significant mood disturbances following birth and that postpartum unhappiness is associated with self-reports of pain, anxiety, and disturbed sleep patterns. These findings suggest that many of the mothers in our sample are experiencing post-partum unhappiness at levels similar to or higher than those reported for low to middle income countries in general, including Tanzania. These data are critical for improving our understanding of the etiologies of postpartum mood disturbances cross-culturally.