Aggressive Mimicry and the Evolution of the Human Cognitive Niche.

The evolutionary origins of deception and its functional role in our species is a major focus of research in the science of human origins. Several hypotheses have been proposed for its evolution, often packaged under either the Social Brain Hypothesis, which emphasizes the role that the evolution of our social systems may have played in scaffolding our cognitive traits, and the Foraging Brain Hypothesis, which emphasizes how changes in the human dietary niche were met with subsequent changes in cognition to facilitate foraging of difficult-to-acquire foods. Despite substantive overlap, these hypotheses are often presented as competing schools of thought, and there have been few explicitly proposed theoretical links unifying the two. Utilizing cross-cultural data gathered from the Human Relations Area Files (HRAF), we identify numerous (n = 357) examples of the application of deception toward prey across 145 cultures. By comparing similar behaviors in nonhuman animals that utilize a hunting strategy known as aggressive mimicry, we suggest a potential pathway through which the evolution of deception may have taken place. Rather than deception evolving as a tactic for deceiving conspecifics, we suggest social applications of deception in humans could have evolved from an original context of directing these behaviors toward prey. We discuss this framework with regard to the evolution of other mental traits, including language, Theory of Mind, and empathy.

Reciprocity and beyond: Explaining meat transfers in savanna-dwelling chimpanzees at Fongoli, Senegal.

OBJECTIVES: To understand the function of food sharing among our early hominin ancestors, we can turn to our nonhuman primate relatives for insight. Here, we examined the function of meat sharing by Fongoli chimpanzees, a community of western chimpanzees (Pan troglodytes verus) in southeastern Sénégal. MATERIALS AND METHODS: We tested three non-mutually exclusive hypotheses that have been used to explain patterns of food sharing: kin selection, generalized reciprocity, and meat-for-mating opportunities. We analyzed meat sharing events (n = 484) resulting from hunts, along with data on copulations, age-sex class, and kinship to determine which variables predict the likelihood of meat sharing during this study period (2006-2019). RESULTS: We found full or partial support for kin selection, direct reciprocity, and meat-for-mating-opportunities. However, the analyses reveal that reciprocity and a mother/offspring relationship were the strongest predictors of whether or not an individual shared meat. CONCLUSIONS: The results of this study emphasize the complexity of chimpanzee meat sharing behaviors, especially at a site where social tolerance offers increased opportunities for meat sharing by individuals other than dominant males. These findings can be placed in a referential model to inform hypotheses about the sensitivity of food sharing to environmental pressures, such as resource scarcity in savanna landscapes.

A comparison of intestinal parasite infections and activity budgets in three free-ranging vs. enclosure-housed lemur species (Lemur catta, Varecia rubra, and Eulemur mongoz).

This study examines how range size affects intestinal parasite infections, and how such infections affect activity budgets in captive lemurs. There were no differences in parasite richness or intensity attributable to habitat type. However, there was a strong suggestion that parasite loads reduce activity levels.

Small-scale utilitarianism: High acceptance of utilitarian solutions to Trolley Problems among a horticultural population in Nicaragua.

Researchers often use moral dilemmas to investigate the specific factors that influence participants' judgments of the appropriateness of different actions. A common construction of such a dilemma is the Trolley Problem, which pits an obvious utilitarian solution against a common deontological dictum to not do harm to others. Cross-cultural studies have validated the robustness of numerous contextual biases, such as judging utilitarian decisions more negatively if they require contact with other individuals (contact bias), they force others to serve as a means to an end (means bias), and if they require direct action rather than inaction (omission bias). However, such cross-cultural research is largely limited to studies of industrialized, nation-state populations. Previous research has suggested that the more intimate community relationships that characterize small-scale populations might lead to important differences, such as an absence of an omission bias. Here we contribute to this literature by investigating perceptions of Trolley Problem solutions among a Mayangna/Miskito community, a small-scale indigenous population in Nicaragua. Compared to previously sampled populations, the Mayangna/Miskito participants report higher levels of acceptance of utilitarian solutions and do not exhibit an omission bias. We also examine the justifications participants offered to explore how Mayangna/Miskito culture might influence moral judgments.

M-AAA-nsplaining: Gender bias in questions asked at the American Anthropological Association's Annual Meetings.

A large body of research has revealed the challenges that disproportionately affect women as they climb the academic ladder. One area that has received relatively little attention is women's experiences at academic conferences, which are often integral to academics' professional development. As conferences are attended by professional colleagues and influential players in specific fields, the professional consequences of any gender bias in criticism are likely to be amplified at such venues. Here, we explore the degree to which the likelihood of audience members asking a question and offering criticism is associated with the gender of a presenter. Audience questions were tabulated during the authors' visits to the three American Anthropological Association Annual Meetings. The results suggested that men were indeed marginally more likely to ask a question, both when considering all types of questions and when considering only critical questions. However, there was no evidence that they differentially targeted women for these questions. Future research might explore what motivates assertive and critical speech in men and women and how their experiences in receiving it might differ, particularly in academic settings in which critical speech might be considered more acceptable.

Exploring the Great Schism in the Social Sciences: Confirmation Bias and the Interpretation of Results Relating to Biological Influences on Human Behavior and Psychology.

The nature-nurture debate is one that biologists often dismiss as a false dichotomy, as all phenotypic traits are the results of complex processes of gene and environment interactions. However, such dismissiveness belies the ongoing debate that is unmistakable throughout the biological and social sciences concerning the role of biological influences in the development of psychological and behavioral traits in humans. Many have proposed that this debate is due to ideologically driven biases in the interpretation of results. Those favoring biological approaches have been accused of a greater willingness to accept biological explanations so as to rationalize or justify the status quo of inequality. Those rejecting biological approaches have been accused of an unwillingness to accept biological explanations so as to attribute inequalities solely to social and institutional factors, ultimately allowing for the possibility of social equality. While it is important to continue to investigate this topic through further research and debate, another approach is to examine the degree to which the allegations of bias are indeed valid. To accomplish this, a convenience sample of individuals with relevant postgraduate degrees was recruited from Mechanical Turk and social media. Participants were asked to rate the inferential power of different research designs and of mock results that varied in the degree to which they supported different ideologies. Results were suggestive that researchers harbor sincere differences of opinion concerning the inferential value of relevant research. There was no suggestion that ideological confirmation biases drive these differences. However, challenges associated with recruiting a large enough sample of experts as well as identifying believable mock scenarios limit the study's inferential scope.

The Fitness Effects of Men's Family Investments : A Test of Three Pathways in a Single Population.

Men's investments in parenting and long-term reproductive relationships are a hallmark feature of human reproduction and life history. The uniqueness of such male involvement among catarrhines has driven an extensive debate surrounding the selective pressures that led to and maintain such capacities in men. Three major pathways have been proposed through which men's involvement might confer fitness benefits: enhancing child well-being, increasing couple fertility, and decreasing likelihood of partner desertion. Previous research has explored the impact of father involvement on these factors individually, but here we present novel research that explores all three pathways within the same population, the Mayangna/Miskito horticulturalists of Nicaragua. Furthermore, we expand the traditional dichotomous measure of father presence/absence by using a continuous measure of overall male investment, as well as two continuous measures of its subcomponents: direct care and wealth. We find that men's investments are associated with children's growth and possibly with wife's marital satisfaction; however, they are not associated with couple fertility.

Inclusive fitness and differential productivity across the life course determine intergenerational transfers in a small-scale human society.

Transfers of resources between generations are an essential element in current models of human life-history evolution accounting for prolonged development, extended lifespan and menopause. Integrating these models with Hamilton's theory of inclusive fitness, we predict that the interaction of biological kinship with the age-schedule of resource production should be a key driver of intergenerational transfers. In the empirical case of Tsimane' forager-horticulturalists in Bolivian Amazonia, we provide a detailed characterization of net transfers of food according to age, sex, kinship and the net need of donors and recipients. We show that parents, grandparents and siblings provide significant net downward transfers of food across generations. We demonstrate that the extent of provisioning responds facultatively to variation in the productivity and demographic composition of families, as predicted by the theory. We hypothesize that the motivation to provide these critical transfers is a fundamental force that binds together human nuclear and extended families. The ubiquity of three-generational families in human societies may thus be a direct reflection of fundamental evolutionary constraints on an organism's life-history and social organization.

Anonymity versus privacy in the dictator game: revealing donor decisions to recipients does not substantially impact donor behavior.

Anonymity is often offered in economic experiments in order to eliminate observer effects and induce behavior that would be exhibited under private circumstances. However, anonymity differs from privacy in that interactants are only unaware of each others' identities, while having full knowledge of each others' actions. Such situations are rare outside the laboratory and anonymity might not meet the requirements of some participants to psychologically engage as if their actions were private. In order to explore the impact of a lack of privacy on prosocial behaviors, I expand on a study reported in Dana et al. (2006) in which recipients were left unaware of the Dictator Game and given donations as "bonuses" to their show-up fees for other tasks. In the current study, I explore whether differences between a private Dictator Game (sensu Dana et al. (2006)) and a standard anonymous one are due to a desire by dictators to avoid shame or to pursue prestige. Participants of a Dictator Game were randomly assigned to one of four categories-one in which the recipient knew of (1) any donation by an anonymous donor (including zero donations), (2) nothing at all, (3) only zero donations, and (4) and only non-zero donations. The results suggest that a lack of privacy increases the shame that selfish-acting participants experience, but that removing such a cost has only minimal effects on actual behavior.

Polygyny among the Tsimane of Bolivia: an improved method for testing the polygyny-fertility hypothesis.

The polygyny-fertility hypothesis states that polygyny is associated with reduced fertility for women and is supported by a large body of literature. This finding is important, because theoretical models of polygyny often differentiate systems based on the degree to which women are forced or willingly choose to enter polygynous marriages. The fact that polygyny tends to be associated with reduced fertility has been presented as evidence that polygyny is often less favourable for women, and that women must, therefore, be pressured into accepting such arrangements. Previous studies, however, have been hampered by the non-random assignment of women into monogamous and polygynous unions (i.e. self-selection), as differences between these groups of women might explain some of the effects. Furthermore, the vast majority of such studies focus on sub-Saharan populations. We address these problems in our analysis of women's fertility in polygynous marriages among the Tsimane of Bolivia. We offer a more robust method for assessing the impact of polygynous marriage on reproductive outcomes by testing for intra-individual fertility effects among first wives as they transition from monogamous to polygynous marriage. We report a significant link between polygyny and reduced fertility when including all cases of polygyny; however, this association disappears when testing only for intra-individual effects.

Infant growth and the thymus: data from two South American native societies.

The thymus plays an important role in the development of the immune system, yet little is known about the patterns and sources of variation in postnatal thymic development. The aim of this study is to contribute cross-cultural data on thymus size in infants from two South American native populations, the Tsimane of Bolivia and the Pumé of Venezuela. Thymic ultrasonography was performed and standard anthropometric measures collected from 86 Tsimane and Pumé infants. Patterns of infant growth and thymus size were compared between the two populations and the relationship between nutritional status and thymus size was assessed. Despite nearly identical anthropometric trajectories, Tsimane infants had larger thymuses than Pumé infants at all ages. Population, infant age, and infant mid-upper arm circumference were significant predictors of thymus area in the Tsimane and Pumé infants. This finding reveals a cross-cultural difference in thymus size that is not driven by nutritional status. We suggest that future studies focus on isolating prenatal and postnatal environmental factors underlying cross-cultural variation in thymic development.

Social aggression and resource conflict across the female life-course in the Bolivian Amazon.

This work explores sources of conflict among forager-horticulturalist women in Amazonian Bolivia, and applies life history theory as a tool for understanding competitive and cooperative social networking behaviors among women. In this study, 121 Tsimane women and girls were interviewed regarding current and past disagreements with others in their community to identify categories of contested resources that instigate interpersonal conflicts, often resulting in incidences of social aggression. Analysis of frequency data on quarrels (N = 334) reveals that women target several diverse categories of resources, with social types appearing as frequently as food and mates. It was also found that the focus of women's competition changes throughout the life-course, consistent with the notion that current vs. future reproduction and quantity-quality trade-offs might have different influences on competition and social conflict over resources within women's social networks across different age groups.

The total cost of father desertion.

OBJECTIVES: The benefits of paternal investment have long been explored by assessing the impact of father's presence on child wellbeing. Previous studies, however, have only examined the average effect of father's presence on child survivorship. Here we assess the total fitness cost to men of deserting (or the benefit of staying), by considering effects on the entire progeny. We estimate the total number of children that a deserting father can expect to lose due to reduced survivorship over the life course in five populations, and compare this loss to the benefit gains from remarrying a younger wife. METHODS: We compiled the observed impacts of father's absence, as well as mortality and fertility schedules, for five foraging or foraging/horticultural populations. We calculate how many additional children a man can expect to lose due to father's absence throughout a marriage. We then calculate the minimum age difference between a first and second spouse that would be necessary to overcome this cost. RESULTS: Because child mortality rates drop so rapidly, the costs that men experience from desertion due to augmented child mortality are modest throughout marriage. Even hypothetically inflated father effects can be overcome with modest age differences between first and second spouses. CONCLUSIONS: Returns to paternal investment in terms of increased child survival are not substantial compared to those received by successfully practicing a serial mating strategy. This suggests that factors other than the ability to enhance child survival, such as female choice, are important to the evolutionary history and continued adaptive functioning of men's unique reproductive strategies.

Spousal violence and paternal disinvestment among Tsimane' forager-horticulturalists.

OBJECTIVES: We develop and test a conceptual model of factors influencing the likelihood of physical wife abuse. The paternal disinvestment model emphasizes that spousal conflict over resource use results from men's attempts to increase individual fitness at a cost to the family (e.g., through pursuit of extramarital affairs). We propose that men use violence to control women's responses to the diversion of resources away from the family: to quell women's objections to male disinvestment, maintain women's parental investment, and to dissuade women from pursuing relationships with other men. METHODS: Interviews were conducted among men and women to determine rates of violence and demographic and behavioral covariates. Structural equation modeling and generalized estimating equations analyses were used to test predictions derived from the model. We also collected data on frequent complaints in marriage and women's perceptions of arguments precipitating violence. RESULTS: Over 85% of women experienced physical wife abuse (n = 49). Indicators of paternal disinvestment positively covary with indicators of marital strife and with rates of wife abuse. The wife's age, matrilocal residence, and presence of joint dependent offspring decrease the likelihood of violence through direct and indirect routes. CONCLUSIONS: Wife abuse is linked to the importance of paternal investment in human families, and is a means by which men control women's responses to a dual reproductive strategy of familial investment and pursuit of extramarital sexual relationships. This framework is more general than traditional sociological and evolutionary perspectives emphasizing patriarchy and men's sexual jealousy, respectively.

Learning, menopause, and the human adaptive complex.

This paper presents a new two-sex learning- and skills-based theory for the evolution of human menopause. The theory proposes that the role of knowledge, skill acquisition, and transfers in determining economic productivity and resource distribution is the distinctive feature of the traditional human ecology that is responsible for the evolution of menopause. The theory also proposes that male reproductive cessation and post-reproductive investment in descendants is a fundamental characteristic of humans living in traditional foraging and simple horticultural economies. We present evidence relevant to the theory. The data show that whereas reproductive decline is linked to increasing risks of mortality in chimpanzees, human reproductive senescence precedes somatic senescence. Moreover under traditional conditions, most human males undergo reproductive cessation at the same time as their wives. We then present evidence that after ceasing to reproduce, both men and women provide net economic transfers to children and grandchildren. Given this pattern of economic productivity, delays in menopause would produce net economic deficits within families.

The Social Strategy Game: Resource Competition within Female Social Networks among Small-scale Forager-Horticulturalists.

This paper examines social determinants of resource competition among Tsimane Amerindian women of Bolivia. We introduce a semi-anonymous experiment (the Social Strategy Game) designed to simulate resource competition among women. Information concerning dyadic social relationships and demographic data were collected to identify variables influencing resource competition intensity, as measured by the number of beads one woman took from another. Relationship variables are used to test how the affiliative or competitive aspects of dyads affect the extent of prosociality in the game. Using a mixed-modeling procedure, we find that women compete with those with whom they are quarreling over accusations of meat theft, mate competition, and rumor spreading. They also compete with members of their social network and with those who were designated as cooperative helpers or as close kin. Women take fewer beads from desired friends, neighbors, and from those viewed as enemies. We interpret favoritism toward enemies as resulting from fear of retribution. Our results suggest that social relations among women are multifaceted and often cannot be simplified by exclusive focus on genetic relatedness, physical proximity, or reciprocity. We argue that a complex understanding of cooperation and competition among women may require important contextual information concerning relationship history in addition to typical features of resource ecology.

Inflammation and infection do not promote arterial aging and cardiovascular disease risk factors among lean horticulturalists.

BACKGROUND: Arterial aging is well characterized in industrial populations, but scantly described in populations with little access to modern medicine. Here we characterize health and aging among the Tsimane, Amazonian forager-horticulturalists with short life expectancy, high infectious loads and inflammation, but low adiposity and robust physical fitness. Inflammation has been implicated in all stages of arterial aging, atherogenesis and hypertension, and so we test whether greater inflammation associates with atherosclerosis and CVD risk. In contrast, moderate to vigorous daily activity, minimal obesity, and low fat intake predict minimal CVD risk among older Tsimane. METHODS AND FINDINGS: Peripheral arterial disease (PAD), based on the Ankle-Brachial Index (ABI), and hypertension were measured in Tsimane adults, and compared with rates from industrialized populations. No cases of PAD were found among Tsimane and hypertension was comparatively low (prevalence: 3.5%, 40+; 23%, 70+). Markers of infection and inflammation were much higher among Tsimane than among U.S. adults, whereas HDL was substantially lower. Regression models examine associations of ABI and BP with biomarkers of energy balance and metabolism and of inflammation and infection. Among Tsimane, obesity, blood lipids, and disease history were not significantly associated with ABI. Unlike the Tsimane case, higher cholesterol, C-reactive protein, leukocytes, cigarette smoking and systolic pressure among North Americans are all significantly associated with lower ABI. CONCLUSIONS: Inflammation may not always be a risk factor for arterial degeneration and CVD, but instead may be offset by other factors: healthy metabolism, active lifestyle, favorable body mass, lean diet, low blood lipids and cardiorespiratory health. Other possibilities, including genetic susceptibility and the role of helminth infections, are discussed. The absence of PAD and CVD among Tsimane parallels anecdotal reports from other small-scale subsistence populations and suggests that chronic vascular disease had little impact on adult mortality throughout most of human evolutionary history.

The goals of direct paternal care among a South Amerindian population.

Human males are remarkable among mammals in the level of investment they provide to their wives and children. However, there has been debate as to the degree to which men actually invest and through which fitness pathways the benefits of familial investment are realized. Much of the previous research exploring these issues has focused on men's roles as providers, but few have explored correlates of men's direct parental care. Although this is reasonable given men's parental emphasis on provisioning, the providing of direct care is more straightforward with a clear provider and recipient and little ambiguity as to the care-giver's intent. Here, we explore contextual correlates of men's direct care among the Tsimane of Bolivia to determine the extent to which such care is patterned to enhance its effectiveness in increasing child wellbeing and the efficient functioning of the family. We also explore whether Tsimane fathers provide care in ways that enhance the positive effect it has on the wife's perception of the care provider. Overall, we find that Tsimane men appear responsive to the needs of children and the family, but show that there is little evidence that men respond to factors expected to increase the impact that men's care has on their reputations with their wives.