Prevalence and predictors of “nesting”: Solutions to adaptive challenges faced during pregnancy

Over the last half century, anthropologists have vigorously debated the adaptive motivations underlying food acquisition choices and food-sharing among hunter-gatherer groups. Numerous explanations have been proposed to account for high-levels of generosity in food-sharing, including self- and family-provisioning, reciprocity, tolerated theft and pro-social- or skill-signaling. However, few studies have asked foragers directly and systematically about the motivations underlying their foraging and sharing decisions. We recruited 110 Hadza participants and employed a combination of free-response, yes/no, ranking and forced-choice questions to do just this. In free-response answers, respondents typically gave outcome-oriented accounts of foraging motive (e.g., to get food) and moralistic accounts of sharing motive (e.g., I have a good heart). In ranking tasks, participants gave precedence to reciprocity as a motive for sharing food beyond the household. We found small but clear gender differences in foraging motive, in line with previous predictions: women were more likely than men to rank family-provisioning highly whereas men were more likely than women to rank skill-signaling highly. However, despite these gender differences, the relative importance of different motivations was similar across genders and skill-signaling, sharing and family-provisioning were the most important motivators of foraging activity for both men and women. Contrary to the expectations of tolerated theft, peer complaints and requests for food ranked very low. There are several compelling reasons that evolutionary thinkers, typically interested in ultimate-level adaptive processes, have traditionally eschewed direct and explicit investigations of motive. However, these data may yet provide important insights.

Male androphilia (i.e., sexual attraction and arousal to adult males) is considered an evolutionary puzzle because it reduces direct reproduction, but is influenced by genetic factors, reliably occurs across cultures, and has persisted over evolutionary time. The kin selection hypothesis states that genes for male androphilia can be maintained in a population if the costs of not reproducing directly are offset by enhancing the reproduction of kin. We tested this hypothesis among the Istmo Zapotec of Oaxaca, Mexico, where transgender and cisgender androphilic males are known as muxe gunaa and muxe nguiiu, respectively. We compared altruistic tendencies towards kin and non-kin children between muxe nguiiu (n = 106), muxe gunaa (n = 106), gynephilic men (i.e., men sexually attracted and aroused to adult females; n = 172), and androphilic women (n = 130). We also assessed whether the sisters of muxes (n = 96) reported receiving more childcare support from their muxe sibling compared to women with only gynephilic brothers (n = 65). The results showed that cisgender and transgender muxes reported more kin-directed altruistic tendencies than men. Muxe nguiiu also reported more kin-directed altruistic tendencies than women. When controlling for altruistic tendencies towards non-kin children, both muxe types exhibited more kin-directed altruistic tendencies than men and women. Women with muxe siblings reported receiving more childcare support from these relatives compared to women with only gynephilic brothers. These findings provide support for the kin selection hypothesis and highlight its potential role in elucidating the evolutionary paradox of male androphilia.

This study explores generosity among Mongolian pastoral nomads using a recipient identity-conditioned heuristic (RICH) allocation game in which study participants could allocate experimental funds among themselves and non-anonymous individuals in their local community. Allocation games were conducted with a sample of forty-six male and female pastoral household heads in Tosontsengel, Mongolia. Results indicate there were positive relationships between players' social reputations for being hardworking, skilled pastoralists, and generous and the amount of money they received from others. Players also gave more money to kin than non-kin. The results also indicate that players' allocation decisions were driven primarily by kinship and players' assessment of others' relative need.

Sex-biased dispersal has long been of interest to anthropologists and biologists, as it can structure populations and determine patterns of kinship, relatedness and cooperation. In most contemporary human societies, females usually disperse at marriage. In a minority of human societies, male dispersal, bisexual philopatry, or both sexes dispersing is practiced. Previous studies suggest that emergence of either agriculture, cattle pastoralism, or patriliny is associated with female-biased dispersal in certain language families. The ancestral patterns of sex-specific dispersal and its ecological correlates in Sino-Tibetans remain uncertain. Here we use comparative phylogenetic methods to infer the evolutionary history of sex-specific dispersal in Sino-Tibetan groups, and tested for coevolution between subsistence (agriculture and cattle-keeping), descent and sex-specific dispersal. We use a variety of ethnographic and historical sources to identify dispersal strategies across Sino-Tibetan phylogenetic trees (n = 97). We found that 1) earliest Sino-Tibetan groups were likely patrilocal; 2) agriculture likely co-evolved with only female dispersal patterns, but the result is sensitive to alternative coding strategy; 3) there is no evidence that domestic cattle co-evolved with dispersal patterns of either gender; and 4) kinship descent likely co-evolved with female dispersal, but not with male dispersal. Moreover, change from state of “patrilineal” to “non-patrilineal” triggered change in female dispersal patterns, from “female non-stay” to “female stay”. Our results suggest that change in descent drove change in female-specific dispersal pattern in Sino-Tibetans. Our findings illustrated how subsistence or descent can play different roles in shaping sex-biased dispersal patterns.

Based on the idea that women are especially attracted to ancestral markers of male genetic quality when conceptive in their cycle, scholars have conjectured that increases in women's extra-pair sexual interests during the conceptive phase of the cycle are moderated by their primary partners' sexual attractiveness. Multiple studies have examined this prediction, with largely supportive but mixed results. The current study is the first to examine whether hormonal influences—thought to mediate cycle shifts—on women's extra-pair sexual interests are moderated by partner attractiveness. 213 naturally cycling, romantically involved women (181 attending multiple sessions) participated in up to four sessions over about a month. Estrogen and progesterone levels were measured multiple times across women's cycles. Male partner attractiveness moderated associations between progesterone levels (though not estrogen levels) and women's extra-pair sexual interests. A negative association between progesterone levels and extra-pair sexual interests, a composite measure, was observed for women with relatively unattractive partners. For women with relatively attractive partners, no clear association emerged. The interaction between progesterone and partner attractiveness was robust for women's interest in extra-pair sex, a component preregistered as exploratory. The interaction effect was also significant for the absolute intensity of women's extra-pair attraction (a component item referenced in the preregistration) but was non-significant for the frequency of women's extra-pair attraction relative to typical days (a composite component specified in the preregistration). These findings inform theoretical understandings of women's sexuality.