Previous studies have found that using software to pitch shift people's voices can boost their perceived attractiveness to opposite-sex adults: men prefer women's voices when pitch-shifted up, and women prefer men's voices when pitch-shifted down. In this study, we sought to determine whether speakers could affect their perceived vocal attractiveness by voluntarily shifting their own voices to reach specific target pitches (+20 Hz or −20 Hz, a pitch increment that is based on prior research). Two sets of Chinese college students participated in the research: 115 who served as speakers whose voices were recorded, and 167 who served as raters who evaluated the speakers' voices. We found that when female speakers increased their pitch they were judged as more attractive to both opposite-sex and same-sex raters. An additional unexpected finding was that male speakers tended to rate other males who shifted their voice up in pitch as more attractive. These findings suggest that voluntary pitch shifts can affect attractiveness, but that they do not fully match the patterns observed when pitch shifting is done digitally.

Waist-to-hip ratio (WHR) and breast size are considered important biological features defining female body attractiveness; but their neurophysiological correlates remain largely unknown. To shed light on this issue, behavioral and electroencephalographic responses were recorded while healthy heterosexual men and women completed an oddball task, which was then followed by an attractiveness judgement task. In both cases participants were presented with female body forms that combined 0.6, 0.7 or 0.8 WHRs with small or large breast sizes. Brain activity dynamics were explored using temporal principal component analysis of the event-related potentials data. Variance in the data was explained mostly by eight temporal factors for the oddball task and six for the attractiveness judgement task. Lower WHRs seemed to gather larger processing resources than higher WHRs at early perceptual-related processes during the oddball task. The less attractive WHRs and breast sizes, in contrast, seemed to boost allocation of resources later in the processing stream, which may be due to a negativity bias enhancing evaluation and decision-making processes. This inference is supported by results from the attractiveness judgement task, although the observer's gender may also play a role. Finally, LORETA analysis of the temporal factors indicate that variations in WHR may modulate activation of frontal regions, such as the anterior cingulate and orbitofrontal cortex, that are related with reward processing and decision-making; while variations in breast size may influence activity in posterior parietal regions involved in human body perception. This paper highlights the biological importance of female body physical features in defining attractiveness, and the neurophysiological correlates that are implicated.

Why do perceivers categorize and stereotype others by their biological sex and age? We suggest that perceivers do so because sex and age interactively shape adaptive goals (e.g., mating, parenting) and strategies. And because such goals and strategies pose different fitness-relevant opportunities and threats, social perceivers use others' sex-age as a cue for predicting others' behaviors. This perspective has multiple implications, which we test in a range of U.S. undergraduate and online survey samples. First, we find that perceivers categorize others not by sex and age independently, but by the interaction of their sex and age (i.e., people mentally group others as females and males of specific ages) (Studies 1 and 2). Second, perceivers hold stereotypes of men and women of specific ages as being differentially oriented towards short- and long-term mating as well as parenting goals (e.g., women are stereotyped to be more oriented towards long-term mating goals than men are, but only at younger ages) (Studies 3 and 4). Finally, providing perceivers with direct information about others' adaptive goals can influence the extent to which perceivers apply stereotypes of agency, communion, and competence, and can even override typical sex stereotypes (e.g., men are generally stereotyped to be more agentic than women, but this sex stereotype disappears when both men and women are presented as engaging in short-term mating goals) (Studies 5 and 6). The current findings challenge existing thinking about sex and age stereotyping, and demonstrate the value of an adaptationist approach for thinking about social perception and stereotypes.

The substantial role of food sharing in human evolution has been widely recognized, and food-soliciting tactics may have been critical in facilitating these transfers. Great apes, our closest living relatives, also use food-soliciting tactics to obtain food from both kin and non-kin. However, the individual and social factors involved in requests for and subsequent transfers of food have been relatively little studied. Here, we examined which tactics (e.g., tactile gestures, taking actions, and vocalizations) infant chimpanzees (Pan troglodytes) employ to solicit food from their conspecifics as well as the success of obtaining food. Using a multimodal approach, we focused on food-related interactions of 14 chimpanzee infants of two different subspecies (P. t. schweinfurthii/verus) living in the communities of Kanyawara, Uganda, and Taï South, Côte d'Ivoire. Overall, we found that infants' solicitation tactics included mainly visual or tactile gestural requests and taking attempts, while vocalizations and gestures involving auditory components were rarely used. When addressing non-maternal conspecifics, infants used more visual gestures with age to solicit food. If food was solicited from mothers or maternal kin, infants predominantly begged for food via (mechanically effective) taking attempts. In terms of subsequent food transfers, taking attempts were more successful than gestures. In light of the prevalent use of non-contact begging despite low rates of success, food solicitation in young great apes might also function to facilitate social tolerance and gain social information. We thus conclude that the food sharing context might represent a critical platform to learn and practice social rules underlying cooperative interactions, which can later be generalized across collaborative domains.

The conditions that propel humans to make sacrifices for groups of unrelated, and often unknown, individuals has received considerable attention across scientific disciplines. Evolutionary explanations for this type of sacrifice have focused on how men form strategic coalitions organized around kin networks and reciprocity when faced with out-group threats. Few studies, however, have analyzed how women respond to external threats. Using data from one of the largest female paramilitary organizations in history we show that women who have more brothers, women whose husbands serve in the military and women without children are more likely to volunteer. These results provide qualified support for the hypothesis that women are more likely to sacrifice for their country when members of their family are at risk. Overall, our analysis suggests that self-sacrifice and intense bonding with an imagined community of unknown individuals, such as the nation state, may arise out of a suite of psychological adaptations designed to facilitate cooperation among kin (i.e. kin psychology). These results can be interpreted within the framework of kin selection showing how individuals come to view unrelated group members as family. They may also shed light on various theories of group alignment, such as ‘identity fusion’ – whereby individuals align their personal identity and interests with those of the group – and on our understanding of evolutionary adaptations that cause women to direct altruism toward in-groups.

Jünger et al. (2018) conducted a preregistered study examining whether women particularly prefer muscular bodies when conceptive in their cycles. Despite an impressive number of participants and within-woman observations, they found no evidence for a preference shift; rather, they claimed, conceptive women find all male bodies more attractive. We preregistered a separate study very similar to Jünger et al.'s, with specified analyses focusing on shifts associated with joint additive effects of log-transformed estradiol and progesterone (ln(E/P)). We performed similar analyses on Jünger et al.'s publicly available data, using an empirically vetted (though not preregistered) measure of Strength/Muscularity. They revealed a ln(E/P) × Strength/Muscularity × Relationship Status interaction effect on sexual attraction. The ln(E/P) × Strength/Muscularity interaction ran in opposite directions for partnered and single women effects largely driven by P levels. Jünger et al.'s null conclusions and claims about general preferences are premature. We offer several observations regarding preregistered analyses.

Adults use features such as facial hair to judge others' social dominance and mate value, but the origin of these judgments is unknown. We sought to determine when these associations develop, which associations develop first, and whether they are associated with early exposure to bearded faces. We presented pairs of bearded and clean-shaven faces to children (2–17 years old; N = 470) and adults (18–22 years; N = 164) and asked them to judge dominance traits (strength, age, masculinity) and mate choice traits (attractiveness, parenting quality). Young children associated beardedness with dominance traits but not mate choice traits. This pattern became more extreme during late childhood and gradually shifted toward adult-like responses during early adolescence. Responses for all traits were adult-like in late adolescence. Finally, having a bearded father was associated with positive judgments of bearded faces for mate choice traits in childhood and both mate choice and dominance traits in adolescence.

The formation of male-female social bonds and the resulting competition among females for male partners is a core element of human societies. While female competition for a male partner outside the mating context is well studied in humans, evidence from non-human primates is scarce, and its evolutionary roots remain to be explored. We studied two multi male – multi female groups of wild Assamese macaques (Macaca assamensis), a species where females gain benefits from selectively affiliating with particular males. Using a behavioral data set collected over several years, we tested whether females competed over access to male social partners, whether success in competition was driven by female dominance rank, and which male traits were most attractive for females. We found assortative bonding by dominance rank between females and males, which together with females initiating and maintaining contact suggests direct female competition over males. Two male traits independently predicted male attractiveness to females: (1) current dominance rank, a measure of “power” or a male's ability to provide access to resources, and (2) prior male affiliation with immatures, a measure of a male's potential paternal proclivity or “commitment” to infant care. Both traits have been consistently identified as drivers of female partner choice in humans. Our study adds to the evidence that female competition for valuable male partners is not unique to humans, suggesting deep evolutionary origins of women's mate choice tendencies for ‘power’ and ‘commitment’.

Pitch is the most perceptually salient feature of the voice, yet it is approximately five standard deviations lower in men than in women, a degree of sexual dimorphism exceeding that of all extant nonhuman apes. Evidence from Western samples suggests that low-frequency vocalizations may have augmented male mating success ancestrally by intimidating competitors and/or attracting mates. However, data are lacking from small-scale societies. We therefore investigated sexual selection on male pitch (measured by fundamental frequency, fo) in a population of Bolivian forager-horticulturists, the Tsimané. We found that experimentally lowering fo in audio clips of men speaking increased perceptions of fighting ability but did not affect perceptions of prestige and decreased their attractiveness to women. Further, men with lower speaking fo reported higher numbers of offspring, and this was mediated by the reproductive rates of men's wives, suggesting that men with lower fo achieved higher reproductive success by having access to more fertile mates. These results thus provide new evidence that men's fo has been shaped by intrasexual competition.

Covariation of life history traits across species may be organised on a ‘fast-slow’ continuum. A burgeoning literature in psychology and social science argues that trait covariation should be similarly organised across individuals within human populations. Here we describe why extrapolating from inter-species to inter-individual trait covariation is not generally appropriate. The process that genetically tailors species to their environments (i.e. Darwinian evolution) is fundamentally different from processes that tailor individuals to their environments (e.g. developmental plasticity), so their outcomes in terms of trait covariation need not be parallel or even related. We discuss why correlational selection, physical linkage, pleiotropy, and non-random mating do not substantively affect this claim in the context of complex human traits. We also discuss life history trade-offs and their relation to inter-individual trait covariation. We conclude that researchers should avoid hypotheses and explanations that assume trait covariation will correspond across and within species, unless they can mount a theoretically coherent argument to support this claim in the context of their research question.

Paternal care in humans is facultative, with investment decisions responsive to socioecological context. In particular, paternity confidence is thought to have a significant impact on men's provisioning. However, various aspects of the relationship a man has with his partner can also influence the way he provides for his children. Previous papers have tended to focus either on these kinds of relationship dynamics or on the impact of paternity confidence. However, these categories are often intertwined and parsing their contributions can be conceptually and methodologically difficult. To better understand how paternity confidence and relationship dynamics impact men's investment decisions, we used a series of pictorial vignettes to assess the resource allocation strategies of Himba men. We focus on three traits: mate fidelity, partner type (marital or non-marital), and relationship status (current or former). Results suggest that men prioritize mate fidelity and current reproductive partners in investment decisions, but social obligations to past and current partners and the presence of other male investors also influence decisions. Himba men appear to be balancing social norms related to marriage and fatherhood with individually-driven incentives to invest in current and more faithful partners.

People often value an item more when they own it than when it is available for purchase, and consequently are relatively reluctant to trade. This is the “endowment effect”, which has been widely documented in human populations and also in some non-human species. This paper develops a simple model in which it is adaptive to have a bias against trade, potentially explaining the basis of the endowment effect. The bias against trade arises from the strategic nature of trade in a moderately competitive environment: the interest of a potential trading partner in making the exchange is evidence that the decision maker already has the more valuable object. The model predicts that an endowment effect is promoted by large uncertainty about the fitness value of items, and also by conditions in which there are on average small gains to be had from trade. Because the model employs a simple bounded rationality heuristic for trade, it explains how the endowment effect could arise in species that lack theory of mind and related strategic reasoning abilities. The model also suggests an explanation for why endowment effects are so rarely observed in biological markets that exist between species. Because the trading classes have very different fitness functions, there is negligible competition across those classes. Consequently, there are substantial mutual gains to trade, so our model predicts there is unlikely to be adaptive pressure for an endowment effect.

Accurately assessing others' reputation, and developing a reputation as a competent, honest, fair individual—a good epistemic and moral reputation—are critical skills. One way to gain epistemic reputation is to display our competence by sharing valuable ideas, especially if we appropriate these ideas—i.e. present them as being our own, and not someone else's, whether that is the case or not (H1). However, idea appropriation should also entail some risks, otherwise it would lose its quality as a reliable signal. In particular, appropriating a bad idea should damage one's epistemic reputation (H2), and being caught appropriating someone else's idea should damage one's moral reputation (H3). As a consequence, people should be more likely to appropriate ideas they think are good when they are motivated to display their competence (H4), and they should refrain from doing so when the odds of getting caught increase (H5). Six online experiments (N = 904) find support for these hypotheses. To assess the reliability and generalizability of these findings, we suggest replicating them with heightened statistical power among similar English-speaking participants (in the UK, US, and Ireland), and among Japanese participants.

Humans exhibit an unusual pattern of sexual behavior compared to other mammalian females. Women's extended sexuality has been hypothesized to be related to a variety of possible benefits, especially non-genetic reproductive benefits, such as securing male investment via reinforced pairbonds or paternity confusion. But sexual behavior also comes at a cost, particularly for pregnant women, in terms of energetic costs, potential disease, and possible harm to the fetus. We hypothesize, therefore, that sexual behavior in pregnant women should reflect adaptive strategies and that pregnant women will be particularly strategic about their sexual behavior in order to maximize potential benefits while minimizing potential costs. One hundred twelve pregnant women completed a survey of their partners' qualities and their sexual desires toward their primary partners and men other than their primary partners. Results showed that women's perceptions of relationship threat positively predicted sexual desire for primary partners, while their perceptions of their partner's investing qualities negatively predicted sexual desire for extra-pair mates. These qualities, as well as cues to partner's genetic quality and gestation age, also interacted in ways that suggest that pregnant women's sexual desires are sensitive to cues of future investment and relationship stability.

A considerable volume of animal research on detecting threat and foraging reveals that the co-presence of conspecifics reduces vigilance and enhances foraging. Monitoring threat is an adaptive process and is of considerable relevance to humans. It is therefore important to understand how the presence of others influences threat monitoring - namely vigilance - and consequently the capacity to detect threats. We examine this with a novel paradigm, that simulates a “foraging under threat” situation, with an eye-tracker (allowing the examination of the allocation of attention). Our results show, as predicted, that participants in the individual condition (versus co-presence) allocated more attentional resources to scanning the environment and thereby sacrificing foraging, which increased their likelihood of detecting threatening events. Thus, the presence or absence of others modulates vigilance strategies in humans. These findings highlight the heuristic value of animal vigilance models to understand humans threat monitoring with considerable applied relevance.

In cooperatively breeding species, extended living in natal families after maturity is often associated with limited breeding possibilities and the ability to gain indirect fitness from helping relatives, with family dynamics, such as parental presence and relatedness between family members, playing a key role in determining the timing of own reproduction. How family dynamics affect marriage and the onset of reproduction in humans is complex and less well-understood. While paternal absence can be associated with both earlier puberty and reproductive behaviour, or with delayed reproduction if marriage requires parental resources, in step-parent families, half-siblings could further decrease the benefits from helping and delaying own reproduction compared to families with only full-siblings. Such costs and benefits are likely age-dependent, but have not been addressed in previous studies. Using data from pre-industrial agrarian Finland, we investigated if parental loss and remarriage affected marriage probabilities of their differently-aged sons and daughters. We found that parental composition had divergent effects across adulthood: loss of a parent resulted in a higher probability to marry in early adulthood, whereas parental presence increased later adulthood marriage probability. Whilst the death of either parent was linked to an overall lowered marriage probability, remarriage of the widowed parent, especially mother, could mitigate this effect somewhat. Additionally, the presence of underage full-siblings lowered marriage probability, suggesting postponement of one's own reproduction in favour of helping parental reproduction. Overall, our results support the idea that humans are cooperative breeders, and show the importance of considering both relatedness and age when investigating family dynamics.

Three autobiographical studies tested the valuable relationships hypothesis of forgiveness. Although previous studies revealed that relationship value predicts interpersonal forgiveness, the measure of relationship value may be conflated with affective assessments of the relationship with the transgressor, which might have caused a criterion contamination problem. Therefore, we assessed the goal-related instrumentality of the transgressor (i.e., how useful the transgressor is for helping the victim to achieve his/her goals in fitness-relevant domains). Three studies, one involving a Japanese student sample (Study 1), a second involving Japanese community sample (Study 2), and a third involving U.S. community sample (Study 3), convergently showed that perceived goal instrumentality, as well as a latent relationship value variable estimated from multiple measures of relationship value, are associated with forgiveness. Moreover, this association could be explained in part by the intermediate association of perceived goal instrumentality with empathy both in Japan and the U.S.

Our long co-evolutionary history with infectious agents likely began soon after the rise of the first single-celled organisms. This ongoing evolutionary arms race has generated complex host adaptations, many highly conserved, for resisting infection (e.g., innate and acquired immune systems, infection-sensitive developmental programs, sexual reproduction). A large body of evidence suggests that, in humans, pathogen-avoidance disgust is an emotion that motivates avoidance of cues associated with pathogens, thereby reducing infection. However, the question of whether there is an emotion that coordinates resistance to active infection has received less attention. We propose that lassitude is such an emotion. It is triggered by cues of active infection and coordinates the fight against infection by: (a) reducing energetically expensive movement to make more energy available to the immune system, (b) reducing exposure to additional infections and injuries that would compound the immune system's workload, (c) promoting thermoregulatory behaviors that facilitate immunity, (d) regulating food consumption to be beneficial for the host but detrimental to pathogens, and (e) deploying strategies that elicit caregiving behavior from social allies. Lassitude exhibits the core features of an emotion – it is triggered by cues of an adaptive problem (i.e., infection), generates a characteristic facial expression (e.g., slack facial muscles, drooping eyelids, slightly parted lips), and has distinct qualia (e.g., profound tiredness, reduced appetite, feelings of vulnerability, altered temperature perception, increased pain sensitivity). We outline the information-processing structure of lassitude, review existing evidence, suggest directions for future research, and discuss implications of lassitude for models of human evolution.

Individuals can gain high social rank through dominance (based on coercion and fear) and prestige (based on merit and admiration). We conducted a cross-cultural developmental study and tested 5- to 12-year-olds, and adults in the UK and China, aiming to determine (a) the age at which children distinguish dominance and prestige, and (b) the influence of cultural values on rank-related reasoning. We specifically tested participants in China because of the value of prestigious individuals modestly yielding to subordinates, a social skill that becomes more salient with age. In both populations, the distinction between dominance and prestige emerged at five years, and improved over childhood. When reasoning about a resource conflict between a high-ranking party and a subordinate, adults in both countries expected high-rank individuals to win, although Chinese adults were less likely to do so regarding prestigious individuals. Across the two countries, younger children (5–7 years) responded similarly to each other, not favoring either party as the winner. Older children (9–12 years), however, diverged. Those in the UK chose the high-rank party, while those in China made no systematic inference. Overall, our findings suggest that while children distinguish prestige and dominance comparably in the two countries, they develop culturally-influenced expectations about the behavior of high-rank individuals.

How do singles' strategies for engaging in sexual activity with a new partner vary across the adult lifespan? Using three large and independent demographically representative cross-sectional samples of heterosexual single adults in the U.S., we found that females approaching the typical age of menopause became less likely to establish relationship exclusivity prior to sexual activity with a new partner. However, after the typical age of menopausal onset, females returned to earlier levels of commitment choosiness. These changes in commitment choosiness surrounding the age of menopause were consistent across two studies (including a larger dataset combining two samples). Findings suggest that single females approaching menopause—a major life history milestone—alter their behavior to achieve reproductively relevant partnering goals but abandon this mating strategy once the typical reproductive period has ended. Males exhibited similar, though attenuated, changes in expected relationship commitment before sexual activity during midlife as well. Age-related changes in commitment corresponded with the amount of stress expressed regarding one's “biological clock”. However, reduced commitment choosiness did not vary with frequency of sexual thoughts, frequency of sexual behaviors, or external pressures to find a romantic partner. Results are discussed in terms of life history theory and sex differences in sexuality.

Teaching is cross-culturally widespread but few studies have considered children as teachers as well as learners. This is surprising, since forager children spend much of their time playing and foraging in child-only groups, and thus, have access to many potential child teachers. Using the Social Relations Model, we examined the prevalence of child-to-child teaching using focal follow data from 35 Hadza and 38 BaYaka 3- to 18-year-olds. We investigated the effect of age, sex and kinship on the teaching of subsistence skills. We found that child-to-child teaching was more frequent than adult-child teaching. Additionally, children taught more with age, teaching was more likely to occur within same-sex versus opposite-sex dyads, and close kin were more likely to teach than non-kin. The Hadza and BaYaka also showed distinct learning patterns; teaching was more likely to occur between sibling dyads among the Hadza than among the BaYaka, and a multistage learning model where younger children learn from peers, and older children from adults, was evident for the BaYaka, but not for the Hadza. We attribute these differences to subsistence and settlement patterns. These findings highlight children's role in the intergenerational transmission of subsistence skills.

Mate choice lies close to differential reproduction, the engine of evolution. Patterns of mate choice consequently have power to direct the course of evolution. Here we provide evidence suggesting one pattern of human mate choice—the tendency for mates to be similar in overall desirability—caused the evolution of a structure of correlations that we call the d factor. We use agent-based models to demonstrate that assortative mating causes the evolution of a positive manifold of desirability, d, such that an individual who is desirable as a mate along any one dimension tends to be desirable across all other dimensions. Further, we use a large cross-cultural sample with n = 14,478 from 45 countries around the world to show that this d-factor emerges in human samples, is a cross-cultural universal, and is patterned in a way consistent with an evolutionary history of assortative mating. Our results suggest that assortative mating can explain the evolution of a broad structure of human trait covariation.

Social information enables individuals to reduce uncertainty and increase decision accuracy across a broad range of domains. Intriguingly, individuals and populations consistently differ in social information use. Understanding the underlying causes of this variation has proven challenging due to the lack of a standardized paradigm to quantify social information use. Here we introduce the BEAST (Berlin Estimate AdjuStment Task); a brief (∼5-min), simple, and incentive-compatible behavioural task to quantify individuals' propensities to use social information. In the task, participants observe an image with a number of animals and estimate the total number. Next, they receive another person's estimate, after which they provide a second estimate. An individual's average adjustment quantifies their propensity to use social information. We found that individuals' propensity to use social information is consistent within the task, has considerable test–retest reliability over 9 months, generalizes to other social learning tasks, and correlates with established self-reported measures of social conformity and social proximity. The BEAST thus reliably captures individual variation in social information use. We conclude by highlighting the BEAST's potential to serve as a flexible framework to assess the determinants of human social information use.

We apply recent adaptationist theories about the emotions “pride” and “shame” to the domain of hierarchical status and test the hypothesis that pride and shame are distinct components of a culturally universal status-management system. Using an international dataset containing ratings of the status impacts of 240 personal characteristics within 14 nations (N = 2751), we found that (i) the status impacts of personal characteristics were strongly intercorrelated across nations (rs = 0.79–0.98); (ii) American's (N = 222) forecasts of the pride or shame they would experience if they exhibited those same personal characteristics closely tracked the status impacts across nations (|rs| = 0.74–0.98); and (iii) pride differentially tracked status gains, while shame differentially tracked status losses. These findings provide strong supporting evidence for the existence of a universal grammar of status criteria, and suggest that pride and shame are key components of a culturally universal status management system.

Loss aversion is often assumed to be a basic and far-reaching psychological regularity in behavior. Yet empirical evidence is accumulating to challenge the assumption of widespread loss aversion in choice. We suggest that a key reason for the apparently elusive nature of loss aversion may be that its manifestation in choice is state-dependent and distinct from a more state-independent principle of heightened attention to losses relative to gains. Using data from process-tracing studies, we show that people invest more attentional resources when evaluating losses than when evaluating gains, even when their choices do not reflect loss aversion. Our evidence converges with previous findings on how losses influence exploratory search as well as physiological, hormonal, and neural responses. Increased attention to losses relative to gains seems to be a necessary but not a sufficient condition for loss aversion in choice.

Plants have developed toxic chemical and physical defenses as a consequence of their co-evolution with herbivores. Humans, like other animal species, have evolved strategies to protect themselves from such plant dangers. For example, recent studies have shown that human infants exhibit a reluctance to manually explore plants and use social learning (SL) to acquire knowledge about plants. However, SL can also be costly under certain circumstances and there is reason to suspect this may be the case for plants. Some plant species are difficult to distinguish from one another. For example, some plants have evolved an adaptive strategy to fight against herbivorous threats, called Batesian mimicry, in which an edible plant mimics features of a poisonous plant to minimize the probability that it is consumed. When SL is prevalent in a population, by proliferating the knowledge about an edible mimic, SL also spreads the risk of consuming its poisonous counterpart. Here we propose a model describing different scenarios where SL is (a) favored, (b) ecologically stable, and (c) expected to evolve. Results show that SL is selected when the proportion of poisonous plants is high. However, this is only true if the edible mimic population is below a certain threshold and its selection depends on the capacity to minimize errors when differentiating edible mimics from their poisonous counterparts.

Humans can use an intuitive sense of statistics to make predictions about uncertain future events, a cognitive skill that underpins logical and mathematical reasoning. Recent research shows that some of these abilities for statistical inferences can emerge in preverbal infants and non-human primates such as apes and capuchins. An important question is therefore whether animals share the full complement of intuitive reasoning abilities demonstrated by humans, as well as what evolutionary contexts promote the emergence of such skills. Here, we examined whether free-ranging rhesus macaques (Macaca mulatta) can use probability information to infer the most likely outcome of a random lottery, in the first test of whether primates can make such inferences in the absence of direct prior experience. We developed a novel expectancy-violation looking time task, adapted from prior studies of infants, in order to assess the monkeys' expectations. In Study 1, we confirmed that monkeys (n = 20) looked similarly at different sampled items if they had no prior knowledge about the population they were drawn from. In Study 2, monkeys (n = 80) saw a dynamic ‘lottery’ machine containing a mix of two types of fruit outcomes, and then saw either the more common fruit (expected trial) or the relatively rare fruit (unexpected trial) fall from the machine. We found that monkeys looked longer when they witnessed the unexpected outcome. In Study 3, we confirmed that this effect depended on the causal relationship between the sample and the population, not visual mismatch: monkeys (n = 80) looked equally at both outcomes if the experimenter pulled the sampled item from her pocket. These results reveal that rhesus monkeys spontaneously use information about probability to reason about likely outcomes, and show how comparative studies of nonhumans can disentangle the evolutionary history of logical reasoning capacities.

Predation risk is a significant concern when social animals including humans engage in foraging. When people search for resources together, individuals often find themselves in a producer–scrounger game, in which some individuals bear the cost of risk monitoring while others can free ride on those efforts. A theoretically rational strategy is to mix foraging and risk monitoring randomly with the same probability across all members, but such uncoordinated action often yields inefficiencies of under- or over-supply of risk monitoring in a group. Here, we examined whether people could spontaneously develop a coordinated risk-monitoring system, alternating vigilance and foraging in a pair. Given that human cooperation is vulnerable to fear of exploitation and emotional arousal under risk, we hypothesized that such sources of anxiety would be potential disruptors to coordination. In a laboratory experiment, two participants worked on a “treasure hunt” task simultaneously, in which they chose between low or high vigilance against predators during foraging without verbal communication. If one chose high vigilance with personal cost, it yielded a spillover benefit to the other. Besides behavioral choices, each participant's physiological arousal (skin conductance response) and cognitive effort (tonic pupil dilation) were measured during the task. Results showed that some pairs were actually able to develop a role-alternating system over time through tacit coordination, but coordinated action was also vulnerable to anxiety and mistrust among participants. Overall, these results imply that, besides the mutual behavioral control that often characterizes repeated interaction, cognitive control of emotional arousal may be a critical psychological factor for the emergence of coordinated cooperation.

Parent-offspring conflict—conflict over resource distribution within families due to differences in genetic relatedness—is the biological foundation for many psychological phenomena. In genomic imprinting disorders, parent-specific genetic expression is altered causing imbalances in behaviors influenced by parental investment. We use this natural experiment to test the theory that parent-offspring conflict contributed to the evolution of vocal music by moderating infant demands for parental attention. Individuals with Prader-Willi syndrome, a genomic imprinting disorder resulting from increased relative maternal genetic contribution, show enhanced relaxation responses to song, consistent with reduced demand for parental investment (Mehr, Kotler, Howard, Haig, & Krasnow, 2017, Psychological Science). We report the necessary complementary pattern here: individuals with Angelman syndrome, a genomic imprinting disorder resulting from increased relative paternal genetic contribution, demonstrate a relatively reduced relaxation response to song, suggesting increased demand for parental attention. These results support the extension of genetic conflict theories to psychological resources like parental attention.

Social emotions are hypothesized to be adaptations designed by selection to solve adaptive problems pertaining to social valuation—the disposition to attend to, associate with, and aid a target individual based on her probable contributions to the fitness of the valuer. To steer between effectiveness and economy, social emotions need to activate in precise proportion to the local evaluations of the various acts and characteristics that dictate the social value of self and others. Supporting this hypothesis, experiments conducted in the United States and India indicate that five different social emotions all track a common set of valuations. The extent to which people value each of 25 positive characteristics in others predicts the intensities of: pride (if you had those characteristics), anger (if someone failed to acknowledge that you have those characteristics), gratitude (if someone convinced others that you have those characteristics), guilt (if you harmed someone who has those characteristics), and sadness (if someone died who had those characteristics). The five emotions track local valuations (mean r = +.72) and even foreign valuations (mean r = +.70). In addition, cultural differences in emotion are patterned: They follow cultural differences in valuation. These findings suggest that multiple social emotions are governed (in part) by a common architecture of social valuation, that the valuation architecture operates with a substantial degree of universality in its content, and that a unified theoretical framework may explain cross-cultural invariances and cultural differences in emotion.

Selection should favor mating preferences that increase the chooser's reproductive success. Many previous studies have shown that the women men find most attractive in well-nourished populations have low body mass indices (BMIs) and small waist sizes combined with relatively large hips, resulting in low waist-hip ratios (WHRs). A frequently proposed explanation for these preferences is that such women may have enhanced health and fertility; but extensive evidence contradicts this health-and-fertility explanation. An alternative view is that men are attracted to signs of nubility and high reproductive value , i.e., by indicators of physical and sexual maturity in young women who have not been pregnant. Here we provide evidence in support of the view that a small waist size together with a low WHR and BMI is a strong and reliable sign of nubility. Using U.S. data from large national health surveys, we show that WHR, waist/thigh, waist/stature, and BMI are all lower in the age group (15-19) in which women reach physical and sexual maturity, after which all of these anthropometric measures increase. We also show that a smaller waist, in conjunction with relatively larger hips or thighs, is strongly associated with nulligravidity and with higher blood levels of docosahexaenoic acid (DHA), a fatty acid that is probably limiting for infant brain development. Thus, a woman with the small waist and relatively large hips that men find attractive is very likely to be nubile and nulliparous, with maximal bodily stores of key reproductive resources.

Abstract Grandmothers provide more childcare for their daughters' children than for those of their sons, almost everywhere. Exceptions occur where virilocal (patrilocal) postmarital residence makes the children of sons more accessible, but even under virilocality, preferential care of daughters' children, net of the effects of proximity, is often demonstrable. A unique counter-example has been reported by Pashos (2000, Evolution & Human Behavior, 21, 97–109) who found that rural Greek grandmothers cared more for their sons' children even when effects of proximity were controlled; however, this result was based on an analysis in which everything from living in the same household to living in nearby villages was treated as equally close. Here, we present new analyses that replicate Pashos's result, based on a large European survey with a finer differentiation of residential proximity. In interviews conducted in 2004–2007, rural, but not urban, Greek women indeed reported more care of sons' than of daughters' children, net of the effects of proximity and other variables, This rural reversal of the usual uterine (matrilateral) bias was not observed elsewhere in Europe. Greeks were not surveyed again until 2015, whereupon the pattern had disappeared, with rural women now exhibiting a strong uterine bias in grandchild childcare. It seems likely that the financial crisis of 2008–2009, which hit Greece especially hard, played some role in this dramatic change, but it cannot readily be traced to increases in either unemployment or multigenerational households.

Some human subsistence economies are characterized by extensive daily food sharing networks, which may buffer the risk of shortfalls and facilitate cooperative production and divisions of labor among households. Comparative studies of human food sharing can assess the generalizability of this theory across time, space, and diverse lifeways. Here we test several predictions about daily sharing norms–which presumably reflect realized cooperative behavior–in a globally representative sample of nonindustrial societies (the Standard Cross-Cultural Sample), while controlling for multiple sources of autocorrelation among societies using Bayesian multilevel models. Consistent with a risk-buffering function, we find that sharing is less likely in societies with alternative means of smoothing production and consumption such as animal husbandry, food storage, and external trade. Further, food sharing was tightly linked to labor sharing, indicating gains to cooperative production and perhaps divisions of labor. We found a small phylogenetic signal for food sharing (captured by a supertree of human populations based on genetic and linguistic data) that was mediated by food storage and social stratification. Food sharing norms reliably emerge as part of cooperative economies across time and space but are culled by innovations that facilitate self-reliant production.

Drawing on the logical principles of life-history theory, it may be hypothesized that—compared to pre-menopausal women—post-menopausal women will spend more time caring for grandchildren and other kin. This hypothesis was tested in two studies, on results obtained from two large datasets documenting altruistic behaviors of pre-menopausal and post-menopausal women in the United States (N = 7, 161) and Australia (N = 25, 066). Results from both studies revealed that (even when controlling statistically for age, health, financial resources, and other pertinent variables), post-menopausal women devoted more time to grandparental caregiving. This effect was specific to kin care: Menopause status was not as strongly related to a measure of non-kin-directed altruistic behavior (time spent volunteering). These results provide the first empirical support for a previously-untested behavioral implication of menopause.

Mate preference research often focuses on traits that indicate a romantic partner's personal worth (e.g., their physical attractiveness, resource potential) rather than their tendency to leverage that worth for mutual vs. zero-sum benefit (i.e., their trustworthiness). No one has assessed the contribution of trustworthiness to perceived mate value relative to other personality dimensions. Here we examined the desirability of a partner's trustworthiness relative to five other personality indicators of mate quality during initial partner selection. Participants (n = 918) ranked multivariate partner profiles constructed from the HEXACO model of personality (i.e., honesty-humility, emotionality, extraversion, agreeableness, conscientiousness, and openness to experience) and provided partner ratings for each trait. Using conjoint analysis, we found that honesty-humility influenced participants' ranking decisions substantially more than each other characteristic (all Cohen's ds > 0.71). This was true for both long- (i.e., committed) and short-term (i.e., purely sexual) partner evaluations, though honesty-humility was relatively more important for long- vs. short-term contexts. There were no sex differences. A different pattern, including sex differences, emerged for partner ratings. Based on these findings, we hypothesize that the challenge of avoiding romantic interpersonal predation may have been a relatively stronger selection pressure during the evolution of human mate preference than has the challenge of identifying other valuable partner traits.

Long-term cooperation between individuals necessitates repairing damage arising from inevitable competing interests. How two members of a valuable relationship switch from competing to cooperating constitutes an important problem for any social species. Observations of non-human animals suggest that affiliative contact immediately following a contest facilitates continued cooperation. Behavioral studies further indicate that winners and losers frequently differ in hormonal changes following a competition. We tested the hypothesis that immediate contact with increases in cortisol (and testosterone for men) for winners following competition would facilitate subsequent cooperation between adult same-sex friends. Results show that contact (versus no contact) immediately following competition enhanced subsequent cooperation between female friends. During contact, increases in winner's cortisol for both sexes, and in testosterone for men, predicted future cooperation. Our results suggest two mechanisms that maintain social bonds following competition between established allies.

It is genuinely difficult to sustain a casual conversation that includes more than four speakers. Add a fifth speaker, and the conversation often quickly fissions into smaller groups. Termed ‘the dinner party problem,’ this four-person conversation size limit is believed to be caused by evolved cognitive constraints on human mentalizing capacities. In this view, people can mentally manage three other minds at any one time, leading to four-person conversations. But whereas existing work has posited and empirically tested alternative accounts of what drives the conversation size constraint, to our knowledge, no work has explored the question of why this capacity is specifically four? In this theoretical paper, we (a) review research demonstrating this cognitive constraint in sociality, (b) review the relevant working memory literature, which has explored the “why four” question at some length, and (c) we begin to pose possible answers to our specific social “why four” question. Using simple mathematical models of small-scale sociality, which we imbue with evolutionarily-relevant content, we present one novel possible explanation for the four-person conversation size constraint.

This study investigated how external negative emotional stimuli influenced 5–6-year-old children's (N = 98) sharing behavior. Children were asked to watch a video that induced either sad or neutral emotion and then share stickers simultaneously with poor and wealthy recipients. Compared to the neutral emotion condition, boys shared less, and offered more self-focused reasons for sharing after being induced sad emotions; girls however, shared equally, and provided self-focused and other-focused reasons equally in both emotional conditions. Results indicated that sadness increased children's tendency to defend their own interest in boys but not in girls, which supports the evolutionary “male warrior hypothesis”. In addition, although children indicated that they liked the wealthy recipient better than the poor one, they shared equally between the two recipients, suggesting an early development of equality concern and charity behavior (i.e., care for the disadvantaged individuals). These findings revealed gender differences in the socio-affective processes underlying children's resource allocation and added developmental evidence for the adaptive function of negative emotions.

Previous reports that women with attractive faces are healthier have been widely cited as evidence that sexual selection has shaped human mate preferences. However, evidence for correlations between women's physical health and facial attractiveness is equivocal. Moreover, positive results on this issue have generally come from studies of self-reported health in small samples. The current study took standardized face photographs of women who completed four different health questionnaires assessing susceptibility to infectious illnesses (N = 590). Of these women, 221 also provided a saliva sample that was assayed for immunoglobulin A (a marker of immune function). Analyses showed no significant correlations between rated facial attractiveness and either scores on any of the health questionnaires or salivary immunoglobulin A. Furthermore there was no compelling evidence that objective measures of sexual dimorphism of face shape, averageness of face shape, or facial coloration were correlated with any of our health measures. While other measures of health may yet reveal robust associations with facial appearance, these null results do not support the prominent and influential assumption that women's facial attractiveness is a cue of young adult women's susceptibility to infectious illnesses, at least in our study population.

The present study explored why married couples periodically exchange gifts. Based on the commitment signal hypothesis, we tested whether relational mobility, which was operationalized as divorce rate in Study 1 and relational opportunities in Study 2, is positively correlated with the frequency of gift exchanges among married couples. In Study 1, we found that married couples in the U.S., which is associated with a relatively high divorce rate, were more likely to give and receive gifts to and from their partners than those in Japan, which is associated with a relatively low divorce rate. This societal difference, in contrast, was not observed among unmarried couples, who were still developing their relationship (and thus, partner changes could frequently happen regardless of the country-level divorce rate). Study 2, a secondary analysis of extant survey data, revealed that Japanese married couples who have more relational opportunities more frequently engage in gift exchanges than those who do not. Together, these results support our hypothesis that periodical gift exchanges work as commitment signals among married couples.

Comparative analyses have sought to explain variation in human marriage patterns, often using predictions derived from sexual selection theory. However, most previous studies have not controlled for non-independence of populations due to shared ancestry. Here we leverage a phylogenetic supertree of human populations that includes all 186 populations in the Standard Cross-Cultural Sample (SCCS), a globally representative and widely-used sample of human populations. This represents the most comprehensive human phylogeny to date, and allows us not only to control for non-independence, but also to quantify the role of population history in explaining behavioral variation, in addition to current socio-ecological conditions. We use multiple imputation to overcome missing data problems and build a comprehensive Bayesian phylogenetic model of marriage patterns with two correlated response variables and eleven minimally collinear predictors capturing various socio-ecological conditions. We show that ignoring phylogeny could lead to both false positives and false negatives, and that the phylogeny explained about twice as much variance as all the predictors combined. Pathogen stress and assault frequency emerged as the predictors most strongly associated with polygyny, which had been considered evidence for female choice of good genes and male intra-sexual competition or male coercion, respectively. Mixed support was found for a polygyny threshold based on variance in male wealth, which is discussed in light of recent theory. Barring caveats, these findings refine our understanding of the evolution of human marriage systems, and highlight the value of combining population history and current socio-ecology in explaining human behavioral variation. Future studies using the SCCS should do so using the present supertree.

Hamilton's rule predicts that altruism should depend on costs incurred and benefits provided, but these depend on the relative needs of the donor and recipient. Rewriting Hamilton's rule to account for relative need suggests an amplifying effect of need on relatedness, but not necessarily other relationship qualities. In a reanalysis of three studies of social discounting and a new replication, we find that relative need amplifies the effects of relatedness on giving in two samples of U.S. adults recruited online, but not U.S. undergraduates or Indian adults recruited online. Among U.S. online participants, the effect of genetic kinship was greater when the partner was perceived to be in higher need than when in lower need. In the other samples, relatedness and greater partner need were associated with greater giving, but the effect of relatedness on giving was not significantly amplified by need. Need never amplified the effect of social closeness on giving, although it did diminish the effect of closeness in U.S. undergraduates, likely reflecting a ceiling effect. These results confirm predictions from a modification of Hamilton's rule in a sample of U.S. adults, but raise important questions about why they hold in some samples but not others. They also illustrate the importance of understanding how contextual factors, such as relative need, can moderate the importance of common variables used in evolutionary cost-benefit analyses.

The transition to low fertility worldwide has led to introduction of diverse frameworks across disciplines to understand its causes and consequences. Previous attempts to compare the relative importance of the key factors influencing women's fertility decision-making largely focused on a single rather than multiple steps of decision-making—an important problem if different factors are associated with different steps. Furthermore, insufficient attention has been paid explicitly to husband's and already-born children's influences, two potentially important factors. Here we introduce a framework covering three steps of reproductive decision-making—ideal family size, fertility desire and fertility intention—and test it using multi-level survey data collected from Chinese one-child mothers. Mother's attitudes towards having two children were paramount factors underlying her ideal family size, and husband's and the firstborn child's attitudes were critical to her desire to have a second child, which in turn played a decisive role in her intention to have a second child. Although husband's attitude was related to all steps, most factors were only relevant to one step; e.g., perceived child mortality and value for old-age security predicted ideal family size, admiration—a prerequisite for social learning—for two-child families predicted fertility desire, and physical/economic constraints primarily predicted fertility intention. Our study emphasizes multiple decision-makers in family reproduction; indicates the relative importance of fertility-influencing factors could vary with steps of decision-making; and has important implications for population policy in low-fertility societies.

Previous research has demonstrated that consanguineous marriage is a vector for socioeconomic inheritance and for the maintenance of family structure and property. On the basis of reconstituted families from the Krummhörn, Ostfriesland in the 18th and 19th centuries, we examine statistical correlations between ascertained inbreeding coefficients (F) based on family trees and socioeconomic status as well as the intergenerational transmission of landholdings. Semiparametric copula/bivariate regression models with non-random sample selection were applied to estimate F and the proportion of medium (0.0625 > F ≥ 0.0156) or high consanguineous unions (F ≥ 0.0625), respectively. Our estimates for F as well as for the proportion of medium (0.0625 > F ≥ 0.0156) or high consanguineous unions (F ≥ 0.0625) are significantly higher among socioeconomically privileged large farmer families than among the landless portion of the population. At the same time, our analyses show that a high level of consanguinity is associated with an increased intergenerational transmission of landholdings through the patriline (but not the matriline). We discuss the reproductive consequences of consanguinity among large farmers in connection with local resource competition, intensive kinship, and potential in-law conflicts.

Recognizing that two elements within a sequence of variable length depend on each other is a key ability in understanding the structure of language and music. Perception of such interdependencies has previously been documented in chimpanzees in the visual domain and in human infants and common squirrel monkeys with auditory playback experiments, but it remains unclear whether it typifies primates in general. Here, we investigated the ability of common marmosets (Callithrix jacchus) to recognize and respond to such dependencies. We tested subjects in a familiarization-discrimination playback experiment using stimuli composed of pure tones that either conformed or did not conform to a grammatical rule. After familiarization to sequences with dependencies, marmosets spontaneously discriminated between sequences containing and lacking dependencies (‘consistent’ and ‘inconsistent’, respectively), independent of stimulus length. Marmosets looked more often to the sound source when hearing sequences consistent with the familiarization stimuli, as previously found in human infants. Crucially, looks were coded automatically by computer software, avoiding human bias. Our results support the hypothesis that the ability to perceive dependencies at variable distances was already present in the common ancestor of all anthropoid primates (Simiiformes).

The theory of facultative calibration, which explains personality differences as responses to variation in other phenotypic traits of individuals, received mixed results throughout the last years. Whereas there is strong evidence that individual differences in human behavior are correlated with the self-perception of other traits, it still needs to be questioned whether they are also adjusted to objective differences in body condition (i.e. formidability). In two independent studies (N1 = 119 men and 124 women, N2 = 165 men) we tested hypotheses of facultative personality calibration in an integrative way, assessing various outcomes of previous studies in the same samples (including Anger Proneness, Extraversion, Neuroticism, Narcissism, Shyness, Vengefulness, and Sociosexual Orientation). Formidability was derived from assessments of physical strength and various anthropometric measures from full-body 3D scans and paired with measures of self-perceived and other-rated physical attractiveness (based on rotating morphometric 3D body models and facial photographs). We could replicate positive correlations with self-perceived attractiveness across outcomes, though these were not corroborated by more objective assessments of attractiveness: an effect of other-rated attractiveness was clearly not supported in our results for either sex, regardless of the personality outcome. Anthropometric measures and physical strength were also largely unrelated to personality, with the exception of Extraversion, Utility of Personal Aggression, and Sociosexual Orientation. While the two samples differed in their results for domain-level Extraversion, at least the Extraversion facets Activity and Assertiveness were related to strength and masculinity in men. For Sociosexual Orientation the results of our two samples varied more substantially, a positive association was only present in Study 2. Future studies need to clarify whether formidability, potentially an indicator of genetic quality for males, enhances their orientation and success in short-term mating. Furthermore we propose longitudinal twin-difference studies as means to evaluate the theory of personality recalibration in a more controlled manner.

Sociosexuality is defined as an individual's interest in uncommitted sexual activity and can be measured in terms of both psychological orientations and behavioral expression. In socio-ecological contexts in which adults monogamously partner and cooperate to raise children, individuals with unrestricted sociosexuality are likely to prioritize mating/competition over committed partnering and parenting. Given the importance of mother-father cooperation in the evolutionary past, humans may have the capacity to facultatively and opportunistically downregulate sociosexuality to focus on priorities related to invested partnering and parenting. To date, no prior studies have used longitudinal data to track within-individuals changes in sociosexuality as it relates to such life history transitions. Given the lack of prior longitudinal research in this area, it is likewise unknown what physiological mechanisms might mediate within-individual changes in sociosexuality through time but testosterone is a plausible candidate. To explore these questions, we drew on a large, long-running study of Filipino men (n = 288), who were single non-fathers at 25.9 years of age and were followed up 4–5 years later. We found that men with more unrestricted sociosexuality at baseline were more likely to experience relationship dissolution by follow-up, consistent with past work. Compared to men who remained single non-fathers at follow-up, men who became married residential fathers showed shifts towards more restricted global sociosexuality as well as sociosexual behavior. Relative to their own baseline values, married residential fathers also had more restricted sociosexuality in all domains at follow-up. They were the only group for whom this was found. We found theoretically-consistent but modest support for positive correlations between men's testosterone and their sociosexuality, but no evidence that the two change in tandem together through time. Our results suggest that some amount of between-individual differences in sociosexuality are not stable and can facultatively shift alongside other aspects of male reproductive effort.

Eye cues have been shown to stimulate rapid, reflexive, unconscious processing and in many experimental settings to cue increased prosocial and decreased antisocial behaviour. Eye cues are being widely applied in public policy to reduce crime and antisocial behaviour. Recently, failed replication attempts and two meta-analyses examining the eye cue effect on generosity have raised doubts regarding earlier findings. Much of the wider evidence on eye cues has still not been systematically reviewed, notably that which is most relevant to its practical application: the effect of eye cues on antisocial behaviour. Given the evidence of humans' heightened sensitivity to threat and negative information, we hypothesized that the watching eyes effect would be more consistent in studies examining antisocial behaviour. In our meta-analysis of 15 experiments from 13 research papers we report a reduction in the risk of antisocial behaviour of 35% when eye cues are present. By contrast, systematic reviews have suggested CCTV cameras reduce crime by only 16%. We conclude that there is sufficient evidence of a watching eyes effect on antisocial behaviour to justify their use in the very low-cost and potentially high-impact real-world interventions that are proliferating in public policy, particularly in the UK. Public significance statement Our meta-analysis of 15 experiments involving 2035 participants shows that photographs and/or stylized images of eyes reduced antisocial behaviour by 35%. Our findings support public policy initiatives employing pictures of ‘watching eyes’ to reduce crime. Furthermore, in an age when we are watched more than at any time in modern history – both online and on the street – our findings highlight an urgent need to fully understand the effect that perceived surveillance, feeling watched, has on our decisions and actions.

Women's mating psychology may have evolved to track reproductive conditions, including conception risk, across and between ovulatory cycles. Alternatively, within-woman correlations between mating psychology and ovarian hormones may be byproducts of between-women relationships. Here, we examined associations between steroid hormones and two facets of sexual psychology with putatively different adaptive functions, sociosexual orientation and general sexual desire, in a sample of naturally cycling women (NC; n = 348, 87 of whom completed 2 sessions) and hormonally contracepting women (HC; n = 266, 65 of whom completed 2 sessions). Across two sessions, increases in estradiol predicted elevated sociosexual desire in NC women, and this relationship was stronger in women whose progesterone simultaneously decreased across sessions. Changes in hormones were not associated with changes in general sexual desire. Between-subjects differences in testosterone robustly, positively predicted sociosexuality and general sexual desire among NC women. Hormones were not consistently related to changes or differences in sexual psychology among HC women. The present results are consistent with testosterone contributing to individual differences, or modulating relatively long-term changes, in women's mating psychology. Further, our within-woman findings are consistent with the hypothesis that shifts in women's mating psychology may function to secure genetic benefits, and that these shifts are not byproducts of between-women associations.

Research on mate preference have often taken a theory-driven approach; however, such an approach can constrain the range of possible predictions. As a result, the research community may inadvertently neglect traits that are potentially important for human mate choice if current theoretical models simply do not identify them. Here, we address this limitation by using a data-driven approach to investigate mating-relevant self-concepts (i.e., what individuals believe to be attractive about themselves). Using Latent Dirichlet Allocation (LDA; a clustering method developed in computer science) and a large sample of written descriptions from online personal advertisements (N = 7973), we identify 25 common topics that individuals use when advertising themselves. Men were more likely to advertise education/status, while women were more likely to discuss being honest/nurturing and caring for pets. We also assessed patterns of universal and compatible mate preferences for these 25 topics by collecting ratings of desirability from a separate group of 100 participants on a subset of these profiles (N = 468). Participants were also asked to write a personal description of themselves as if they were writing for a dating website. Overall, both male and female profiles that discussed outdoor activities, and music/art were rated as more desirable, while women that discussed a healthy lifestyle and friends/family were also rated as more desirable. Both men and women who discussed sex or mentioned being a parent were rated as less desirable. When comparing the topic probabilities between profiles collected online and those written by the raters, we found that raters preferred profiles that were more similar to their own, particularly for topics to do with being outgoing and agreeable.

Life history theory anticipates that organisms trade offspring quantity for offspring quality. In modern human societies this tradeoff is particularly acute because of increased returns on investments in embodied capital. Religious people, however, despite having more children than their secular counterparts, do not appear to suffer lower quality offspring. To explain this apparent paradox of religious fertility, we propose a religious alloparenting hypothesis, which hypothesizes that higher levels of alloparenting in religious communities enable religious individuals to support larger families without reducing offspring quality. Using data from a large national sample whose population is roughly half religious and half secular (N = 12,980; New Zealand), we demonstrate that, after adjusting for denominational, environmental, ethnic and other demographic differences, religious identification is associated with an increased likelihood of having at least one child, and religious identification and ritual frequency are positively related to offspring number among people with at least one child. Consistent with the religious alloparenting hypothesis, religious identification and ritual frequency are also positively associated with alloparenting among community members who do not currently have young children of their own. These are the first findings to reveal that religious cooperation extends to alloparenting; however, whether or not the levels of alloparenting in religious communities are sufficient to mitigate the costs of higher relative fertility remains a critical consideration for future research.

Studies have shown that participants can adequately take into account several cues regarding the weight they should grant majority opinions, such as the absolute and relative size of the majority. However, participants do not seem to consistently take into account cues about whether the members of the majority have formed their opinions independently of each other. Using an evolutionary framework, we suggest that these conflicting results can be explained by distinguishing evolutionarily valid cues (i.e. they were present and reliable during human evolution) from other cues. We use this framework to derive and test five hypotheses (H1 to H5). Our first three experiments reveal that participants discount majority opinion when the members of the majority owe their opinions to the same hearsay (H1), owe their opinions to having perceived the same event (H2), or owe their opinions to a common motivation (H3). Experiment 4 suggests that, by contrast, participants do not discount majority opinion when the members of the majority owe their opinions to sharing similar cognitive traits (H4). Finally, Experiment 5 suggests that participants adequately discount majority opinion when one of the members of the majority is untrustworthy (H5). This set of experiments shows that participants can be quite skilled at dealing with informational dependency, and that an evolutionary framework helps make sense of their strengths and weaknesses in this domain.

Friendship naturally leads to treating some people differently from the way we treat everyone else. One manifestation of such preferential treatment is in the domain of prosociality: we are more likely to extend favors towards our friends. Little is known about the developmental and evolutionary roots of such preferential prosociality. Here, we investigate whether young children and chimpanzees show partiality towards friends in helping contexts. Results show that young children at the age of three – when they first form preferential peer relationships – already bias their helping decisions in favor of their friends, both when they have to make a choice whether to help a friend or a neutral peer (Study 1) and when measuring their overall motivation to help (Study 2). In Study 3, by combining observational and experimental methods, we demonstrate similar though less robust motivations to provide help preferentially to friends in our closest living relatives, chimpanzees. Taken together, these studies suggest that partiality towards friends is grounded early in ontogeny and human evolution.

Although genetic relatedness has been shown to be an important determinant of helping and other forms of cooperation among kin, it does not correspond well to the different types of kin designated by the kin terminologies used in human societies. This mismatch between genetic relatedness and kin terms has led some anthropologists to reject the idea that kin terms have anything to do with genetic relatedness or anything else biological. The evolutionary and cultural anthropological approaches can be reconciled through an appreciation of the concept of fitness interdependence, defined as the degree to which two or more organisms positively or negatively influence each other's success in replicating their genes. Fitness interdependence may arise for a variety of reasons, including not only genetic relatedness but also mating and marriage, risk-pooling, mutual aid, and common group membership. The major kin term systems correspond to cross-culturally variable but recurrent patterns of fitness interdependence among different types of kin. In addition, changes from one kin term system to another are associated with corresponding changes in recurrent patterns of fitness interdependence among kin, and kin terms are often used metaphorically in situations in which fitness interdependence has arisen among non-kin.

Across human societies, female sexuality is suppressed by gendered double standards, slut shaming, sexist rape laws, and honour killings. The question of what motivates societies to punish promiscuous women, however, has been contested. Although some have argued that men suppress female sexuality to increase paternity certainty, others maintain that this is an example of intrasexual competition. Here we show that both sexes are averse to overt displays of female sexuality, but that motivation is sex-specific. In all studies, participants played an economic game with a female partner whose photograph either signalled that she was sexually-accessible or sexually-restricted. In study 1, we found that men and women are less altruistic in a Dictator Game (DG) when partnered with a woman signalling sexual-accessibility. Both sexes were less trusting of sexually-accessible women in a Trust Game (TG) (study 2); women (but not men), however, inflicted costly punishment on a sexually-accessible woman in an Ultimatum Game (UG) (study 3). Our results demonstrate that both sexes are averse to overt sexuality in women, whilst highlighting potential differences in motivation.

Partner preferences are formed by several mechanisms, including an imprinting-like effect (parent-similarity) and homogamy (self-similarity). It is still unknown, however, whether these preferences remain stable throughout an individual’s lifetime. We have therefore tested the consistency of mate choice in eye and hair colour both in a shortand long-term context. In other words, we tested whether people systematically choose partners with a particular eye and hair colour. We asked 1,048 respondents to indicate the eye and hair colour of themselves, their opposite-sex and same-sex parent, and all the romantic partners they had in their lives. Our results show that people consistently choose partners of a particular eye and hair colour in both short- and long-term contexts, which suggests that people do have their ‘types’. Nevertheless, the consistency was significantly higher in a long-term context than in a short-term context. Furthermore, the eye colour of one's partner was predicted by the eye colour of one's opposite-sex as well as same-sex parent, but the strongest parental effect was found when both parents had same eye colour. There were no significant results for hair colour. Our results thus suggest that preferences for eye colour are determined by the imprinting-like effect rather than by homogamy, and that they remain stable over time. These findings also indirectly support an assumption of stability of this imprinting-like effect in humans, since people consistently choose partners with their opposite-sex parent's eye-colour.

Faced with punishing severe offenders, why do some prefer imprisonment whereas others impose death? Previous research exploring death penalty attitudes has primarily focused on individual and cultural factors. Adopting a functional perspective, we propose that environmental features may also shape our punishment strategies. Individuals are attuned to the availability of resources within their environments. Due to heightened concerns with the costliness of repeated offending, we hypothesize that individuals tend towards elimination-focused punishments during times of perceived scarcity. Using global and United States data sets (studies 1 and 2), we find that indicators of resource scarcity predict the presence of capital punishment. In two experiments (studies 3 and 4), we find that activating concerns about scarcity causes people to increase their endorsement for capital punishment, and this effect is statistically mediated by a reduced willingness to risk repeated offenses. Perceived resource scarcity shapes our punishment preferences, with important policy implications.

In collaborations, group productivity typically increases with more cooperators, but is also often subject to diminishing returns. This pattern provides a different view about cooperation from traditional social dilemmas: defection is not necessarily the dominant strategy. Rather, a frequency-dependent “anti-conformist” strategy (cooperate if many others defect, and vice versa) is often individually rational. This study addresses human cooperation under such marginally diminishing group productivity, focusing on the plasticity of cooperative choices. We conducted a two-part “team foraging” experiment, in which the most- or least-cooperative members in the first part were re-grouped separately for the second part. We observed that cooperating and defecting “types” emerged within a group over time but did not completely persist across groups, with some of the most cooperative members switching to become the least cooperative (and vice versa). Risk attitude was a key factor in this switching behavior: greater risk-takers showed greater behavioral plasticity. These results imply that human cooperation may be more context-dependent and behaviorally plastic than previously thought.

Cooperation often comes with the temptation to defect and benefit at the cost of others. This tension between cooperation and defection is best captured in social dilemmas like the Prisoner's Dilemma. Adult humans have specific strategies to maintain cooperation during Prisoner's Dilemma interactions. Yet, little is known about the ontogenetic and phylogenetic origins of human decision-making strategies in conflict scenarios. To shed light on this question, we compared the strategies used by chimpanzees and 5-year old children to overcome a social dilemma. In our task, waiting for the partner to act first produced the best results for the subject. Alternatively, they could mutually cooperate and divide the rewards. Our findings indicate that the two species differed substantially in their strategies to solve the task. Chimpanzees became more strategic across the study period by waiting longer to act in the social dilemma. Children developed a more efficient strategy of taking turns to reciprocate their rewards. Moreover, children used specific types of communication to coordinate with their partners. These results suggest that while both species behaved strategically to overcome a conflict situation, only children engaged in active cooperation to solve a social dilemma.

Why do humans help others? Many theories focus on dimensions like kinship or reciprocity. On their surface, these theories seem unable to explain help directed at fleeting strangers. In response to this puzzle, researchers have proposed that the mind has ecologically rational systems for providing aid. These systems respond to cues that predicted adaptive behavior during human evolution, regardless of whether such cues continue to be predictive in modern environments. In three studies, we test for two cues that might predict whether a potential benefactor will help a potential recipient: the need of the recipient and the extent to which the recipient is willing to sacrifice for the benefactor. Both cues, in ancestral environments, have the potential to predict whether a long-term relationship might be established. Consistent with past research, we find that both cues matter: Needy people and people willing to sacrifice are helped more. However, the cues are not merely additive: In some cases, the cue of need is ignored and only willingness to sacrifice is used. We discuss these results in terms of recent evolutionary theories of emotions.