Humans routinely evaluate the romantic relationships of others, a capacity that carries adaptive value for social learning, mate assessment, and coalition management. The present study examined how two cooperative norms, fairness and altruism, shape third party judgements of perceived relationship longevity, and whether these judgements vary according to relationship context and observer sex. Using an observer based experimental design, undergraduate participants viewed a short video of a couple engaged in a resource allocation task, framed as being in either a short term or long-term relationship. One partner behaved either fairly by dividing resources equally or altruistically by giving a greater share to the partner. Participants then rated how likely they believed the relationship was to last. Results revealed a significant three-way interaction between cooperative behavior, relationship length, and participant sex. Fair behavior was associated with greater perceived longevity in short term relationships, whereas altruistic behavior was associated with greater perceived longevity in long term relationships, particularly among female observers. These findings indicate that cooperative behaviors are interpreted in context sensitive ways, and that the capacity to infer relationship stability from cooperative cues may serve evolutionarily relevant functions for observers.
According to the bounded generalized reciprocity theory (BGR), people are endowed with psychological devices that facilitate within-group cooperation under a situation of conflicting interests. When interacting with ingroup members, they approach the situation as one of aligned interests and expect their partner to follow suit. So far, the role of expected cooperation in ingroup favoritism remains elusive. Against this backdrop, we probed whether expected cooperation is necessary for ingroup favoritism to occur. To this end, we employed a stag hunt game (SH), which instantiates a situation of aligned interests, and tested a BGR-derived prediction that ingroup favoritism in this game would be associated with ingroup-specific elevation of expected cooperation. We found support for this view. Unexpectedly, people also cooperated less in SH with outgroup members than with unclassified strangers. This pattern has seldom been observed empirically and cannot be accounted for by existing theory.
Previous research has suggested that infant faces capture adult attention, and this effect has been interpreted as reflecting the biological salience of infant faces as positively valenced stimuli, to which humans may have evolved to respond preferentially. However, subsequent studies have not consistently replicated this phenomenon. The present study investigated attentional capture by infant faces in a sample of university students, while controlling for relevant covariates, including parental readiness and the perceived attractiveness of infant faces. The results confirmed the attention-capturing effect of infant faces; however, no significant effects of pre-parental readiness or infant facial attractiveness were observed.
Human cooperation depends on selecting reliable partners under conditions of uncertainty and limited information. I propose that personality traits function as an informational architecture that compresses high-dimensional behavioral observations into low-dimensional representations optimized for prediction and social communication. From this perspective, the Big Five can be understood not merely as a psychometric taxonomy, but as a representational format that facilitates efficient partner choice in cooperative contexts. Compressed trait representations reduce cognitive load, enhance transmission through gossip, and promote reputational consensus. Within this framework, Agreeableness plays a central role in cooperation-dominant settings. I argue that Agreeableness may signal cooperativeness through two complementary mechanisms: by structuring intrinsic cost asymmetries between altruistic and exploitative behavior, and by increasing susceptibility to social learning and norm internalization. This account integrates personality psychology with evolutionary models of partner choice and indirect reciprocity.
Languages evolve through diffusion processes similarly to biological evolution, forming linguistic clusters based on geographic proximity. A few mathematical modeling studies have tested the classical theory on the formation of a peripheral distribution of dialects, which originally assumed two conditions: (i) new words are innovated exclusively at a cultural center and (ii) these words spread outward due to the prestige of the cultural center. However, it is known that these special conditions are often not met. We examined whether and how the presence or absence of each condition influences the outcome using an extended Bayesian Iterated Learning Model. Our mathematical analyses revealed that peripheral distributions can emerge not only when both conditions are present but also when one of the two is absent. Furthermore, the satisfaction of one or both conditions in a population can be predicted by investigating the word age distribution there.
Evolutionary psychology and comparative cognition both pursue evolutionary accounts of cognition and behavior, yet they meet recurring interface problems that blunt strong inference: anthropomorphic projection, unvalidated human baselines, weak hypothesis construction, and conflation of proximate with ultimate explanations. I recast “comparative evolutionary psychology” as a study-first, field-scaling bridge rather than a disciplinary merger and propose a practical toolkit: design tasks calibrated to each species’ sensory and motor capacities, establish constraint-matched human baselines, and present a priori predictions that pit specific adaptationist predictions against specified domain-general process models where they make divergent predictions. These predictions are adjudicated with diagnostic probes such as transfer to novel situations. Treating analysis as explicit model competition can raise the evidential bar, help reduce anthropomorphic bias, and better connect functional and mechanistic levels. These practices provide a principled path toward discovering genuine evolutionary continuities, convergences and divergences of cognition and behavior.
An obstacle in explaining the evolution of cooperation through third-party punishment is the second-order free rider problem, that is, if punishing others is costly, it is adaptive for any individual not to punish. One hypothesis to resolve the issue states that third-party punishment is advantageous for the punisher, because it is evaluated as socially good and thereby enhances future cooperation from the observers. However, it is thus far unclear whether cooperation is truly promoted by third-party punishment in the presence of a social norm favoring punishing behavior, and under what circumstances such social norm is established. In this paper, we present two mathematical models to explore the reputation-enhancing effect of third-party punishment and its role in the evolution of cooperation. Our results suggest that third-party punishment can indeed facilitate the evolution of cooperation when individuals base their evaluation of others on their punishing behavior, and such punishment reputation dependency can coevolve with third-party punishment during the process where large-scale cooperation is established within a population.
Generosity does not always result in a positive reputation. Kawamura and Kusumi (2020) found that altruistic behavior that deviates from social norms was evaluated less favorably than moderately altruistic behavior, and that this tendency was more pronounced in Japan than in the United States. This may be attributed to a lower tolerance for deviance among Japanese individuals. Relational mobility, which influences tolerance for deviance, is generally higher in the United States than in Japan, potentially accounting for these cultural differences. In the present study, Japanese participants preferred an equal distributor over one who gave away everything, perceiving the former as more normative. This finding replicates the results of the previous study. Furthermore, the equal distributor was also newly found to be preferred as a business partner. However, individual differences in perceived relational mobility did not influence participants’ evaluations of the distributors. The effects of the degree of participants’ altruism and general trust were also examined.
Human social behavior spans a continuum from self-serving actions to other-oriented sacrifices. Underlying these behaviors are reputational concerns, specifically rejection avoidance and praise seeking. However, it is unclear how reputational concerns relate to pathological altruism, which involves excessive, irrational self-sacrifice, and healthy selfishness, which prioritizes one’s own fulfillment with consideration of potential negative effects on others. This study examined whether rejection avoidance and praise seeking are associated with pathological altruism and healthy selfishness. Data were collected from 787 participants between the ages of 15 and 59. The results indicated that both rejection avoidance and praise seeking were positively correlated with pathological altruism and healthy selfishness, even after controlling for empathy. Most associations were stable across all ages, although the association between praise seeking and healthy selfishness was moderated by age. These findings provide initial empirical evidence that reputational concerns may drive individuals toward both pathological altruism and healthy selfishness. This result enhances the understanding of the motivational role of social evaluation in complex human social functioning.
This study examines whether men exhibit outgroup aggression in an intergroup situation created in a laboratory setting. According to the male warrior hypothesis, men have psychological mechanisms that allow them to engage in intergroup conflict and outgroup aggression. However, there is little empirical evidence that men behave aggressively toward outgroup members. To test whether men engage in outgroup aggression in intergroup situations, we conducted a laboratory experiment using a modified Intergroup Prisoner’s Dilemma–Maximizing Differences (IPD-MD) game. The game consisted of three pools: ingroup cooperation, outgroup exploitation, and outgroup aggression. Participants decided how much of their endowment to allocate to each pool. We also manipulated whether participants were informed of the group's gender composition prior to the game, testing the theoretical prediction that men are more likely to form coalitions for outgroup aggression. The results showed no gender differences in contributions to the outgroup aggression or exploitation pools, nor was there an effect of the information manipulation. However, both men and women anticipated that outgroup members would engage in outgroup aggression.
As one of the prerequisites for maladaptive cultural evolution, oblique transmission has drawn attention. However, even though maladaptive cultural evolution could occur if children choose oblique transmission frequently, can oblique transmission be selected by children in the course of genetic evolution? In addressing the question above, in this study, we conducted agent-based simulations focusing on the evolution of “oblique transmission bias,” the tendency of children to choose oblique transmission when they can choose between oblique and vertical transmission. At first, we analyzed how the oblique transmission bias evolves by comparing models with two cultural traits versus five traits, manipulating the probability of environmental changes and the strength of natural selection, respectively. As a result, the oblique transmission rate evolved under limited conditions. Second, we conducted simulations under the setting of the oblique transmission rates as exogenous variables; maladaptive cultural evolution did not occur because of oblique transmission when oblique transmission bias is as strong as one evolved in the previous simulation. In addition, we show that if maladaptive culture is more likely to be imitated by children, maladaptive cultural evolution occurs.
Lower social status is one of the social factors associated with paranoia—the belief that others have harmful intentions. From an evolutionary perspective, paranoia is an adaptive psychological mechanism for coping with social threats. Two types of social status have been theorized based on differences in the social influence of superiors on subordinates: prestige and dominance. Dominant superiors force their subordinates to obey them through threats, and subordinates are motivated to avoid these superiors. Conversely, subordinates voluntarily approach prestigious superiors to gain beneficial expertise. If paranoia functions as a response to social threats, it is strongly associated with evaluations of superiors’ dominance in an immediate environment. A web-based survey of full-time workers (N = 300) examined the correlations between paranoia, evaluations of dominance and prestige toward leaders, and willingness to learn from leaders. We distinguished between a familiar leader (a workplace supervisor) and a distant leader (a leader in the broader community). In support of our hypothesis, paranoia was positively associated with the evaluation of a familiar leader’s dominance, but not with that of a distant leader’s dominance. However, contrary to our prediction, paranoia was positively associated with willingness to learn from leaders regardless of the type of leader.
Individuals responsible for child care are consistently exposed to children’s excretion pathogens, yet they must still take care of their children, especially when the children are infected. Our objective was to investigate whether exposure to infected infants would negatively impact people’s emotions towards cute babies, and vice-versa. Moreover, as cooperative breeding is a hallmark of our species, we also explored whether this response would differ between child carers and non-child carers. In two studies, 128 participants rated how much disgust they felt (disgust score) when watching 10 pictures of infected babies after viewing cute babies or furniture pictures (study1) and how much tenderness they felt (tenderness score) when watching 10 pictures of cute babies after viewing infected babies or furniture pictures (study 2). Priming participants with pictures of cute babies did not significantly impact their disgust response towards infected children. In contrast, priming with infected babies led to a reduction in tenderness felt towards cute babies, but only for non-carers. This result is consistent with the possibility that child carers desensitize themselves through exposure or that people less prone to disgust when exposed to pathogens related to caregiving situations invest more in child care.
Various disciplines, including evolutionary biology, anthropology, archaeology, and psychology, have studied the evolution of rituals. Archaeologists have typically argued that burial practices are one of the most prominent manifestations of ritual practices in the past and have explored various aspects of burial practices, including burial directions. One of the important hypotheses on the cultural evolution of burial practices in Japan is the kitamakura hypothesis, which claims that burial directions (including Kofuns and current burials) were intended to be oriented toward the north after the Kofun period under the influence of Confucianism or Buddhism. This hypothesis would be more plausible if burial directions were not oriented northward before the Kofun period. This research focused on the burial directions in the northern Kyushu area of the Yayoi period, i.e., the directions of the kamekan jar burials. The results are almost consistent with the hypothesis, although one notable exception is found, and its possible interpretations and implications are discussed.
Perceived threats from immigrants have been associated with heightened anti-immigrant attitudes among host country citizens (Esses et al., 2001, 1998). However, most studies have focused on assessing attitudes using scales, with few investigating behavioral responses. This study developed a new experimental game based on the Preemptive Strike Game (Simunovic et al., 2013) to measure reactions to perceived threats of immigrant attacks. A sample of Japanese participants (N = 1,153; pre-registered) was divided into two groups: one group faced a Japanese opponent (Japanese condition), whereas the other faced an immigrant opponent (Immigrant condition). The results did not show a significant increase in defensive behaviors toward immigrants. However, at a descriptive level, the findings suggest the potential for defensive behaviors to arise from perceived threats. The study discusses the implications for future research including the need to accurately identify threat-induced behaviors.
Individuals with high psychopathic traits tend to use short-term reproductive strategies, such as having many short-term relationships and committing sexual offenses. These strategies are maladaptive in the long term, but a certain number of individuals with high psychopathic tendencies exist in modern societies. This study focuses on the effects of social value orientation (SVO) on the enhancement of cooperative tendencies in highly psychopathic individuals. We hypothesized that highly psychopathic individuals with pro-social orientation would cooperate more than those with pro-self orientation. The results showed that individuals with high psychopathic traits behaved uncooperatively regardless of the different types of SVO. In addition, they were most cooperative with family members, followed by friends, and least with strangers. This finding suggests an exaggerated kin bias in which psychopathic individuals prefer kin as a cooperation partner.
People often refuse to touch objects that were once in contact with an immoral person because they find the idea of residue transfer disgusting. Previous research has revealed that objects in contact with immoral persons are often rejected because of disgust contagion. However, it is unclear whether the importance of contact with a disgusting presence also applies when moral disgust contaminates people. Thus, this study examined whether moral disgust can spread to a person without direct physical contact with a moral transgressor. Participants evaluated their impressions of individuals either in contact with or separated spatially from a moral transgressor. The results indicate that negative evaluations were significant only for individuals who had direct contact with moral transgressors. This suggests that physical contact is essential for the transmission of moral disgust. Although further research is needed to confirm the human-to-human contagion system, this study offers a significant step toward understanding the mechanisms of moral disgust contagion.
The purpose of this study is to investigate whether conformist bias can be observed in information-seeking situations. We replicated the experiment conducted by Fujikawa et al. (2024), in which conformist bias was observed, using a between-participants design. In Experiments 1 (N = 116) and 2 (N = 157), participants responded to “yes/no” questions in which the answer was objectively and not objectively fixed as correct or incorrect. After being shown one of four patterns of others’ responses (9, 6, 3, or 0 others answering “yes”), participants answered the same questions. The results of both Exp. 1 and Exp. 2 showed a conformist bias for the questions with objectively fixed answers, but not for the questions with non-objectively fixed answers. The differences between the results of Fujikawa et al. (2024) and our results were discussed.
Signaling theory offers an interpretation of how one’s impression of a potential mate’s personality affects mating strategies. It remains unclear how people perceive the (dis)similarity of a potential mate to one’s own personality and use this information in mating contexts. We focus on this distance in personality similarity between a potential mate and oneself, particularly in deciding whether to interact. We present a novel framework for investigating this issue via an online chatting tool. The actual personality of 24 participants was measured according to the Big Five Inventory. Then, participants were paired to have a conversation. Their face, occupations, and income were hidden, and their voice transformed to be neutral in pitch. They talked about their own experiences as related to the Big Five dimensions. Analysis of their communications showed their willingness to consider their partners as a friend, lover, and spouse was most explained by the distance between self-recognition of the impression of one’s own personality presumably held by the partner and the impression of the partner’s personality. Our results suggest people rely on the meta-perception of their personality rather than their actual personality, at least when considering potential mates when they first meet.
The purpose of this study is to incorporate an evolutionary perspective into the field of ethnohistory. Specifically, I developed a statistical model to estimate the polygyny rate among hunter-gatherers and applied it as a supplementary method of historical source criticism. The records of Reverend Thomas Bridges, who began his research in the late 1850s, are considered the most reliable accounts of the Yamana (Yahgan) people of Tierra del Fuego, South America. However, his records on the practice of polygyny are inconsistent, stating that polygyny was “very general” at times and denying its existence at others. While historical source criticism necessitates efforts to discover new sources, in cases such as that of the Yamana where reliance on inaccurate records by missionaries and travelers of the time is unavoidable, the evaluation and selection of conflicting sources become crucial. Therefore, I attempted to estimate the polygyny rate among the Yamana using a generalized linear mixed model with hunter-gatherer societies as the population. The results suggest that Bridges’s records, particularly those from the early stages of his research, are likely to be unreliable, even when considering Bayesian credible intervals.
