How do people decide whom to help when information about others is limited? From an evolutionary perspective, personality traits may function as social cues that facilitate evaluations of potential interaction partners. The present study examined whether the Big Five traits of both potential helpers and targets predict helping intentions toward strangers. A total of 120 Japanese adults evaluated 40 experimentally constructed personality profiles and rated both their willingness to help and their liking for each target. Linear mixed-effects models revealed trait-specific patterns. Openness and extraversion showed no significant participant or target effects on helping intentions. In contrast, targets higher in conscientiousness and agreeableness received more help, and significant participant × target interactions emerged for both traits. Supplementary response surface analyses indicated that these interactions were not driven by personality similarity per se. Rather, individuals higher in conscientiousness or agreeableness were particularly responsive to the corresponding traits in targets. Additional mediation analyses showed that the interaction effects on helping were fully accounted for by liking, suggesting that personality traits influence helping primarily through evaluative processes. These findings indicate that agreeableness and conscientiousness function as socially interpretable cues that shape evaluations of potential partners under conditions of uncertainty. More broadly, the results suggest that social evaluation may represent a psychological mechanism through which personality traits contribute to assortative patterns in cooperation.

Purpose This research examined whether similarity in moral foundations—particularly reciprocity-related Fairness—predicts altruistic behavior toward friends and acquaintances. Drawing on multilevel selection theory and partner choice models of reciprocal altruism, we tested the hypothesis that moral similarity functions as a cue for strategic compatibility, thereby facilitating cooperative investment. We further distinguished between help provision and help receipt and examined whether effects vary by helping cost. Methods Two online studies were conducted with Japanese adults (Study 1: N = 255; Study 2: N = 243). Participants identified a same-gender friend living nearby and reported perceived similarity in moral foundations and other traits. Study 1 measured retrospective frequencies of low-, medium-, and high-cost helping. Study 2 replicated the design using multi-item Moral Foundations Questionnaire scales and assessed willingness to help via a behavioral proxy (checkbox method). Data were analyzed using ordinal logistic regression. Results Across both studies, similarity in Fairness consistently predicted provision of low-cost help and, in Study 2, high-cost help. In contrast, moral similarity did not reliably predict help received. Medium-cost helping was more strongly associated with similarity in appearance or contact frequency. Conclusions Similarity in reciprocity norms appears to guide individuals’ cooperative investment decisions rather than predict behavioral symmetry. Shared moral commitments may facilitate positive assortment among non-kin by signaling long-term strategic compatibility.

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.

Competence and prosociality of the person being helped are important indicators for ensuring a return on help, as reciprocity would not be possible if the person being helped lacked either the competence or the inclination to reciprocate in the future. A previous study that measured compassion using a Likert scale found that the controllability of the cause of the difficulty and the level of the sufferer’s prosociality independently influenced the degree of compassion. In this study, we aimed to examine how the probability of reciprocity affects physiological responses (such as heart rate), as well as psychological responses, to the suffering of others. Participants’ heart rates were monitored while they watched a video of a man monologuing about his difficulty. Compassion was also measured using a Likert scale. In Study 1, we investigated the effect of the controllability of the difficulty on heart rate. In Study 2, we provided participants with additional information about the prosociality of the man in the video. Controllability affected compassion ratings, replicating the results of previous studies. Although participants’ heart rates decreased when they learned that the man was suffering, the controllability of the cause did not influence the degree of decline. The results suggest that people experience compassion at a physiological level, while the controllability of the difficulty (i.e., a cue for reciprocity) is processed subjectively and reflected only in consciously reported psychological measures, which may reflect the evolutionary history of compassion.

In the checkbox method, participants check boxes in a grid of 10 × 10 checkboxes on a web page by pointing the mouse on each box and clicking. Each box is numbered from 1 to 100 and it is only possible to check them in order. This method is useful for quantitatively measuring the extent to which respondents are willing to pay a cost, such as the willingness to help or to express gratitude. It needs to be tested whether the checkbox method of measurement shows similar results to traditional methods such as the Likert scale in contexts not directly related to cost. However, no studies have compared the results of a Likert scale with those of a checkbox scale in such a context. We asked participants to complete both a Likert scale and a checkbox scale of the Big Five personality traits using the TIPI-J and investigated relationships between the scales. Despite the time-consuming nature of the responses, there were statistically significant correlations between the number of boxes checked and the Likert scale scores, which revealed that the checkbox method can measure the Big Five personality traits correctly to some extent.

In order to explain helping strangers in need in terms of reciprocal altruism, it is necessary to ensure that the help is reciprocated and that the costs of helping are thus compensated. Competence and willingness to make sacrifices for the benefactor of the person being helped are important cues for ensuring a return on help because reciprocity would not be possible if the person being helped had neither the competence nor the inclination to give back in the future. In this study, we used vignettes and manipulated the cause of suffering strangers’ difficulties and prosociality to investigate participants’ compassion for and willingness to help the stranger. In Study 1, we measured willingness to help by using hypothetical helping behaviors that were designed to vary in cost. In Study 2, we measured willingness to help by using the checkbox method in which participants were asked to sequentially check 10 × 10 checkboxes on a webpage, which asked the participants to pay a small but real cost. In both studies, the controllability of the cause and the prosociality were found to independently affect compassion. These two factors also independently affected willingness to help, as measured by both the hypothetical questions and the checkbox method. We consequently discussed the reasons for the independent processing of the competence and behavioral tendency cues.

Altruistic behavior is defined as actions by an individual that enhance the fitness of another individual while reducing the fitness of the actor. In other words, altruism entails a willingness to incur costs for the benefit of others. While previous research on altruism has measured the willingness to incur costs in hypothetical scenarios, in these studies, respondents merely selected options presented without actually incurring any costs. In this study, we measured willingness to help others by using the "checkbox method" in which participants were asked to sequentially check 10 x 10 checkboxes on a webpage, which asked the participants to pay a small but real cost. The scale consisted of a combination of three recipient categories and four help categories. As participants' relationship with the recipient grew more distant, they became less willing to incur a cost for help. This finding aligns with the multilevel selection theory, supporting the validity of the checkbox method. Furthermore, there were statistically significant correlations between the total number of checked boxes and the score of the Self-Report Altruism Scale Distinguished by the Recipient, which measures the frequency of altruistic behavior in daily life, for all recipient categories, while the correlation was weak when the recipient was a stranger. These results suggest that the checkbox method provides a new response format to measure both intraindividual variability and interindividual differences in the level of altruism.

Illusion of control is a form of self-deception in which individuals overestimate their ability to control events and have inappropriately higher expectations of success than is justified by objective probability. It is unclear why this illusion occurs in cases where obtaining accurate information is adaptive. The module hypothesis posits that obtaining accurate information and signaling one’s usefulness involve distinct modules. These modules are in operation separately when it is important to make accurate decisions and when it is advantageous to signal one’s own usefulness to others. According to this hypothesis, the illusion of control is involved in signaling one’s ability to control events. We examined this in the context of individual differences. We measured the sense of control in a task in which performance was randomly determined and manipulation had no effect. Then, we analyzed the correlations of sense of control with the degree of praise seeking and need to avoid rejection. In contrast to the hypothesis, our results demonstrated that the degree of praise seeking and need to avoid rejection were not associated with the sense of control.

The Knobe effect (side-effect effect) is a tendency to ascribe intentionality in cases of negative, but not positive, side effects. From an adaptive point of view, one possible function of attributing intentionality is to make it easier to predict the actor’s future actions. When the cost of false negatives (i.e., missing an existing intentionality) is high, the need to deal with future actions of the perpetrator increases, which would lead to an increase in the degree of intentionality attribution. This study uses the “lieutenant scenario” to examine whether increasing the severity of side-effect outcome, as the cost of false negatives, facilitates intentionality attribution. Although the side-effect effect was replicated, the results show that the severity of the consequences of the effect did not affect the magnitude of intentionality attribution. Only the positive or negative outcome known to the actor affected the magnitude of intentionality attribution, which might be consistent with the idea that the Knobe effect arises as a result of responding to different mental states of the actor.

Based on a series of vignette studies, DeScioli et al. (2011) proposed that omission bias, i.e., the preference for harm caused by omissions over harm caused by commissions even when the outcome is the same, may be occurred as omission produces little public evidence of wrongdoing. In this study, the effects identified by DeScioli et al. (2011) were examined using an opt-in policy for organ donation in Japan. The results revealed that omission in the context of donating organs was not regarded as immoral, and the individual unwilling to donate and record this decision (public evidence of omission) was rated as morally superior to the individual unwilling to donate and do not record the decision (no public evidence of omission). Considering the Dynamic Coordination Theory, leaving public evidence might be itself morally highly regarded because one can determine which side the actor takes according to the evidence when the act in question (omission in the context of organ donation) is the default and does not have particularly serious consequences.

The Big-Five personality traits contribute to altruistic behaviors in daily life; the degree to which each trait contributes varies according to the recipient. Similarly, factors of the HEXACO model are also expected to contribute to altruistic behaviors according to the relationship between the parties involved. Furthermore, the Dark Triad is another important model of personality structure that may influence altruistic behavior. This pre-registered study investigated the relationship between the traits of the HEXACO and Dark Triad personality models and the frequency of altruistic behaviors measured by the SRAS-DR. Extraversion contributed to altruistic behaviors toward all three types of recipients (family, friends/acquaintances, strangers). Agreeableness contributed only to altruistic behaviors toward strangers, while openness to experience significantly contributed to altruism toward friends/acquaintances and strangers. Previous reports that extraversion and openness from the five-factor model contribute to altruism toward strangers were replicated for a broader range of participants using HEXACO traits for extraversion and openness. Our results suggest that the primary factor explaining individual differences in altruistic behaviors toward strangers is the tendency to be actively involved in the external world, reflected in a person's sociability and inquisitiveness, rather than empathy or the tendency to be fair.

Toyama and Sakurai (2001) reported self-enhancement in the context of agreeableness and conscientiousness among Japanese students, while they observed self-effacement in the context of extraversion and openness. However, their study did not include standardized scales or report the age of the participants. In the present study, self-enhancement and self-effacement in the Big Five personality traits were investigated among a wider age range of participants to conceptually replicate the previous study. In addition to extraversion and openness, self-effacement was newly observed in neuroticism. In contrast to the previous study, there was no self-enhancement in agreeableness and conscientiousness. There were no significant changes in self-enhancement or self-effacement by age group. These results raised doubts about the robustness of the findings of Toyama and Sakurai (2001).

In this study, we investigated the cognitive processes and nonverbal cues used to detect altruism in three experiments based on a zero-acquaintance video presentation paradigm. Cognitive mechanisms of altruism detection are thought to have evolved in humans to prevent subtle cheating. Several studies have demonstrated that people can correctly estimate levels of altruism in others. In this study, we asked participants to distinguish altruists from non-altruists in video clips using the Faith game. Participants decided whether they could trust allocation of money to the targets who were videotaped while talking to the experimenter. In our first experiment, we asked the participants to play the Faith game under cognitive load. The accuracy of altruism detection was not reduced when participants simultaneously performed a cognitive task, suggesting that altruist detection is rapid and effortless. In the second experiment, we investigated the effects of affective status on the accuracy of altruism detection. Compared with participants in a positive mood, those in a negative mood were more hesitant to trust videotaped targets. However, the accuracy with which altruism levels were detected did not change when we manipulated participants’ moods. In the third experiment, we investigated the facial cues by which participants detected altruists. Participants could not detect altruists when the upper half of the target’s face was hidden, suggesting that judgment cues exist around the eyes. We also conducted a meta-analysis on the effect size in each experimental condition to verify the robustness of altruism detection ability.

In humans, education is characterized by active teaching in a wide range of knowledge domains. The subjects taught include how to think and behave, norms and morals, and knowledge and skills, all of which directly increase the fitness of the pupils. Although many comparative and developmental studies have examined active teaching in humans, few have considered its ultimate factors, especially its adaptive significance. The evolution of education can be considered in the context of the evolution of altruistic behavior because tutors transmit adaptive information to pupils at a cost. The evolution of altruism can be explained by the positive assortment of genes related to altruism. However, a genetic basis for reciprocity is not a necessary condition for the evolution of reciprocal altruism. Reciprocal altruism is possible without positive assortment if mutually beneficial social relationships can be created culturally through education. Active teaching of reciprocal norms could be interpreted as niche construction for a social environment. The adaptive perspective presented here could lead to an evolutionary science of education in the future.

Moral emotion is thought to have evolved to guide our behavior and control our impulse to achieve immediate rewards, thus serving to enforce pro-social behavior. Guilt, one of the moral emotions, is a social, other-oriented emotion that is experienced primarily in interpersonal situations, although it may also be experienced in non-interpersonal situations. We predicted that the intensity of the sense of guilt would differ depending on the relationship between a witness and the person who performed the antisocial behavior because building a good reputation plays an important role in the evolution of reciprocal altruism through indirect reciprocity. Participants were asked to imagine that they had been observed by a third party while committing five kinds of moral transgression based on moral foundation theory, and to describe the intensity of their sense of guilt when witnessed by parents, a cordial friend, a neighbor, or a stranger. The intensity of guilt was significantly lower when the act was witnessed by a stranger regardless of the moral foundation involved. The effects of the kind of witness, however, differed for each moral foundation. The results support the hypothesis that guilt functions to guide our behavior, to achieve cooperation.

A reconciliation signal may have evolved because it can reduce the uncertainty of defection, which might be caused by an error in a repeated prisoner’s dilemma situation. Ohtsubo and Watanabe (2009) proposed the “costly apology model” and argued that transgressors should make apologies in a costly way to convey sincerity to victims. Ohtsubo and Yagi (2015) showed that people are more likely to make a costly apology to valuable partners than to less valuable ones. The cost of apology was measured by the strength of willingness to suffer some inconvenience (e.g., cancellation of an important meeting) for it. Such a method of measuring costs is, however, dependent on culture, context, and the participants’ personal situations, and can only measure costs indirectly. We attempted to replicate the results of Ohtsubo and Yagi (2015) by using the checkbox method as a measurement of cost of apology. Among the factors expected to affect the number of checkboxes checked, the primary factor was the instrumentality of the friend, which replicated the results of Ohtsubo and Yagi (2015). Because the participants paid real costs of effort and time for fictitious mistakes, the costly apology might be triggered quickly and intuitively by a heuristic.

Assortative mating must be important for maintaining morality in a population, as moral principles are shared by most people in a group. Breeding by a pair with similar morals results in genetic and cultural transmission of these morals to the next generation, which maintains the moral norms of the group. In this study, we investigated absolute and relative mate preferences in relation to particular moral foundations, as represented by five general moral values. In both sexes, correlations between ratings for self and an ideal romantic partner on these factors were rather high (.67 ≤ r ≤ .84). Differences between self-ratings and ratings for the ideal romantic partner did not deviate significantly from zero for any of these factors.

The watching-eye effect is a phenomenon wherein ar t i f ic ial surveil lance cues enhance prosocial behavior toward strangers. Two meta-analyses on the effect, which focused on generosity measured by donations and economic games, suggest that ar tif icial surveillance cues do not inf luence the donation amount. However, the factors possibly affecting the watching-eye effect should be examined more carefully. The most important factor that can bias results is the number of people around when the participants make a donation decision.

Prosocial lie is an interesting subject in evolutionary psychology because generosity toward others is sometimes inconsistent with adherence to norms. People use justification to tell a lie without threatening the positive self-concept. Previous studies have demonstrated that observing desired counterfactuals encourages justified lying. For example, in the ‘die-under-the-cup’ experimental paradigm, where no one except the participant knew the number of a rolled die, participants who rolled the die once lied less than those who rolled multiple times. This suggests that participants in the multiple-roll condition reported the highest value they observed on all rolls. However, the effects of counterfactuals were only assessed indirectly. In this study, the number that was actually rolled in the ‘die-under-the-cup’ paradigm was determined using a mechanical die. Participants were given an incentive of a donation to charity. Only seven of 133 participants (5.2%) were prosocial liars (i.e., reported a larger number than their actual first roll). Six of the seven liars were female. Honesty decreased when the first roll was a lower number (1, 2 or 3). Not all prosocial liars reported the largest number they had rolled.

Although both dynamic (i.e., facial expressions) and static facial traits are used as cues of altruism, only static facial traits have been shown to be cross-culturally readable with respect to altruism detection skills. To investigate whether dynamic facial cues of altruism are also cross-cultural, we asked French subjects to estimate the altruism of Japanese individuals on the basis of silent video clips. These video clips were taken from a previous experiment, which found that Japanese raters were able to accurately estimate the altruism of a videotaped Japanese individual. By using the same design and stimuli in France, we found that French raters were unable to assess the altruism of a Japanese individual. Hence, our results suggest that dynamic facial cues of altruistic intent are culturally specific rather than universally readable.