The effects of exogenous testosterone and facial attractiveness on men’s altruistic punishment behavior

Altruism plays a crucial role in social interactions, fostering social cohesion and strengthening interpersonal relationships. It may also be considered an adaptive trait in mate selection, with individuals being more likely to display altruistic behaviors toward attractive potential partners (Bhogal et al., 2019, Margana et al., 2019, Boog et al., 2024). Altruistic (costly) punishment is one type of special altruistic behavior (Fehr, Gächter, 2002), which plays a pivotal role in maintaining social cooperation, upholding social norms, and fostering moral development. The third-party punishment (TPP) paradigm has emerged as a powerful tool for investigating altruistic punishment behavior. In this paradigm, a third party (the bystander) can punish the a party (the dictator) who has made an unfair allocation to a second party (the recipient) (Fehr and Gächter, 2002, Fehr and Fischbacher, 2003, Fehr and Fischbacher, 2004, Kanakogi et al., 2022). Previous studies on TPP have found that the dictator's allocation intentions, the harm caused to the recipient, and personal information influence the punishment decision (Krueger and Hoffman, 2016, Feng et al., 2021, Feng et al., 2023, Yang et al., 2021, Yang et al., 2024). Unlike second-party punishment, which serves personal interests, TPP signals desirable traits such as credibility and altruism through costly punitive behaviors (Jordan et al., 2016, Chen et al., 2020). Moreover, when in mate selection, altruistic punishment of a third party may function as a costly mating signal, as high-cost altruistic behaviors can make individuals more attractive in society (Niu et al., 2020).

Previous studies have demonstrated that altruism plays a significant role in mating preferences. Altruistic individuals who scored higher on altruism or displayed more altruistic behaviors self-reported having more potential partners (Arnocky et al., 2017). Women, in particular, have been found to prefer partners with altruistic traits more than men (Griskevicius et al., 2007, Oda et al., 2013). However, more recent research indicates that men also value altruism in potential partners. For instance, both men and women rate altruistic individuals as more attractive, especially in the context of long-term relationships (Farrelly and King, 2019). Further evidence suggests that altruistic behaviors, such as costly pro-environmental actions, can serve as mating signals. One study found that both men and women find individuals who engage in costly pro-environmental behavior more attractive and that people are more likely to endorse such behaviors when in the presence of attractive opposite-sex individuals. (Farrelly and Bhogal, 2021). Similarly, men demonstrate greater generosity when women are present (Latané, 1970). Additionally, men are more likely to behave altruistically in the presence of women rather than men in online tasks (Farmer and Farrelly, 2023), suggesting that mating motivations may increase prosocial behavior in social contexts. Although the link between altruistic behaviors and mate selection is well established, the effect of mating motivations on altruistic punishment remains largely unexplored.

Facial attractiveness is an important factor that can elicit mating motivation, and attractiveness appearance significantly impacts both the suitors and the pursued (Riggio et al., 1991, Peters et al., 2007, Currie and Little, 2009). Several researchers have found that individuals behaved more altruistically towards attractive opposite-sex individuals. For example, in social networking site interaction situations, men were more prosocial in their requests to attractive women than to unattractive women (Schwarz and Baßfeld, 2019). In charitable contexts, men donated more when attractive women were present and with no effect on the amount of money donated by women (Iredale et al., 2008). A study using pictures of faces found that men were more cooperative and altruistic towards attractive women (Bhogal et al., 2016). Previous research using dictator and ultimatum games has explored how attractiveness influences altruistic behavior. For example, in the ultimatum game, men were more willing to accept unfair offers from highly attractive female proposers compared to those from less attractive female proposers (Ma et al., 2015). Similarly, the effect of facial attractiveness on altruistic behavior has been found in third-party punishment. In the third-party punishment game, men participants punished men proposers more than women proposers. In addition, individuals were less willing to punish attractive proposers of the opposite sex (Li and Zhou, 2014).

Other studies provide further evidence supporting the role of attractiveness in influencing altruism and mating-related behaviors (Farrelly et al., 2016; Novakova et al., 2021). One study extended this line of research by introducing more complex social decisions involving both punishment and compensation in third-party contexts. They found that under romantic priming, men were more likely to choose compensatory or dual strategies (compensating and punishing), and women showed a preference for men who compensated rather than punished (Ferguson et al., 2019). These findings suggest that mate-related altruistic behavior is not limited to one form (e.g., costly punishment), but flexibly adapts to social context and signaling opportunities. Nevertheless, some research indicates that facial attractiveness does not always exert a significant influence on altruistic behavior, depending on the specific context. For example, Bhogal et al. (2017) found no effect of facial attractiveness on allocation fairness in a face-to-face ultimatum game using chocolate coins as currency. Taken together, these findings highlight that while attractiveness often enhances altruistic responses, its influence is nuanced and context-dependent. In conclusion, most studies support the idea that individuals demonstrate more altruistic behavior toward highly attractive members of the opposite sex.

Testosterone, a primary men sex hormone, influences men’s mating behavior and altruistic behavior (Arnocky et al., 2018, Carré et al., 2023, Novakova et al., 2024). Testosterone levels vary according to relationship status, with unmarried men in relationships showing lower levels of testosterone than single men, and married men exhibiting even lower levels (Burnham et al., 2003, Gray et al., 2004, Van Anders and Goldey, 2010, Holmboe et al., 2017). Facial attractiveness plays an important role in mate selection. Previous studies found that exogenous testosterone administration on men can modify their facial preference (Han et al., 2020) and increase the attraction of low-attractiveness women to single men (Geniole et al., 2022). These findings suggest that testosterone levels affect men’s cognition of female faces and that higher testosterone levels may increase individuals' mating motivation. Testosterone not only affects men’s mating behavior but also affects altruistic behavior. Men with higher levels of endogenous testosterone behave more altruistically in the dictator game (Novakova et al., 2024). However, increased testosterone levels can enhance individuals’ aggressive response, and aggressive behavior is moderated by other factors such as personality and social background (Carré et al., 2023). In third-party intervention games, it has been found that individuals with high endogenous testosterone levels are more likely to punish dictators, while individuals with low endogenous testosterone levels are more likely to help recipients (Wang et al., 2022). In the context of altruistic behavior, the role of testosterone is complex, with some studies suggesting that it can either promote or inhibit altruistic behavior depending on the specific circumstances (Burnham, 2007, Eisenegger et al., 2010, Dreher et al., 2016, Reimers et al., 2019, Ou et al., 2021, Novakova et al., 2024). Given these complexities, the influence of testosterone on altruistic punishment behavior remains unclear and requires further investigation to determine how these effects may interact with other contextual factors, such as mating motivation and facial attractiveness.

Both testosterone levels and the potential partner's facial attractiveness can elicit men’s mating motivation; however, how testosterone and a potential partner's facial attractiveness influence men’s altruistic punishment behavior is still unexplored. The goal of this study was to examine the effects of testosterone administration and the facial attractiveness of female recipients on men's altruistic punishment behavior implementing a TPP game paradigm. To address this, we employed a modified third-party punishment (TPP) game in a double-blind, placebo-controlled, within-subjects crossover design. Men participants acted as third-party observers who could choose to punish male dictators for making unfair monetary allocations to female recipients. The key experimental manipulations involved (1) varying the facial attractiveness of the female recipients and (2) using a within-subjects crossover design, where each men bystander participates in the experiment twice, once with exogenous testosterone administration and once with a placebo, effectively controlling individual differences. This design choice enhanced the study's internal validity and enabled more precise comparisons of the effects of the two independent variables on altruistic punishment behavior.

Based on the preceding discussion, we were planning to test three hypotheses in this study. Firstly, we hypothesized that men participants will display more altruistic punishment behavior when the recipient is a highly attractive woman, compared to when the recipient is a low-attractive woman, in a TPP situation. This hypothesis is grounded in the idea that men may use costly altruistic punishment as a signal to display desirable traits, such as altruism (Jordan et al., 2016, Chen et al., 2020, Niu et al., 2020). Previous studies have shown that individuals tend to behave more altruistically toward attractive opposite-sex individuals (Schwarz and Baßfeld, 2019, Iredale et al., 2008, Bhogal et al., 2016; Ma et al., 2019; Li and Zhou, 2014). To isolate the effect of attractiveness on altruistic behavior without the influence of testosterone, we focused on men participants under placebo conditions only.

Secondly, we predicted that testosterone administration will increase men participants' altruistic punishment behavior in TPP situations, and this increase will be significantly greater when the recipient is a highly attractive member of the opposite sex. Specifically, testosterone administration will elevate both the proportion and intensity of punishment directed toward other men (the Dictator) when interacting with highly attractive female recipients, compared to less attractive ones. Our hypothesis is based on the idea that testosterone influences men's mating motivation, coordinating behavioral responses such as competition for resources, which are important for mate selection (Roney and Gettler, 2015). Therefore, exogenous testosterone administration may enhance short-term mating motivation, which could amplify altruistic TPP behavior, particularly when directed toward highly attractive individuals.

Finally, we assumed that testosterone administration will increase men participants’ facial attractiveness ratings and dating intentions toward women across varying levels of attractiveness. Specifically, based on previous research showing that exogenous testosterone can enhance attractiveness ratings even for low-attractiveness faces (Geniole et al., 2022), we expected testosterone to generally elevate these ratings and intentions regardless of the women’s baseline attractiveness. This hypothesis is supported by evidence that testosterone plays a significant role in mating-related behaviors (Roney and Gettler, 2015).