The role of prosocial behaviors as courtship displays has received a great deal of attention (e.g., Farrelly et al. Reference Farrelly, Lazarus and Roberts2007; Iredale et al. Reference Iredale, Van Vugt and Dunbar2008; Miller Reference Miller2000; Phillips et al. Reference Phillips, Barnard, Ferguson and Reader2008). As such, it has contributed greatly to our understanding of why humans behave altruistically from a Darwinian perspective. That the target article not only recognizes this, but also suggests from the body of evidence that mating motives are a key cause of prosocial biases to attractive individuals is pleasing to see, and I agree fully. However, I believe that the target article curtailed its investigation of this too early, and a more interesting and revealing understanding can be gleaned when one goes further. This commentary aims to do just that, by focusing on the contexts in which prosocial biases are used in mate choice as courtship displays (as suggested in the conclusion of the target article) and also, importantly, on the different types of prosocial behavior that exist. As part of this, it is essential to not just concentrate on research findings of actual prosocial behaviors, which the target article has done comprehensively. Instead, this commentary will also discuss research that shows the counterpart to this, if indeed a key cause is mating motives; namely, what is it about prosocial behaviors that all individuals (including, of course, physically attractive ones) find desirable in mate choice?

A particularly important context to consider is the role of length of relationship. Here there is very strong experimental support for prosocial behaviors being more desirable for longer relationships (Barclay Reference Barclay2010; Farrelly Reference Farrelly2011; Reference Farrelly2013; Farrelly et al. Reference Farrelly, Clemson and Guthrie2016; Guo et al. Reference Guo, Feng and Wang2015; Moore et al. Reference Moore, Wigby, English, Wong, Szekely and Harrison2013; Oda et al. Reference Oda, Okuda, Takeda and Hiraishi2014), as well as having an important role cross-culturally in actual long-term relationships (Stavrova & Ehlebracht Reference Stavrova and Ehlebracht2015; Tognetti et al. Reference Tognetti, Berticat, Raymond and Faurie2014). This suggests that prosocial behavior is signaling good phenotypic quality, that is, the ability of the signaler to provide and support as a good partner or parent (Farrelly Reference Farrelly2011; Kokko Reference Kokko1998; Miller Reference Miller2007). Furthermore, the lack of preferences for prosocial males for short-term relationships among females at the fertile stage of their menstrual cycle (Farrelly Reference Farrelly2011; Oda et al. Reference Oda, Okuda, Takeda and Hiraishi2014), as well as non-prosocial men being preferred by women for short-term relationships (Farrelly et al. Reference Farrelly, Clemson and Guthrie2016), suggests that an alternative signal, that of good genetic quality (Miller Reference Miller2000), cannot account for mate choice preferences for prosocial behavior. Therefore, these findings offer support for the sexual-signaling hypothesis of the target article because, as the authors highlight, it suggests that prosocial behaviors are signaling desirable mate choice traits. More importantly, however, the role of relationship length makes an important contribution as we are now able to surmise more precisely what is being signaled.

Additionally, more can be revealed about the target article's aims if we consider that there exist a myriad of types of behaviors that can be considered “prosocial.” Once this is recognized, a more in-depth investigation of the role of such behaviors in mate choice can be revealing. For example, from the research in the target article that looks at economic games, it is suggested that the parameters of different games mean different types of prosocial behaviors are being signaled. Although, as the target article recognizes, different games tend to produce similar findings with regard to the effects of physical attractiveness, this is not always the case. For example, Jensen (Reference Jensen2013) found no increased prosociality to attractive opposite-sex individuals with the trust game. This raises a particularly interesting question: Can behavior signaled in this game, trustworthiness, be considered clearly distinctive from that of other games such as the dictator or ultimatum game, which can perhaps be associated with generosity, or the prisoner's dilemma game, which is often considered a measure of reciprocal cooperation? In a similar vein, fairness in a particular interaction, rather than indiscriminate prosociality (i.e., helping anyone), will be interpreted differently, so is there a difference in individuals' biases to display these to physically attractive observers? Little research exists to answer this, but a recent study found that behaving fairly occurred more than overall prosociality toward physically attractive partners in an ultimatum game (Bhogal et al. Reference Bhogal, Galbraith and Manktelow2016). Elsewhere, Guo et al. (Reference Guo, Feng and Wang2015) found that a cultural norm among Chinese undergraduates had a great influence over the role of kin altruism, making this type of prosocial behavior unusual in mate choice as it was preferred more by men (and signaled more by women). Finally, heroism can be considered an additional category of prosocial behavior, which, although it too has been shown to be an important trait in mate choice (Farthing Reference Farthing2005; Reference Farthing2007; Kelly & Dunbar Reference Kelly and Dunbar2001) and therefore more likely to be biased toward attractive individuals, is unfortunately not addressed in the target article. Overall this is not to say that the premise of the target article and the body of research is flawed, as the majority of prosocial behaviors researched do indeed show their value in mate choice scenarios, suggesting that they signal a similar value. However, a more nuanced and careful view of what “prosocial behavior” may constitute in future research that examines such biases toward attractive individuals is clearly warranted.

To conclude, the aims of this commentary were not only to support the argument of the target article that evolutionary explanations can best account for biases in prosocial behavior toward physically attractive individuals, but also to build on this further with a more detailed analysis of research into the role of such behaviors in mate choice. The analysis of the latter aim suggests that one should expect such biases to be more prominent in mating contexts where individuals may be seeking more long-term, committed relationships (perhaps the modern workplace, which the target article does show often happens) and also pay attention to what aspect of “prosociality” the biases are signaling, to enlighten us and further aid our understanding.