“Purity” – a heterogeneous set of phenomena encompassing health, sexuality, and self-control – has been an anomaly for cooperative theories of morality (Gray, DiMaggio, Schein, & Kachanoff, Reference Gray, DiMaggio, Schein and Kachanoff2022). Hence, Fitouchi et al. have done a great service in providing a cooperative explanation.

According to their account, many aspects of “purity” can be understood as cues of self-control, and self-control is moralised because it predicts a person's likelihood of reciprocating in a social dilemma (whereas impurity can be understood as a cue of a lack of self-control, which predicts a person's likelihood of cheating in a social dilemma). We agree.

However, social dilemmas are not the only type of cooperative problem, and reciprocity is not the only solution. There are other types of cooperation (kin altruism, mutualism, conflict resolution), that explain other types of morality (family values, solidarity, heroism, deference, fairness, and property rights) (Curry, Reference Curry, Shackelford and Hansen2016; Curry, Mullins, & Whitehouse, Reference Curry, Mullins and Whitehouse2019). These other types of cooperation may explain other aspects of “purity” that are not explained by Fitouchi et al.'s self-control theory.

Take conflict resolution. Organisms often come into conflict over food, territory, mates, and other resources (Huntingford & Turner, Reference Huntingford and Turner1987). Contestants have a common interest in minimising the mutual costs of conflict – time, energy, injury – hence these interactions are modelled as non-zero-sum hawk–dove games (Maynard Smith & Price, Reference Maynard Smith and Price1973). One strategy for minimising costs is to engage in “ritual contests”: contestants display conflict-winning traits (that indicate their probability of winning the conflict were it to escalate); contestants with inferior traits defer to those with superior traits, and withdraw from the contest (Maynard Smith, & Parker, Reference Maynard Smith and Parker1976). In stable social groups, these contests lead to the formation of dominance hierarchies (Preuschoft & van Schaik, Reference Preuschoft, van Schaik, Aureli and de Waal2000).

Many organisms, including humans, engage in such contests and form hierarchies (Mazur, Reference Mazur2005). The traits humans display in contests include: strength, health, beauty, bravery, generosity, intelligence, skill, industriousness, and coalition size (Buss et al., Reference Buss, Durkee, Shackelford, Bowdle, Schmitt, Brase and Trofimova2020; Gintis, Smith, & Bowles, Reference Gintis, Smith and Bowles2001; Riechert, Reference Riechert, Dugatkin and Reeve1998). Emotions are important regulatory mechanisms in these contests. For example, people are proud of, and motivated to display, superior traits; and they are ashamed of, and motivated to conceal, inferior traits (Sznycer et al., Reference Sznycer, Tooby, Cosmides, Porat, Shalvi and Halperin2016, Reference Sznycer, Al-Shawaf, Bereby-Meyer, Curry, De Smet, Ermer and Tooby2017). (One function of shame, then, is to motivate people to withdraw from contests they have little chance of winning.)

This conflict-resolution theory predicts that these superior and inferior traits will be moralised because they help to solve a cooperative problem – they help to minimise or forestall conflict – quite apart from any other function they might perform (Curry, Reference Curry and Sinnott-Armstrong2007). This theory predicts that superior traits will be considered morally good – honourable virtues, worthy of respect. And inferior traits will be considered morally bad – dishonourable vices that degrade those who possess them by lowering their social value in the eyes of others.

This theory can explain why, for example, cues of high and low mate-value have been considered morally good and bad, “pure” and “impure.” People compete for mates by signalling cues of high mate-value that are attractive to the opposite sex (such as fertility, fidelity, chastity, beauty, industry), and concealing cues of low mate-value that are unattractive (such as infertility, infidelity, promiscuity, poor health, a history of failed relationships). Sexually attractive traits will be considered morally good, sexually unattractive traits will be considered morally bad.

As an initial test of this hypothesis, we asked an online sample in the United States (MTurk; n = 98; 66% male; mean age = 33 years) to rate the degree to which 20 “impure” traits (including promiscuity, masturbation, laziness, and drinking alcohol): (1) indicate a lack of self-control; (2) are sexually unattractive; and (3) are morally bad (1–100). We regressed “moral badness” onto “lack of self-control” and “sexual unattractiveness” using a mixed model, with traits nested within participants. (All materials, data and analysis are available on OSF: https://osf.io/g52w6/.)

Both “lack of self-control” (β = 0.26) and “sexual unattractiveness” (β = 0.25) predicted the “moral badness” of the traits (marginal R ² = 0.24). The two predictors together explained more variance in moral badness than either do alone.

These results support the self-control theory; and they also support the conflict-resolution theory. They show that a broader cooperative theory of morality can better explain why traits are moralised. Future research should develop and test predictions from all available theories of cooperation when attempting to explain moral psychology. Advancing in this way, cooperation may provide a comprehensive explanation of moral phenomena, including those previously labelled “purity.”