Fitouchi et al. forward an account of puritanical morality that rejects a link to disgust in favor of one focused on self-control and cooperation. Although we find their skepticism of moral foundations theory and the disgust priming literature well justified, we believe that, in line with our titular quote by Friedrich Nietzsche, they have overlooked multiple ways that disgust might still inform morality.

Consider the example that arguably sparked the decades-long interest in the concept of purity: Sibling incest. Haidt's (Reference Haidt2001) moral intuitionist model argued that people experience a flash of affect (here, disgust) when considering incest, which in turn leads to condemnation, which is justified by “rational” arguments. A deeper adaptationist analysis inspires further questions, such as: Why are people disgusted by the thought of having sex with their close kin? Why do some people experience greater disgust toward incest than others? And why would these personal feelings of disgust influence condemnation of others who engage in (even consensual) incest?

A small literature (summarized by Lieberman & Smith, Reference Lieberman and Smith2012) has sought to answer these types of questions by considering: (1) the ultimate function of avoiding sex with close kin, and (2) the proximate mechanisms required for recognizing close kin and experiencing such disgust. Briefly, this literature proposes that a domain-specific type of disgust, sexual disgust (Tybur, Lieberman, & Griskevicius, Reference Tybur, Lieberman and Griskevicius2009), functions to reduce fitness-compromising sexual behaviors (e.g., the higher risk of combining deleterious recessive alleles inherent to incest; Bittles & Neel, Reference Bittles and Neel1994). Individuals who categorize each other as close genetic relatives reliably develop a mutual sexual disgust via a process that relies upon the detection of ancestrally valid kinship cues. For siblings, these cues include: (1) observations of an individual being cared for as a newborn by one's mother (e.g., nursing), and (2) observations of repeated shared parental investment over the duration of dependency (Lieberman, Tooby, & Cosmides, Reference Lieberman, Tooby and Cosmides2007; Westermarck, Reference Westermarck1891). The second cue is more important when the first cue is absent, because the first cue is presumably higher validity but not available to everyone (e.g., for younger siblings in a sib-pair). In line with predictions, both cues predict the degree of personal sexual aversion to sex with a sibling (Lieberman et al., Reference Lieberman, Tooby and Cosmides2007).

Remarkably, the cues discussed above also predict one's condemnation of others' sibling incest. Earlier work argued that this phenomenon is a by-product of personally felt disgust (Fessler & Navarrete, Reference Fessler and Navarrete2004; Lieberman, Tooby, & Cosmides, Reference Lieberman, Tooby and Cosmides2003); later work built upon this idea by proposing that felt disgust might inform the value of strategically supporting (or, at least, not resisting) particular norms (DeScioli & Kurzban, Reference DeScioli and Kurzban2013; Tybur, Lieberman, Kurzban, & DeScioli, Reference Tybur, Lieberman, Kurzban and DeScioli2013). Put simply, people have little to lose by endorsing rules against behaviors they find disgusting given that they are unlikely to engage in such behaviors and thus be targeted by resulting sanctions.

Multiple other sexual behaviors are similarly morally condemned. Consider why people stigmatize and punish (sometimes by death) not only those who have sex with a sibling, but also those who have sex with someone of the same sex. The argument forwarded in the target article suggests that such behaviors are moralized because they are diagnostic of self-control failures and, consequently, uncooperative tendencies. We are deeply skeptical of this interpretation. Individuals with same-sex sexual preferences remove themselves from the pool of intrasexual competitors in the majority mating market. What could be more cooperative in a cutthroat sexual marketplace than removing oneself from the competition?

As Lieberman and Patrick (Reference Lieberman and Patrick2018) explain, adaptations that regulate personal decisions in the domains of food choice, physical contact, and mate choice can influence the perception of the social affordances and externalities that others hold. Broadly speaking, individuals place lower social value on those who (1) eat foods of lower consumption value, (2) regularly touch contaminated objects, or (3) select sexual partners perceived as lower reproductive value – that is, those who engage in disgust-eliciting behaviors. These considerations only relate to personal partner choice – they don't explain the time and energy invested in condemning third parties for engaging in the consumption, contact, and sexual behaviors often lumped under the umbrella of purity. Other systems are required to explain condemnation.

Bearing similarity to aggressive behaviors present in our chimpanzee cousins (Wrangham, Reference Wrangham1999), humans might have coalitional adaptations that monitor for, exploit, and potentially eliminate vulnerable resource competitors (Kurzban & Leary, Reference Kurzban and Leary2001; Tooby & Cosmides, Reference Tooby, Cosmides and Hogh-Olesen2010). If disgust-eliciting behaviors inform low social value, then individuals who engage in such behaviors might be especially prone to exploitation. The proximate, experiential aspects of such systems comprise our moral sense, which in turn shapes our perceptions of concepts such as “responsibility,” “blame,” “harm,” and (especially pertinent to the target article) “self-control.” Such concepts facilitate the mental and physical coordination of groups of people for the express purpose of targeting individuals viewed as holding low value. From this perspective, perceptions of self-control failures are often the outputs of other systems designed for moral condemnation, and feelings of disgust often serve as inputs.

The target article's dismissal of disgust is largely based on findings that (1) priming disgust (e.g., via exposure to a disgust-eliciting odor) does not lead people to generally find actions more morally wrong (Landy & Goodwin, Reference Landy and Goodwin2015), and (2) disgust expressed toward moral violations shares features with anger expressed toward identical moral violations (e.g., Piazza, Landy, Chakroff, Young, & Wassermann, Reference Piazza, Landy, Chakroff, Young, Wassermann, Strohminger and Kumar2017; cf. Molho, Tybur, Güler, Balliet, & Hofmann, Reference Molho, Tybur, Güler, Balliet and Hofmann2017). These observations do not inform the phenomena or accounts described above. They do mirror other recent accounts that, to us, have thrown the disgust baby out with the contaminated bath water in favor of an overly credulous focus on the idea that morality (perhaps exclusively) functions to promote cooperation (e.g., Curry, Mullins, & Whitehouse, Reference Curry, Mullins and Whitehouse2019) or, relatedly, punish harms (Schein & Gray, Reference Schein and Gray2018). Although cooperation is relevant to morality, good evidence suggests that it cannot explain everything in this area (DeScioli & Kurzban, Reference DeScioli and Kurzban2009). A more complete understanding of morality might require a long look into the abyss of the darker side of human nature, with disgust being an important part of this investigation.