In Bad Taste: Evidence for the Oral Origins of Moral Disgust

The last issue of Science reports a fascinating piece of research on the facial display of disgust among participants whom have been treated unfairly in an economic game. Here is the abstract:

In common parlance, moral transgressions “leave a bad taste in the mouth.” This metaphor implies a link between moral disgust and more primitive forms of disgust related to toxicity and disease, yet convincing evidence for this relationship is still lacking. We tested directly the primitive oral origins of moral disgust by searching for similarity in the facial motor activity evoked by gustatory distaste (elicited by unpleasant tastes), basic disgust (elicited by photographs of contaminants), and moral disgust (elicited by unfair treatment in an economic game). We found that all three states evoked activation of the levator labii muscle region of the face, characteristic of an oral-nasal rejection response. These results suggest that immorality elicits the same disgust as disease vectors and bad tastes.

Following anthropologist Richard Schweder, psychologist Jonathan Haidt has proposed that some of our moral judgments (about incest, purity, or chastity) come from our “sense of disgust”. Do these results argue in favour of such a theory of morality ?


Surprisingly, the authors are very cautious about interpreting their results in favour of a theory of morality based on disgust (and in their commentary, in the same issue, Rozin, Haidt and Finchner are quite cautious too in using these results to support their theory). It may be because such results go actually against Schweder and Haidt’s theory. Indeed, in Chapman et al.’s experiment, moral disgust is elicited by a moral transgression (unfairness) unrelated to the “ethics of purity”. Furthermore, these results go in the same direction than recent experiments which have shown that disgust is not specifically linked to the ethics of purity (Schnall, Haidt, Clore, & Jordan, 2008; Simone, Benton, & Harvey, in press).

If moral judgment is not based on disgust, then why has disgust been exapted from contamination to moral cognition? Following Darwin, the authors suggest that

Configuration of emotional facial expressions has evolved from a functional role in regulating sensory intake. These ancestral configurations may later have proven useful as social signals, assuming a new function without needing to change their basic form.

The authors do not speculate further. But we can complement their suggestion. Disgust might have been exapted for communication. The old facial display of rejection (selected to signal contaminated substances) could have been reused to communicate rejection of some behaviour or some immoral individuals. To sum up, it may be the case that moral judgment have nothing to with disgust. Disgust is just a way to convey moral evaluations.

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