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 oralnasal rejection response. These results suggest that immorality elicits the same disgust as disease vectors and bad tastes.A summary graphic from the review of this work by Rozin et al. and some of their comments:
Domains of disgust. The schematic represents routes by which eliciting situations may trigger the disgust output program. Those that run through the disgust evaluation system--which includes appraisal of the elicitor, feelings, and contamination ideation--trigger the full disgust emotion. Solid lines represent routes through which an elicitor can activate the disgust evaluation-output program. Dashed lines (green) represent direct elicitation of the disgust output program. The dotted line (brown) represents a metaphoric, indirect route.
According to the principle of preadaptation, a system that evolves for one purpose is later used for another purpose. From this viewpoint, disgust originates in the mammalian bitter taste rejection system, which directly activates a disgust output system. This primal route (e.g., bitter and some other tastes) evokes only the output program, without a disgust evaluation phase. During human evolution, the disgust output system was harnessed to a disgust evaluation system that responded not to simple sensory inputs (such as bitter tastes) but to more cognitively elaborated appraisals (e.g., a cockroach). Initially, the evaluation system was a food rejection system that rejected potential foods on the basis of their nature or perceived origin. This was the first "true disgust," because it engaged this evaluation system. Later, through some combination of biological and cultural evolution, the eliciting category was enlarged to include reminders of our animal nature, as wel as some people or social groups. This process had adaptive value, because by making things or thoughts disgusting a culture could communicate their negativity and cause withdrawal from them.