Univ of Toronto says:

In everyday language, people sometimes say that immoral behaviours “leave a bad taste in your mouth.” But this may be more than a metaphor according to new scientific evidence from the University of Toronto that shows a link between moral disgust and more primitive forms of disgust related to poison and disease.
…
The research employed electromyography, a technique that uses small electrodes placed on the face to detect electrical activation that occurs when the facial muscles contract. In particular, they focused on movement of the levator labii muscle, which acts to raise the upper lip and wrinkle the nose, movements that are thought to be characteristic of the facial expression of disgust.

“We found that people show activation of this muscle region in all three situations – when tasting something bad, looking at something disgusting and experiencing unfairness,” said Chapman.

Fascinating stuff.