Immoral behaviour really does leave a 'bad taste in the mouth', claims a new study, which found that disgust over an unfair social situation is hard-wired into the human body as strongly as the reaction to a foul taste.

The University of Toronto study shows a link between moral disgust and more primitive forms of disgust related to poison and disease.

"Morality is often pointed to as the pinnacle of human evolution and development," says lead author Hanah Chapman, a graduate student in the Department of Psychology. 

"However, disgust is an ancient and rather primitive emotion which played a key evolutionary role in survival.  Our research shows the involvement of disgust in morality, suggesting that moral judgment may depend as much on simple emotional processes as on complex thought," the expert added.

The study has been published in Science.

To reach the conclusion, scientists examined facial movements when participants tasted unpleasant liquids and looked at photographs of disgusting objects such as dirty toilets or injuries. 

Then they compared these to their facial movements when they were subjected to unfair treatment in a laboratory game. From analysis, the research team found that people make similar facial movements in response to both primitive forms of disgust and moral disgust.

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," Chapman added.