Computer-generated average facial expressions of the five most expressive individuals tasting neutral, sweet, and bitter solutions. The upper lip and nose areas are highlighted to show the action of the levator labii muscle (upper lip raise and nose wrinkle) across conditions. (Science/AAAS)

Moral disgust in the face of unfair treatment and primitive disgust in reaction to a poison or disease may be more closely linked than we believe, say Canadian researchers.

University of Toronto investigators suggest people literally turn up their noses in response to both moral and primitive disgust.

"Morality is often pointed to as the pinnacle of human evolution and development," Hanah Chapman, a graduate psychology student and lead author of the study, said in a release. (Listen to the CBC Radio Quirks and Quarks interview with Chapman.)

"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."

Chapman and her colleagues will publish research backing up their claim in the journal Science on Friday.

In the study, they attached small electrodes to the faces of 20 participants to record muscle movement. They then asked them to taste unpleasant liquids and look at photos of disgusting things — such as dirty toilets or injuries. The researchers also subjected the participants to unfair treatment in a laboratory game.

In the game, two players split $10, with one proposing how it should be split, and the other either accepting or rejecting. If the participant rejected it, neither player got money.

The researchers measured muscle movement reactions for both fair proposals, such as splitting the money equally, and unfair proposals, such as $1 for one person and $9 for the other.

The team then compared participants' facial movements, finding similarities in their reaction to the unpleasant liquids, disgusting photos and unfair treatment.

Specifically, participants moved their levator labii muscle, which raises the upper lip and wrinkles the nose. The researchers took these movements to be characteristic of the facial movements we make when expressing disgust.

"Disgust is a somewhat surprising candidate for a moral emotion, given its hypothesized origins in the … adaptive functions of rejecting toxic or contaminated food and avoiding disease," they write. "In the moral domain, this rejection impulse might have been co-opted to promote withdrawal from transgressors, or even from the thought of committing a transgression."

Or as Adam Anderson, principal investigator on the project and the Canada Research Chair in affective neuroscience, put it: "Surprisingly, our sophisticated moral sense of what is right and wrong may develop from a newborn's innate preference for what tastes good and bad, what is potentially nutritious versus poisonous."