Literally a bad taste in the mouth

We often use the phrase "It left a bad taste in my mouth" to describe an activity or situation we find unpleasant. But now researchers writing in the journal Science have shown that there may be more to this metaphor than meets the eye.

The researchers, led by Hanah Chapman, wondered if there was any kind of link between the facial movements made when we eat disgusting food, when we see a disgusting picture, or when we experience unpleasant behaviour. So they carried out some experiments using volunteers.

To start with, the researchers gave the volunteers different drinks - either neutral tasting, sweet or bitter, and took close-up video images of their faces. In particular, they focused on the actions of a group of muscles called the levator labii - these make us wrinkle up our noses and raise our upper lips when we taste something nasty.  Unsurprisingly, they found that the bitter taste caused a big movement of these muscles, compared to sweet or neutral tastes.

The scientists then showed people pictures of disgusting things, including poo, injuries, insects and so on. They compared these with pictures of sad things, and neutral pictures for contrast.  The team found that only the disgusting pictures led to a movement of the levator labii muscles, and the stronger the disgust that the person felt, the more their muscles moved.

Intrigued by this, the team went on to look at situations where people experienced unpleasant or unfair situations. They played a game with the volunteers in which two players split ten dollars. One person suggests how the money should be split, then the other agrees or disagrees. If they reject the offer, then both players lose the money.  The researchers discovered that unfair offers, say of 9 dollars to one dollar, were met with the same facial movements of disgust  seen with the nasty liquid or unpleasant pictures.

Well the researchers suggest this means that moral disgust and outrage has similar evolutionary roots to physical disgust. They think that the physical response to something nasty has probably been co-opted during our social evolution to express disgust at social and moral situations we don't like. Obviously this would be an important social signal and code to help keep people in line and prevent unfairness, building a stronger, more equitable society.