Yuk!

Why do human beings screw up their faces in disgust and why do we do this at more or less the same things? Putrid meat, bad body odours, faeces, maggots, flies, vomit, pus running from sores – people around the world respond with revulsion and similar facial expressions to these. True, there are different cultural overlays that emerge around particular sorts of disgusting objects and practices —different sorts of meat are avoided in different cultures, for instance—but at a deep level we all say and feel ‘Yuk!’ in very similar situations.

Dr Valerie Curtis, an anthropologist, epidemiologist and specialist in hygiene at the London School of Hygiene and Tropical Medicine, and author of Don’t Look, Don’t Touch, Don’t Eat: the science behind revulsion, has a plausible explanation of where this basic disgust reaction came from. The answer, lies, she argues, in natural selection. The things humans find disgusting are all associated with disease and parasites of one kind or another. These would have been serious threats to living to a breeding age for our early ancestors. Even today in parts of Africa as many as 50 percent of deaths come from infections. In the not-too-distant past only those who managed to avoid serious contagion or infection would be likely to survive to pass on their genes to another generation, but before that, our non-human ancestors would have had to avoid parasites and diseases in their environment to flourish. So it is hardly surprising that gradually, over millennia, humans evolved disgust reactions that lead us to steer clear of situations that might give rise to harmful infection.

Diarrhea is a major killer worldwide, responsible for nearly a million child deaths a year. It’s caused by a range of viruses, bacteria, and gut parasites. One of the most effective ways of preventing its spread is hand washing with soap after using the toilet. Professor Curtis and her team managed to harness the strong disgust reaction to improve hygiene and save lives. They persuaded a Ghanaian advertising agency to make a commercial showing a Ghanaian woman coming out of the toilet without washing her hands and then making food which she served to her children. The film was doctored so that a purple smear appeared on her palms, which she then transferred to the food. This commercial evoked widespread revulsion—which is exactly what was intended. After the advertising campaign had been run on national television for a year, hand washing using soap after using the toilet had gone up by 13 per cent, and 41 per cent before eating.

Listen to Professor Valerie Curtis discussing the Sources of Disgust on the Social Science Bites podcast.

For a different angle on the Yuk Factor, listen to Julian Savalescu on the Philosophy Bites podcast.

Martha Nussbaum discusses Disgust with Nigel Warburton on Multiculturalism Bites podcast (requires iTunes).