Disgust evolved to combat disease

Parasites are a consistent cause of disgust around the world

Scientists claim to have found evidence that the emotion of disgust evolved to protect us from the risk of disease.

People found images of things that pose a disease risk consistently more disgusting than similar images of things that hold no risk of disease.

In the web-based experiment, women were also found to be more sensitive to disgust than men and as people aged, their disgust sensitivity dropped.

Details of the research are published in the science journal Biology Letters.

The findings suggest that humans may be biologically programmed to avoid certain things such as faeces, wounds, rotting matter and bodily fluids.

Other researchers have proposed that people largely learn to be disgusted by these things.

Nature and nurture

The study, which used data from around 40,000 people, showed people all over the world generally found the disease-causing objects disgusting.

You can't teach a child to have an aversion to oranges or sweets Dr Val Curtis, London School of Hygiene and Tropical Medicine

According to the report's authors, this casts doubt on the idea that ideas about what and what is not disgusting are cultural rather than innate.

"We have a predisposition to being disgusted by faeces. But you can't teach a child to have an aversion to oranges or sweets," lead scientist Dr Val Curtis, of the London School of Hygiene and Tropical Medicine, told BBC News Online.

Although this predisposition to basic disgust stimuli is in our genes, claim the authors, there is also a certain amount of disgust behaviour instilled in us by experience.

"Every one of our behavioural patterns can be explained by learned and biological components. As with most of these things it's a bit of nature and a bit of nurture," added Dr Curtis.

The study found that women have a higher disgust sensitivity than men, perhaps because they are traditionally the main carers for children.

Cultural bias

But Professor Clark McCauley, of Bryn Mawr University in Pennsylvania, US, said that although disgust probably did have an evolutionary foundation - mainly to protect people from eating rotting meat - it was heavily influenced by culture.

"What people today find disgusting goes far beyond what can be understood in the evolutionary sense," Professor McCauley told BBC News Online.

"This biological mechanism was taken up and extended to produce a much broader mechanism of revulsion at different cultural horizons.

"For example, what counts for appropriate care of hair in our society is not the same as in some other societies."

Participants in Dr Curtis's study carried out a test on the BBC website which asked them to mark how disgusting different images were.

The images included a muddy football; a clean and infected wound; a louse; a wasp and plates of goo with different colours.

In the last case, for instance, people consistently report the green and yellow goo as more disgusting than the blue, presumably because the former colours resemble bodily fluids.

There is also evidence that animals have a disgust response. They, like humans, tend to avoid each others' faeces.