What would you do if you had the misfortune, like a friend of mine, to drop your mobile phone into a lavatory which was full of someone else`s shit? She said the very idea of fishing it out was utterly disgusting, and when she took it back to the shop and told them what had happened, they did not try even the simplest test, but threw it straight into the bin.

In Emotional Rollercoaster this week (xxx zzz) Claudia Hammond explores the science of disgust. Why do some things disgust us, and is the emotion of disgust hard-wired into us by our genes?

Charles Darwin wrote at length about emotions: `The term 'disgust' means something offensive to the taste, as actually perceived or vividly imagined; and anything which causes a similar feeling, through the sense of smell, touch, and even of eyesight. A smear of soup on a man's beard looks disgusting, though there is of course nothing disgusting in the soup itself.

He described the expression of disgust as appearing `chiefly in movements round the mouth. The nose may be slightly turned up, which apparently follows from the turning up of the upper lip; or the movement may be abbreviated into the mere wrinkling of the nose. But as disgust also causes annoyance, it is generally accompanied by a frown, and often by gestures as if to push away or to guard oneself against the offensive object.`

Disgust varies from culture to culture; I understand the Japanese find quite disgusting our habit, when we have a cold, of blowing the nose, collecting the snot in a handkerchief, and carefully storing it away in a pocket.

A recent scientific survey of 40,000 people showed that young women are most easily disgusted, and that disgust is produced by things associated with disease – not only dodgy food but also some insects, and bodily fluids. The scientists concluded that disgust may be a defence mechanism against disease, most important for women of child-bearing age.

Drinking water contaminated by sewage has always been one of the world`s deadliest killers. I am intrigued, therefore, by the fact that the Romans built the great sewer in Rome – the cloaca maxima – not because they knew of the link between dirty drinking water and disease, but simply to get rid of the smell. Perhaps those hard-wired emotions saved them, and us, from extinction.