Is hygiene in our genes?.

Disgust is a response that protects us from danger, which we have inherited from our ancestors, says Dr Val Curtis.

Dog shit, dirty nappies, vomit, bad breath, stained towels, lice, nasal mucus, half-eaten food, saliva, worms, rotten meat, maggots, sores, urine, rats and sweat. What do all these things have in common? The answer is that we find them disgusting. And surprisingly enough people everywhere seem to find them disgusting too. In Africa, India and Europe people say such things turn their stomach and make them recoil. Touching excreta or maggots is hard for most of us and we go to great lengths to remove the evidence of such revolting yucky stuff from our lives.

Though disgust has been recognised as one of the six basic emotions since the days of Darwin, it has been very little studied. So much so that one researcher called it ‘the forgotten emotion of psychiatry’. Nevertheless, disgust is something we are all familiar with. We easily recognise the facial expression: a wrinkled nose and the corners of the mouth pulled down. We know the feeling of nausea, the shudder, the urge to drop whatever it is that is disgusting, and the way we almost automatically say ‘Yuck!’.

However it is not at all clear why we have an emotion of disgust. We know that fear keeps us safe from danger and hunger helps keep us nourished. Does disgust also have a purpose?

Out of place

A number of writers have tried to explain disgust. Foremost among them is Mary Douglas, who suggests that disgust is a by-product of the way that we organise our lives. We put objects into categories, and objects that don’t fit tend to be classed as dirty, polluting or disgusting and thrown out. Hence earth in the garden is in the right place and not particularly dirty, but once we bring earth into the kitchen, or put it on a plate, then it is very much in the wrong place and so tends to disgust us. Faeces, which are neither inside nor outside the body, threaten our notion of what is and what is not us, and are hence classed as dirty and polluting. Paul Rozin, the leading psychologist of disgust, has another explanation. He suggests that disgust originated as a food aversion, but has become a means of protecting ourselves, body and soul, from pollution.

A protective device

Our work suggested that there might be another explanation for disgust. Our collections of what people found disgusting in six countries and an international airport showed common themes. These included bodily excretions and body parts, decay and spoilt food, and a number of living creatures, especially insects and worms. Whilst working on another project I flipped through the index of disease carriers in a textbook on infectious disease. To my surprise I noticed the same list: excreta (causing at least 25 diarrhoeal diseases) saliva and breath (carrying measles, colds, scarlet fever, flu and chicken pox), wounds and sores (sepsis, pneumonia, gonorrhoea), spoilt food (carrying food toxins and diarrhoeal diseases). Rats, lice, snails and worms were all there too, involved in causing over 20 known diseases. Could it be that humans have evolved a disgust of all these things as a way to keep us healthy?

It is readily accepted that humans evolved physical defences against infection. Our complex immune system, our gut with apertures at each end, and antibiotics in our tears clearly evolved to protect us from disease, which is caused by the bugs which are trying to break us down and use us for food. It therefore seems reasonable to suppose that we have also evolved behavioural defences to disease. Human ancestors with a heightened repulsion from faeces, saliva and parasites would have been healthier, and thus more likely to pass on their genes. As a result, the tendency to avoid such things would have spread, so that it is now common in humans everywhere. This behavioural drive is what we call the emotion of disgust.

An instinctive drive

So it may be that people avoid smelling, seeing or touching human excreta more by an instinctual drive for self-preservation than by a need to order and classify, to avoid pollution or because of a rational calculation that germs cause disease. We humans like to think we are logical, but we are driven more than we care to admit by a set of emotions that were shaped by the challenges faced by our ancestors, mammal and primate. Disgust is one of these drives. Disgust might turn out to be a prime candidate to help investigate the role that emotions and culture play in our lives.

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