When I was a child there was a joke that went:
Q. "What's the definition of disgusting?"
A. "A tramp sitting on top of a compost heap, drinking diarrhoea through a sweaty sock."
Not particularly hilarious now, but at the time it never failed, although the resultant laugh was forced through a grimace; it was funny but it was also disgusting.
The psychology of disgust has a patchy history. Darwin considered it in his classic Expressions of the Emotions in Man and Animals and Freud used it as part of his theory of infant development (especially in relation to potty training), but it was largely ignored in mainstream psychology for most of the 20th Century. Recently, however, psychologists have started to reconsider this long-neglected emotion. In particular how disgust develops, its relationship to other emotions (such as the closely-related contempt) and why it evolved.
On the last point most researchers agree that disgust evolved as a mechanism to the prevent contamination by pathogens such as bacteria. And it is true that most of the things that elicit a digust response do tend to be things that normally present a risk of illness; bodily secretions, for example, or rotten meat. Although all cultures exhibit the disgust response (read more about this here) there is a large degree of variation in the objects of disgust. In some culutures insects and their larvae are considered delicacies (not just for celebrities on reality TV) and many Chinese find cheese disgusting. The reason for this is what is called the omnivore's dilemma. Unlike many other animals (for some reason giant pandas spring to mind) humans are capable of eating a wide range of foodstuffs. This flexibility in our diet bring great benefits (if one foodstuff is not available rather than starving we can switch to another) but it also carries a risk: how do we know that the new food item is not poisonous? One way is to fix to the diet according to what is available when we are young, this would prevent us from going off an eating something that might disagree with us. According to this theory, young children will try (pretty much) anything with diet becoming increasingly less flexible as we get older due to cultural learning.
A belief related to disgust is that of contagion. It is not necessary to actually touch a disgusting object in order to become contaminated by it, it can be had merely by touching something or someone who touched the object. This vicarous form of contamination is again illusrated by another childhood experience of mine. This time a "game" which probably didn't have a name but let's call it the "Jets" game.
The Jets game began when someone inadvertently (or intentionally!) touched a disgusting item: a dead bird, for example, or a dog poo. This "infected" person would then chase after the others attempting to touch them. Once contact had been made, the child that had been touched was "infected" and he or she would try to contaminate someone else and everyone else would try to avoid his or her touch. Interestingly once an infected person touched someone else they were no longer infected in sharp constrast to the truth of real contagion. If a child was tired of running, it was permissable for them to take a breather without fear of being infected, so long as they performed the following. (1) they crossed their arms in front of their chest -- Egyptian mummy-like -- moving them so that the tips of their fingers struck their shoulders repeatedly, (2) they repeated the word "jets". This thereby offered a form of immunity to infection for a limited time. The notion of contagion is again common throughout the world and was noted by Anthropologist James Frazer in The Golden Bough. Contagion is a powerful belief, so powerful that people will refuse to wear a (laundered) sweater that was reputed to have been worn by Adolf Hitler, and will baulk at the thought of drinking water than once contained a (dead and sterilized) cockroach.
As the joke at the top of this posting points out, disgust is closely related to humour, this is true for adults as well as children. The British comedy programme Little Britain relies on disgust for much of its humour. For example, the pensioner who spontanously and unknowingly urinates on the floor of whatever shop she happens to be in at the time, the psychiatric patient Anne who, among other things, licks her hand and wipes it down the faces of visitors and Maggie the WI woman who projectile vomits after (for example) tasting jam made by an Asian lady. Maggie is an interesting case because she bridges the gap between what is often called "core disgust" and more complex manifestation of the same (or at least similar) emotion.
We laugh at Maggie because her vomitting triggers our basic disgust response, but Maggie vomits because she is disgusted by ethnic minorities, homosexuals and other folk whom she considers "unsavoury" (not also that contagion plays a significant role, the jam she consumes has been contaminated by being made by someone she finds repellant). It has been suggested by Rozin and his colleague, Jonathan Haidt that the disgust mechanism is expanded from its initial domain of bodily products and rotten meat and used to invoke similar emotional responses towards people who are culturally and ethnically different from us. The paradigm case of this was the attitude of the Nazis towards the Jews in the first half of the last century, but this is by no means uncommon, as demonstrated by the Rwandan massacre, "ethnic cleansing" in the former Yugoslavia and continuing conflict in Darfur. The website Genocide Watch identifies eight steps to genocide the third of which, dehumanisation, is relevant to my point here it states:
"One group denies the humanity of the other group. Members of it are equated with animals, vermin, insects or diseases. Dehumanization overcomes the normal human revulsion against murder."
It can be seen that the powerful emotion of disgust (invoked by alliusions to vermin/disease, etc.) and the concomitant motivations to cleanse or purify can play a powerful role in ethnic conflict and genocide. The question is are these the very same processes that are at work? Is the part of the mind/brain responsible for maintaining the purity of the body the same part as that which is responsible for maintaining the purity of the culture/race? Or is it simply a metaphor? In Metaphors We Live By Lakoff and Johnson reveal a host of metaphors that we use to structure our world. Argument as war (I "attack your argument", you "defend your viewpoint"), time as money (spend time, waste time, save time) and various orientation metaphors ("up" is associated with happiness, consciousness and high status; down with sadness, unconsciousness, and low status) are so engrained in the way we think about the world that we hardly see them as metaphors but they are, nonetheless, only metaphors. No-one is arguing that the parts of the brain that engage when we think about real space are used in thinking about metaphorical spaces such as "waking up", "falling asleep" and so on.
I would also be interested to know why disgusting things can be so funny. The neuroscientist V.S Ramachandran suggests that we laugh as a signal to others that an event is inconsequential, a kind of "I'm OK" signal that might have evolved before language. This might explain why it is OK to laugh at people's misfortune, so long as they are OK at the end (as in the TV show You've been framed -- most of us don't find it funny if people really hurt themselves. If this is the case then disgusting things are funny when they are harmless, and the laughter signals this harmlessness to others. I'm not sure about this, but I don't know of a better explanation (actually I do have a rather different theory of humour which I will discuss in a future post).
So there we have it. Disgust is very serious (it protects us from contamination and seems to underpin xenophobic and racist behaviour) but it is also closely related to humour. BTW if anyone knows how you might suck diarrrhoea through a sock, sweaty or otherwise, keep it to yourself.