Social disgust operates much the same way, according to Haidt: "If physical disgust is about distinguishing ourselves from animals, then social disgust is about distinguishing ourselves from `demons.' `Human being' is a charged category, and we want to keep its boundaries clearly defined. Someone who cheats on his taxes can be human; someone who eats human flesh cannot. Socially disgusting acts are those that reveal that you have inhuman motives."
The reason such reminders of our "animality" are so harrowing may be equally uncomplicated: any reminder of our animal nature is also a reminder of our own mortality. Certainly, we can coolly discuss death and even come to terms with it; indeed, the knowledge of life's precariousness is singularly human. But it is also the most crucial threat to the psyche, and as such must be repressed. No wonder so much of what we find disgusting relates to death and illness: blood, boils, amputations, and mutilations suggest the fragility of life; corpses and body parts simply verify it.
This theory neatly ties in with related research. People who are disgust sensitive also tend to display a greater than average fear of death. Conversely, people who are tough to gross out are likely to score high on measures of "thrill-seeking." Haidt, McCauley, and Rozin have discovered that in general women have higher disgust sensitivity than men and tend to score lower on "thrill-seeking" tests. Another correlation involves neuroticism and psychoticism: individuals scoring high on tests for the former tend to be more disgust sensitive than normal; those scoring high in psychoticism less than normal. The probable explanation: neurotics, who tend to be oversocialized, are likely to be very concerned with social customs, traditions, and the like--things distinctly human. A genuine sociopath couldn't care less. Researchers have also discovered a correlation among disgust "domains"--people who are grossed out by holding a dead cat also tend to be very unhappy to find a B.M. left unflushed in a public toilet.
Surprisingly little is known, however, about why such things repulse some people more than others. It does appear that sensitivity to disgust runs in families, says Rozin. "Food preferences are not well transmitted in American families--if parents like broccoli, that doesn't tell you much about their kids. But if the parents are disgust sensitive, that does tend to get passed on."
Brain physiology may also play a role. A team of British and German scientists recently reported that patients with Huntington's disease show a remarkable inability to recognize expressions of disgust, yet can identify other expressions with ease. Huntington's, an inherited disorder that impairs muscle movement, ravages a brain region called the basal ganglia--one of two brain areas activated when healthy people see expressions of disgust. The other, the anterior insula, also plays a role in taste, hinting at disgust's origins in taste rejection. "Who would have expected dedicated brain areas for [recognizing] disgust faces?" asks Rozin.
But there's more. A third part of the puzzle--which Rozin finds most surprising of all--is that the ability to identify expressions of disgust is also impaired in people with obsessive-compulsive disorder (OCD). Given that OCD patients are often obsessed with cleanliness, however, one might expect them to be more sensitive to disgust, not less so. Rozin admits that it's not clear what to make of these unexpected findings--but he suggests that they may be precursors to breakthroughs in how we conceptualize disgust.
UNANSWERED QUESTIONS
The brain's role in disgust isn't the only area ripe for exploration. Rozin, for one, is eager to discover whether a deadening of disgust in one domain also extends to others: "Hospital orderlies, for example, must deal with body waste and death on a regular basis. Does that affect their sensitivity towards food or interpersonal disgust? We don't know." He also remains intrigued by the abilities of people to suspend or frame their sense of disgust, depending on the context. "The smell of decay coming from a house in France could either be ripe cheese or a lavatory. Whichever you think it is will make a big difference in how you react to the odor."
Perhaps the greatest area remaining to be explored is the cross-cultural aspect of disgust. "Certain cultures, like India, have muted core and animal origin disgusts and exaggerated moral and interpersonal disgust," Rozin notes. This means the focus of disgust is on social relations, such as those between members of different castes; protecting the body from "core" disgusts is of less concern. (In the 1970s, a prime minister of India became infamous in the West for waking up each morning to a glass of his own urine.) The example of India suggests to Rozin that the central role assigned to fear of animal origins in the study of disgust will have to be revisited. "Remember, if you believe in reincarnation, then animal origins and death itself don't hay. the same status."
But while value-free disgust might be the explicit social and political aim of our society, we also clearly do still moralize about what we consider disgusting or off-putting--generally areas of personal hygiene--and, in some areas, like obesity and smoking, we have actually begun--in a somewhat medieval way--to re-link disgust to morality.
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