Put a cockroach in a beverage, and most people will refuse to drink it--even after the cockroach is removed. It's not surprising, perhaps, that this is the trend uncovered in the research of psychology professor Paul Rozin, PhD, of the University of Pennsylvania, an expert in food choice and disgust. But what underlies such universal reactions of revulsion?

At their core, Rozin explained at APA's 2003 Annual Convention, those reactions evolved from instinctual bad-taste reflexes to a more abstract culture-molded emotion. In other words, disgust shifted from a reaction to avoid bodily harm to one that wards off harm to the soul, Rozin said. However, the basic mechanisms of disgust--the physiology (e.g., nausea), behavior (e.g., withdrawal) and facial and body expressions (e.g., a gaped lower jaw or extension of the tongue)--have remained mostly constant in human history, Rozin pointed out.

In the last 15 years, scientists have devoted more time to studying disgust. Unlike the other basic emotions, disgust has no obvious pathology, as fear and anger do, Rozin noted. But some people are more disgust-sensitive than others, such as those with obsessive compulsive disorder.

Disgust research has also grown in popularity since researchers have revealed--through brain-imaging studies--that certain parts of the brain are activated when people are disgusted. Researchers have also found that people with Huntington's disease--or those who carry its gene--cannot recognize the typical facial expressions associated with disgust.

"This is fascinating because it allows us to study what it is like to be socialized in a world where we cannot recognize this powerful signal that is so important to making us a civilized person," Rozin said.

How disgusting

To test his theories of disgust, Rozin and his colleagues have tried the dead-cockroach-in-drink test with children and adults. Most adults refused the drink even after the cockroach's removal because the juice--in their eyes--was contaminated, Rozin said. The same results held true when people were asked to drink apple juice from a bed pan.

This raises a problem for most people because much of what we encounter in the world contains elements that can spur disgust, Rozin said.

How, then, do we get through life? "We handle this by framing--where we don't think about the contaminating history of an object unless we are explicitly reminded of it," Rozin explained. For example, some people kiss their dog and do not think anything of it, unless they first see the dog drinking out of the toilet bowl, Rozin noted.

Children, on the other hand, lack this sense of contamination, and if offered juice containing a cockroach, will often drink it, Rozin said. While disgust--in a distaste sense--is present at birth, its more emotional aspects are acquired culturally as children learn what foods are dangerous and what foods make them sick.

But to understand how disgust really functions, Rozin said psychologists must first understand an evolutionary, biological concept based on Ernst Mayr's idea of preadaptation, in which something evolves for one purpose and is used later for another. For example, the mouth evolved as an eating organ but later became involved in speech. Similarly, disgust evolved into an emotion that can be triggered by interpersonal and moral events, not just physical ones, Rozin said.

For instance, people in the United States often do not remove their shoes when walking into a house. However, in Japan, people would consider it disgusting to bring dirt from the outside world into their house, he said.

Rozin and his colleagues have found that some of the biggest triggers of disgust are dead bodies, poor hygiene and inappropriate sex, probably because they all remind people of their animal nature, Rozin said.

"What has happened in the human cultural evolution is that a whole bunch of meanings and changes have been piled on to what elicits disgust," he said. "The system that has been originally used as a form of food rejection has come to be used in general for the process of socialization. We use the system 'get this out of my mouth' for all kinds of purposes now."