Nicholas D. Kristof in the New York Times:
Some evolutionary psychologists believe that disgust emerged as a protective mechanism against health risks, like feces, spoiled food or corpses. Later, many societies came to apply the same emotion to social “threats.” Humans appear to be the only species that registers disgust, which is why a dog will wag its tail in puzzlement when its horrified owner yanks it back from eating excrement.
Psychologists have developed a “disgust scale” based on how queasy people would be in 27 situations, such as stepping barefoot on an earthworm or smelling urine in a tunnel. Conservatives systematically register more disgust than liberals. (To see how you weigh factors in moral decisions, take the tests at www.yourmorals.org.)
It appears that we start with moral intuitions that our brains then find evidence to support. For example, one experiment involved hypnotizing subjects to expect a flash of disgust at the word “take.” They were then told about Dan, a student council president who “tries to take topics that appeal to both professors and students.”
The research subjects felt disgust but couldn’t find any good reason for it. So, in some cases, they concocted their own reasons, such as: “Dan is a popularity-seeking snob.”
More here. [Take the tests, they are interesting.]