You've probably heard the one about heavy metal singer Ozzy Osbourne decapitating rats (or was it puppies?) during concerts in the 1980s.

Urban legends are fantastical stories that spread like syrup among urban and not-so-urban areas alike. But these anecdotes contradict sociologists' beliefs about the survival of stories—that they provide insightful social commentary.

Psychologists at Stanford and Duke universities had another theory. "We proposed that ideas are selected and retained in part based on their ability to tap emotions that are common across individuals," explains Chip Heath, Ph.D., an associate professor of organizational behavior at Stanford. Heath and his colleagues decided to examine anecdotes that inspire disgust (some 25 percent of urban legends fit the bill). They took 12 urban legends and presented undergraduates at Duke University with three increasingly revolting versions of each story. For example, someone develops vacation photos and discovers that their toothbrush has been photographed in close proximity to a hotel-worker's (a) fingernail, (b) armpit or (c) anus.

Amused undergrads consistently repeated the version that elicited the most disgust. "Emotion matters," says Heath. "It's not informational value alone that causes these things to succeed."

Many urban legends up the emotional ante by acting as cautionary tales. "There's the story of sadistic people booby-trapping Halloween candy," offers Heath, who published his findings in the Journal of Personality and Social Psychology. "That's classic emotional selection. It made people less able to trust their neighbors."