Americans are both privacy-mad, yet blithely self-revealing online. We post all kinds of embarrassing information about ourselves, but periodically erupt in a rage when we feel that our online privacy has been compromised. When do we care about privacy, and when don't we?

A team of researchers from Carnegie Mellon University has found that people's decisions to reveal information about themselves online can depend on unexpected and even irrational cues. In one experiment, college students filled out Web surveys about various questionable activities, like cocaine use and drunken driving.

When the Web page had a crisp, official look, used the university seal, and bore the name of a fictitious university body, the students were sparing with their admissions.

But when the Web site was colorful and blurry, and the survey was introduced by a red devil's head and the words "How BAD Are U???", the students were far more likely to say they had taken part in nefarious activities. The students, however, conceded that the unprofessional Web site seemed to be a far less safe place to give revelatory information.

"The little devil face sort of winks at you, suggesting that it's O.K. to do bad stuff," said George Loewenstein, a professor of economics and psychology at Carnegie Mellon and one of the paper's authors. "All of a sudden, people feel that it's O.K. to divulge all kinds of personal and incriminating information."

First Published: August 30, 2010, 6:00 a.m.