Why would anybody sing in public without being able to carry a tune? Or bore people senseless with unfunny jokes? Or, say, march on Moscow in the winter? Don't these people realize how stupid they are?

Having pondered this central question about their fellow human beings, psychologists David Dunning and Justin Kruger decided to investigate. Their results, published in the Journal of Personality and Social Psychology, bear out the obvious: Les incompétents have no clue.

Dunning, a professor of psychology at Cornell University, tested students there on a range of subjects, from logical reasoning to grammar to the ability to spot a funny joke. "Then," he says, "we compared how well people thought they did against how well they actually did. Overall, people overestimated themselves." And those who did worst were most likely to think they had outperformed everyone else. "Incompetent people," says Dunning, 40, the son of a machinist and a nurse, who grew up in Midland, Mich., "don't know they're incompetent. The skills that allow for competence allow for the recognition of competence."

So what's a dummy to do? "We don't necessarily know when we're about to make the wrong decision," says Dunning, "so we should ask for a second opinion." And to make sure the other person isn't just being polite, Dunning cautions, "Don't tell them your opinion first."

Of course, the obvious question is: How does Dunning know the study was competent? "It was published," he says. "It went through the wringer that scientific articles go through. But I'm waiting for someone to say, 'You made a mistake.' "