He's brash, cocky and non-conformist, the cad everybody says is bad news, but women love him anyway. What's up with the enduring sex appeal of the proverbial "bad boy" image?

A new University of British Columbia study may help shed some light. It's confirmed the cliché that women dig bad boys and that nice guys really do finish last.

More than 1,000 study participants were asked to rate the sexual attractiveness of photos of the opposite sex showing a variety of expressions from happiness to confidence to moodiness.

Women found swaggering and brooding men significantly more sexually attractive than happy, smiling guys.

The opposite was true for men: Male participants were most sexually attracted to women who looked happy, and least attracted to women who appeared proud and confident.

"We were not asking participants if they thought these targets would make a good boyfriend or wife -- we wanted their gut reactions on carnal, sexual attraction," says study co-author Alec Beall.

But why do women love bad boys, while men prefer happy women? Evolutionary theorists suggest females are attracted to male displays of pride because they imply power, competence and an ability to provide for his family. Smiling, on the other hand, is associated with a lack of dominance -- a trait consistent with traditional gender norms of the "submissive and vulnerable" woman.