A note to single dudes: If you're looking to pick up a woman at a bar, whatever you do -- don't smile at her.

Women are actually less sexually attracted to smiley, happy men, suggests a new University of British Columbia study, published online today in the journal Emotion. If that's surprising to you -- it was surprising to lead researcher Jessica Tracy, too. "I wouldn’t have believed it if we didn’t go out and replicate it three times," says Tracy, an assistant psychology professor at UBC.

Researchers asked more than 1,000 volunteers to rate the sexual attractiveness of hundreds of images of the opposite sex. (All were heterosexual, ages 17 to 49 years, with a median age of 21. Fifty-two percent of participants were Asian, and 48 percent were Caucasian.) In the images, the men and women pictured were demonstrating one of three emotions: happiness, pride or shame -- plus a "neutral" image thrown in there, too. They found that women ranked the smiling guys as less attractive -- but they were into the prideful and ashamed men. But the male participants were most attracted to the smiling women, and least attracted to the ones who seemed proud.

More research is needed to determine why this might be, but Tracy has a few hunches. Past research has shown that smiling increases perception of femininity, so that might be one reason smiles worked on women, but not on men. Also, "smiling indicates availability, or interest. For men, that's a really important thing to know about a woman, so it makes sense that men would find smiling really attractive," Tracy says. "For women, that's not as important. There's the general assumption that men are more generally receptive."

But the fact that women find shame more attractive may also help explain the attractiveness of the "bad boy" -- the one who seems like he can be turned around. "The bad boy who feels shame, women have always found that attractive -- that’s the James Dean look. He’s the bad boy, but he wants to change," Tracy says.

One thing to keep in mind: The study measured just sexual attractiveness, not whether women are interested in carrying on a relationship with a sullen, unsmiling dude. But Tracy adds, "If a (man's) sole aim is to be as sexually attractive as possible, smiling may not be his best bet."

Follw Melissa Dahl on Twitter: @melissadahl.