Glenn Reynolds linked to this http://www.livescience.com/20294-women-choose-bad-boys.html
I'm not saying the research is wrong (although much, if not most, academic research is flawed). But the biggest reason women want bad boys is no great secret. Women will say they want the nice guy, the sensitive guy, the guy who cries at movies and calls his mom regularly. Then they'll pass on the nice guy and chase the bad boy.
It's all about the fantasy. Deep down, women want to think that their beauty, sex appeal and overall desirability will be just what it takes to entice the bad boy to give up his bad ways and allow her to domesticate him. She alone possesses the female essence that will tame the wild beast.
Of course, she's wrong. But that's why girls are girls and boys are boys; why girls are complicated and impossible to understand; why boys are as simple as an off/on switch.