Kevin, a 24-year-old recent college graduate from Denver, wants to get married someday and is "almost 100% positive" that he will. But not soon, he says, "because I am not done being stupid yet. I still want to go out and have sex with a million girls." He believes that he's figured out how to do that:

Girls are easier to mislead than guys just by lying or just not really caring. If you know what girls want, then you know you should not give that to them until the proper time. If you do that strategically, then you can really have anything you want … whether it's a relationship, sex, or whatever. You have the control.

Kevin was one of 100 men and women, from a cross-section of American communities that my team and I interviewed five years ago as we sought to understand how adults in their 20s and early 30s think about their relationships. He sounds like a jerk. But it's hard to convince him that his strategy won't work—because it has, for him and countless other men.

People like Kevin expect to make the transition from this selfish outlook to a committed relationship, but it isn't that easy. Psychologist Scott Stanley of the University of Denver sees visible daily sacrifices, such as accepting inconveniences in order to see a woman, as the way that men typically show their developing commitment. It signals the expectation of a future together. Such small instances of self-sacrificing love may sound simple, but they are less likely to develop when past and present relationships are founded on the expectation of cheap sex