Not so long ago, the average American man in his 20s had achieved most of the milestones of adulthood: a high-school diploma, financial independence, marriage and children. Today, most men in their 20s hang out in a novel sort of limbo, a hybrid state of semi-hormonal adolescence and responsible self-reliance. This "pre-adulthood" has much to recommend it, especially for the college-educated. But it's time to state what has become obvious to legions of frustrated young women: It doesn't bring out the best in men.

But for all its familiarity, pre-adulthood represents a momentous sociological development. It's no exaggeration to say that having large numbers of single young men and women living independently, while also having enough disposable income to avoid ever messing up their kitchens, is something entirely new in human experience. Yes, at other points in Western history young people have waited well into their 20s to marry, and yes, office girls and bachelor lawyers have been working and finding amusement in cities for more than a century. But their numbers and their money supply were always relatively small. Today's pre-adults are a different matter. They are a major demographic event.

What also makes pre-adulthood something new is its radical reversal of the sexual hierarchy. Among pre-adults, women are the first sex. They graduate from college in greater numbers (among Americans ages 25 to 34, 34% of women now have a bachelor's degree but just 27% of men), and they have higher GPAs. As most professors tell it, they also have more confidence and drive. These strengths carry women through their 20s, when they are more likely than men to be in grad school and making strides in the workplace. In a number of cities, they are even out-earning their brothers and boyfriends.