It's not a new theory: As women progress in educational and professional opportunities, their odds of finding a committed man appear to go down. Women in their 40s and 50s have long heard this, but new research finds it's true for women just entering adulthood as well.

That's one of the findings in the new book "Premarital Sex in America: How Young Americans Meet, Mate and Think About Marrying," by researchers Mark Regnerus and Jeremy Uecker at the University of Texas at Austin.

They looked at the results from a number of national studies including the National Longitudinal Study of Adolescent Health and the National Study of Youth and Religion, in addition to interviews with young people ages 18 to 23.

Researchers found that since women in the 18- to 23-year-old group feel they don't need men for financial dependence, many of them feel they can play around with multiple partners without consequence, and that the early 20s isn't the time to have a serious relationship. But eventually, they do come to want a real, lasting relationship. The problem is that there will still be women who will have sex readily without commitment, and since men know this, fewer of them are willing to go steady.

"Women have plenty of freedom, but freedom does not translate easily into getting what you want," Regnerus said.

The wide availability of pornography has also influenced the dynamics of relations between men and women, Regnerus said. A segment of 20-something men are content to have their sexual experiences by themselves, removing them from the pool of available partners. That means high-quality men - likely those who want monogamous, committed relationships - are still eligible for dating, but the overall dating pool has shrunk, meaning some women will be left unsatisfactorily single.

Researchers also found that marrying at or before age 20 constitutes the greatest risk for subsequent divorce, the data show. Early marriage doesn’t cause the divorce, but the partners are likely to be unprepared for the kinds of adjustments required, Regnerus said.

And here's perhaps some good news: Sexual behavior among this age group is less salacious than you might think. The "hookup culture" is most prominent when there is a Greek system present; otherwise, college students seem more inclined toward stable relations and have fewer sexual partners.

In case you were wondering, 16% of 18- to 23-year-olds are virgins, according to the surveys used in the book. In that age group there are more men than women who have never had sex. By age 27, the portion of virgins goes down to 8%.