What's behind the current decline in marriage? New research suggests that single women's frequent complaint is actually true--there just aren't enough men worth marrying. At least not if single women require husbands whose education level and income matches or surpasses theirs.

In a fascinating blog post at the Psychology Today website, social psychologist Theresa DiDonato details new research that seeks to explain the phenomenon of declining marriage. In the 1950s, about 70 percent of Americans were married, compared with about 50 percent as of last year. This statistic is especially striking when you consider that same-sex marriage is now legal throughout the United States, removing a barrier to marriage for millions of people who would not have chosen to marry someone of the opposite sex. And, DiDonato notes, the percentage of people who say they have never been married has risen by 10 percent.

To find out why marriage is on the decline, researchers Daniel Lichter, Joseph Price, and Jeffrey Swigert used Census Bureau data to compare the husbands of married women with single men currently available on the dating market. They were, in essence, testing the validity of a frequently heard complaint from single women: All the good men are already taken.

The researchers began by comparing single women with married women of similar ages, demographics, and education levels. They looked at the husbands of these married women to try to determine the characteristics that might make a man marriageable in single women's eyes. Then they compared these theoretical husbands with the single men that the single women in their study might meet.

Less well educated, more likely to be unemployed.

Their findings can only be described as depressing. The available single men turned out to be less likely to have jobs than the husbands single women were presumably seeking. (Theoretical husbands had a 90 percent chance of being employed, whereas only 70 percent of available men were.) They were less likely to have a college degree. And the women appeared to be hoping for husbands with a 58 percent higher income than that of actual available men.

When the researchers analyzed the data further, matching single women's assumed desired qualities in a spouse against actual available men, they found even more disheartening news. Older women would have an especially hard time finding an acceptable mate. The same was true for minority women, especially if they were African American, and for highly educated women. And when the researchers added in geography, comparing a woman's theoretical desirable husband with the pool of available men in her region, the chances of finding a mate got even worse.

Or, at least, the chances of finding an "acceptable" mate. We don't actually know whether American women are holding out for more-likely-to-be-employed, better-educated, higher-earning men than are available on the dating market today. The researchers just constructed a "synthetic husband" they believe single women were seeking; they didn't actually ask any single women for their views. But if the researchers are right about what single women want in a husband, the statistics say many of them will be disappointed.

How will this play out? The researchers take a straightforward view: "This study reveals large deficits in the supply of potential male spouses. One implication is that the unmarried may remain unmarried or marry less-well‐suited partners."

Honestly, neither of those outcomes seems all that bad to me. In the 1950s, marriage was not only a matter of romance, but also a matter of economics. Because incomes back then were higher in relation to living expenses, more couples could afford to have one spouse--usually the mother--as a full-time parent. At the same time, career opportunities for most women were more limited than they are now.

I don't mean to suggest that raising children as a single parent is as easy as sharing parenting with a partner, or that women today earn as much as men do. In fact, research suggests that it will be 51 years until we reach gender pay parity in the U.S. Still, today's women have more choices for their careers, and for co-parenting, than women in the 1950s did. This may mean that staying unmarried isn't such a bad thing after all.

Is a husband who earns less really unsuitable?

And then there's the question of who is or isn't acceptable husband material. This feels personal to me, because my husband of 19 years is definitely what these researchers would call a "less well-suited partner." He has less formal education than I do, although he's certainly as well read and as smart. I've nearly always earned more than he does. Yet ours is one of the happiest marriages I know, and we aren't unique. We've encountered a lot of other happy marriages and partnerships in which the woman earns more than the man.

When we first got together, a well-meaning friend of mine tried hard to talk me out of the relationship precisely because of his limited economic prospects. At the time she was seemingly happily married to a man who earned more than she did. A few years later, that marriage imploded in an acrimonious divorce.

I don't claim to have all the answers about what makes a good marriage, but it does seem to me that basing a relationship even partly on economic expectations can be a bad idea, because things change. Industries shift, companies fail, and a spouse with a high-paying job could decide one day that he (or she) doesn't want to do that job anymore. That actually happened to the wife of a lawyer I know.