Back in the 1970s, it might have been raining men, but today there's a veritable drought -- at least for two groups of single ladies.

The question of whether America's women have enough marriageable men was taken on by Brookings Institution senior fellow Isabel Sawhill and former senior research assistant Joanna Venator. Their answer isn't all that reassuring for two distinct groups of women: college-educated women and black women.

The study, which used data from the Current Population Survey and the General Social Survey, found that recent trends in men's earnings and college graduation rates, as well as the high incarceration rate for black men, are to blame for an uneven balance between the genders in those two groups.

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Overall, men are not only earning less than they used to, but they're failing to enroll in colleges at the same rate as their female counterparts. The result is that there are only 85 men for every 100 women who are 25 to 35 years old and who are college educated.

That's the prime marrying age for Americans, given that men and women tend to be in their late 20s when they marry for the first time, according to the U.S. Census Bureau.

The report may back up what single women with college degrees have been sensing for years: The pickings are slim.

The changing demographics will have a wide-ranging impact on men and women, as well as on how they form -- or don't form -- families. Given the shortage of college-educated men, highly educated women are likely to either look for men who have fewer qualifications (and likely earn less) than them, or else skip marriage entirely, the researchers said.

"Women are now more educated than men, meaning that they will necessarily face a shortage of marriage partners with the same level of education," Sawhill and Venator wrote. "What we are likely to see in the future, then, is either women marrying 'down' educationally, or not marrying at all."

But that idea of "marrying down" embodies a fair amount of elitism that may be hurting both men and women, said Jon Birger, a journalist and the author of "Date-onomics: How Dating Became a Lopsided Numbers Game," published in August by Workman Publishing.

"The whole concept of describing marrying a working-class guy as marrying down is offensive," Birger said. "The notion that a man lacking a college degree is inferior I think is wrong. I believe it's inevitable that we'll have an increase in mixed-collar marriages, where college-educated women marry men who are not college-educated."

Online dating exacerbates the problem, said Birger, because people can check off if they want to see only profiles of potential dates with the same education level that they have. That's increasing "assortative mating" -- when people seek out partners who are similar to them -- at the same time that the dating market is more imbalanced as college-educated women now outnumber their male counterparts, he added.

"Based on my conversations with single female friends, a lot of men take advantage of the lopsided gender ratios," Birger said. "College-educated women who are unwilling to date noncollege-educated guys are giving college-educated men too much leverage in the dating market."

Eventually, Birger predicted, Americans will become more accepting of "mixed-collar" marriages, a trend he already sees in the black community, where black college-educated women are more comfortable marrying men without college degrees.

Not all women are facing the same odds, however. Each woman without children and who has a high school education actually has more than 2.5 men to pick from, the researchers found.

Black women are facing a dearth of marriageable black men because of the "very high rates of incarceration and early death among black men compared to white men," the paper noted. Even though interracial marriage is more common than in previous decades, most Americans tend to marry within their racial group, the researchers added.

The trend toward fewer marriages is already in evidence. About 20 percent of Americans over the age of 25 have never been married, compared with 9 percent a half-century ago, according to the Pew Research Center.

Still, women say education isn't as important for would-be husbands as some other traits, such as having similar ideas about raising children and a steady job, Pew found. Slightly more than one-quarter of women wanted their spouses to have the same level of education, while more than three-quarters wanted them to hold a steady job.

Sawhill, an economist who has been described by The Washington Post as "a longtime proponent of marriage," has argued in her essays and books such as "Generation Unbound," that the decline in marriage rates has a negative economic impact for families, as well as for children who are born to unmarried mothers.

Creating economic opportunities for men, especially those without college educations, should be a priority, such as by expanding technical education programs and reforming the criminal justice system, the researchers noted.