Blame the drop in marriage rates on declining American values, and you may be ignoring what often provides the push for couples to walk down the aisle: education.

Young adults with only a high school diploma aren't earning as much as their parents and grandparents.

That's what Andrew Cherlin, a Johns Hopkins sociologist, argues in his new book Labor's Love Lost. Cherlin told The Huffington Post that the move away from marriage has to do with the fact that those without a four-year college education face fewer job options and less financial stability, which causes them to delay marriage or eschew it altogether. A disregard of morals isn't the culprit here.

And it's not a subconscious choice: 34 percent of young adults cited financial security as the reason they weren't currently married, according to a recent Pew Research Center poll.

Between 1979 and 2007, young male high school graduates saw their real earnings fall by about 30 percent. Unlike their mid-century predecessors, men in this moderately educated group can't expect to be as financially stable as their parents or grandparents, so they end up putting off marriage or avoiding it completely.

Without financial security, couples are taking increasingly complex relationship paths in lieu of marriage.

In 2013, marriage rates hit an unprecedented low, bottoming out at 50.3 percent. Compare that to 1950, when 82 percent of non-widowed women between the ages of 18 and 64 were married, and it's hard to ignore the shift.

Cherlin ties this change to the "slow-motion collapse" of what we used to call the "working class" or "blue collar" family. Forty or 50 years ago, these families took on mid-level jobs that didn't require higher schooling, so men were able to make enough to support a wife and kids. Many of those jobs have been lost with modern technology. (It's worth noting that mid-century women weren't given opportunities to be household breadwinners and often relied on husbands for economic stability.)

Nowadays, young men and women without college degrees are facing dimmer job prospects than ever, so they're avoiding marriage. Without a stable income, it's not advantageous for either partner -- man or woman -- to enter into a seemingly permanent union.

Closing the marriage gap may involve putting more folks without college degrees on the path to stable, mid-level jobs.

"More and more of the moderately educated young adults, those without college degrees, are having kids in living-together relationships, often ending those relationships after a couple of years, starting another one, perhaps having a kid in another relationship and building unstable, complex families," he said.

Of course, this marriage gap wouldn't have happened if we didn't shift our norms since mid-century America. It's now more acceptable and common for couples to cohabit and have children before marriage. But the big difference between college grads and those without a degree is that cohabiting college grads tend to be more economically stable and make it to the altar before they have children, while the less educated are having children before marriage and then breaking up.

It's this "interaction effect between a changing economy and changing attitudes," as Cherlin put it, that he argues has led to the marriage gap.

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The remedies? Cherlin said that more education is needed -- not in the form of four-year college degrees, but vocational education at community colleges that can put young adults on the path to entering the few mid-level jobs that are left. He also said that labor market interventions, like strengthening unions and increasing the minimum wage, might bring the moderately-educated closer to their college graduate counterparts when it comes to marriage.

"As far as I can tell, a college degree is really the closest thing we have to a class boundary in the US," Cherlin said. "Certainly in terms of family life."