Economics professor Melissa Kearney, the study's co-author, wanted to explore how fracking booms have affected the share of babies born outside of marriage. Historically, bursts of prosperity among blue-collar men have reduced the share of kids born to unwed parents.

As the theory goes: Women have more drive to marry their child’s father if he can contribute to the household. They’d rather not tie the knot with an additional dependent.

Kearney checked on the romantic progress of men without college degrees in states at the center of the United States' oil and natural gas boom which have seen spikes in employment and wages. She looked at population data from swaths of Texas, Oklahoma, California and Pennsylvania.

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What she found surprised her.

“There was a different response this time, and it’s sobering,” she said.” The commitment to childbearing with marriage in the '70s and '80s is just no longer there.”

Kearney and her co-author, Riley Wilson, calculated that every $1,000 per capita increase in an area’s fracking production was linked to an additional six births per 1,000 women. About half of those extra babies, she said, were born to married parents.

In other words, more money seemed to bring more kids — regardless of the parents’ marital status.

This baby boom wasn’t as shocking to Kearney as the unofficial relationships. Babies, she explained, are viewed as “normal goods” — a demand that increases when income increases.

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In the past, however, baby booms from employment surges were restricted to married couples.

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During the 1970s and '80s, for example, surging energy prices during what was known as the Appalachian coal boom brought more work to miners. It fattened the wages of similarly skilled men in southern New York, Mississippi and Kentucky and shook up family formation in those areas. More women got married, and fewer women had kids on their own.

A 10 percent increase in earnings was tied to a 9.6 percent decrease in the share of unmarried women (ages 15 to 34) — and a whopping 25 percent reduction of children born to single moms.

Seth Sanders, a public policy professor at Duke University who studied Appalachia’s marital demographics, put it simply: Times have changed.

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“There has been growing acceptance of having children outside of marriage, especially in the white community,” he said. “Appalachia in the '70s and '80s was socially conservative with a high value to marriage.”

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Not that higher wages magically created stronger unions. Deciding to marry is an intimate choice, one that hinges on more than a partner’s wages.

Isabel Sawhill, an economist at the Brookings Institution and author of “Generation Unbound: Drifting into Sex and Parenthood Without Marriage,” said Kearney’s findings reflect a cultural transformation. Marriage, she said, is no longer seen as essential to a woman’s upward mobility. (Until 1974, for example, banks could require women to get a husband’s permission to apply for a credit card.

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“Women couldn’t be very choosy in the past — they had to be married for both social and economic reasons,” she said. “They’d be stigmatized if they weren’t and they might not be able to make it on their own. Now the social context has shifted. They can raise the bar.”

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In the United States, 40 percent of children are born to unmarried women, the most recent data from the Centers for Disease Control and Prevention show. Educational differences skew the share, however: Sixty-two percent of such kids have mothers who lack a college degree.

Academics have long wondered how to encourage marriage, because two parents tend to have more resources than one. A 2015 study from the American Enterprise Institute and the Institute for Family Studies found that states with higher concentrations of married parents also had higher median household incomes and lower rates of child poverty.

Researchers who focus on poor communities have found that women generally prioritize motherhood over marriage.

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“Women who don’t want to forgo being a mother will wait to find a reliable, steady partner — someone who can bring stability to the table,” the economist Kearney said, citing the workof Johns Hopkins sociologist Kathryn Edin.

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Thirty years ago, sociologist William Julius Wilson, who is now a professor at Harvard University, invented the term “marriageable male.”

In his groundbreaking book, “ The Truly Disadvantaged,” Wilson sought to explain why single motherhood was on the rise in predominately black communities and found that employed women were outnumbering employed men. That imbalance, he concluded, reduced women’s incentive to marry.

Five more studies have supported Wilson’s theory, according to a Brookings Institution report on the economics of eligible bachelors. Declining male employment, coupled with stagnating wages, can explain 27 percent of the drop in matrimony since 1980, the researchers wrote.

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Research from MIT’s David Autor suggests the marriageable male theory isn’t outdated everywhere. After Chinese imports started creating tougher competition for American products, employment dropped in the nation’s manufacturing hubs. Autor found these “trade shocks” in Rust Belt states led to a 5 percent drop in marriage among young women (18 to 25) and a 4 percent drop in the birthrate.