Rising income inequality, and the resulting scarcity of certain types of jobs, is a key reason a growing number of young Americans are having babies before getting married.

A study led by Johns Hopkins University sociologist Andrew J. Cherlin is the first to trace how the income gap, a large-scale societal trend, affects individual personal choices about starting a family. The greater the income inequality in an area, the less likely young men and women are to marry before having a first child, according to the study, which is published online today and will appear in the August issue of the American Sociological Review.

"Does income inequality affect a young adult's decision about getting married and starting a family?" asked Cherlin, the Benjamin H. Griswold III Professor of Public Policy in the Krieger School of Arts and Sciences. "We think the answer is 'Yes' for those who don't graduate from college. Places with higher income inequality have fewer good jobs for those young adults. They don't foresee ever having the kinds of well-paying careers that could support a marriage and a family. But they are unwilling to forgo having children. So with good jobs in limited supply and successful marriage looking unlikely, young women and men without college degrees may go ahead and have a child without marrying first."

Cherlin and his fellow authors found that areas with high levels of income inequality have a shortage of jobs available in the middle of the job market. These are jobs available to those without college degrees that pay wages that would keep a family out of poverty—like positions for office clerks, factory workers, and security guards.

Without access to this sort of work, young men can't make an adequate living. They don't see themselves as good marriage material, and their partners agree. Couples like this might live together and have a child, but they are reluctant to make the long-term commitment to marriage, according to Cherlin.

The team studied 9,000 young people of the generation known as millennials, from 1997 when they were 12 to 16 years old, until 2011, when they were 26 to 31. By the end of the study, 53 percent of the women and 41 percent of the men reported having had at least one child—and 59 percent of those births occurred outside of marriage. Most of the first children born outside of marriage were to women and men who didn't graduate from college.

The researchers then matched that information about birth and marriage with census data on income and employment. They found that childless unmarried men and women who lived in counties with greater household income inequality and fewer middle market jobs available were less likely to marry before having a child. In fact, women who lived in an area with high inequality had 15 to 27 percent lower odds of marrying before having a first child than did women who lived in an area with low inequality.

"For many young adults, having a child is still one of the most satisfying experiences they can imagine. And if there's nothing else for a young person to look forward to, at least they can do that," Cherlin said. "They believe that being married is optional. But having a child is mandatory."