A few years ago, researchers published an eye-opening statistic: 57 percent of parents ages 26 to 31 were having kids outside of marriage. Who were these unwed Millennials and why were they forgoing the traditional structure of American family?

New research from sociologists at Johns Hopkins University and the University of Melbourne has started to answer that question. These aren’t a random assortment of Millennials but disproportionately come from a specific group of young Americans who don’t have college degrees, live in areas with high income inequality, and tend not to have very bright job prospects. The study, published this month in the journal American Sociological Review, found that in areas with the greatest income inequality, young men and women were more likely to have their first child before marriage. The areas with the largest income gaps also tended to have the fewest medium-skilled jobs, which researchers define as jobs that only require a high-school diploma but still enable families to live above the poverty level—jobs such as office clerks and security guards.