The U.S. is seeing a steady rise in children born to unmarried, cohabiting couples—a trend that reflects a societal shift away from single parenthood but whose economic implications may not be much better.

Just over a quarter of births to women of child-bearing age—defined here as 15 to 44 years old—in the past five years were to cohabiting couples, the highest on record and nearly double the rate from a decade earlier, according to new data from the Centers for Disease Control and Prevention for 2011 to 2013.

By contrast, the share of births involving women who are unmarried and not cohabiting fell slightly over the same period, data from the CDC’s National Survey of Family Growth show.

The result: Cohabiting parents now account for a clear majority—59%—of all births outside marriage, according to estimates by Sally Curtin, a CDC demographer. In all, 40% of the 3.93 million births in 2013 were to unmarried women.

The figures reframe the stereotype of a single mother caring for kids on her own, yet fuel concerns among sociologists that more children are losing out on the significant economic benefits of the stable households long associated with married parents.

“In terms of instability and uncertainty for kids,” said Columbia University social-work professor Jane Waldfogel, cohabitation “is in some ways closer to single parenthood than it is to marriage.”

Several factors are behind the rise in cohabiting parents.

Earnings of less-educated American men have fallen in recent decades, while education levels have risen among women, making marriage less attractive economically for women, research shows.

And while the vast majority of Americans say they eventually want to marry, according to a recent Gallup survey, many are finding the financial security they perceive as a prerequisite elusive.

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