The rise of unmarried, cohabiting parents is "the largest unrecognized threat" to the stability and well-being of children in the U.S., according to a pro-marriage group's new report, co-authored by University of Minnesota professor William Doherty.

Released Tuesday by the Institute for American Values, the report highlights the fact that American children are now more likely to live with unmarried, cohabiting couples by age 12 than to see their parents divorce by that age. In fact, the parent divorce rate in America has returned to levels it hasn't seen since the 1960s.

In many ways, the report found, children living with two unmarried parents are as likely to experience problems as children living with single parents. While children living with unmarried, cohabiting parents are more likely to be secure economically, they are equally as likely to abuse drugs and have behavior problems as children living with single parents, the report stated. And in the case of physical abuse, federal research has found that children are much more likely to be harmed if they are living with one biological parent and a cohabiting partner than if they are living with married parents, or two unmarried biological parents, or even single parents.

Children do tend to experience stability when they live with unmarried biological parents who don't split up, said the report's lead author, Brad Wilcox of the University of Virginia. Trouble is, the likelihood that unmarried parents split up in the U.S. is 170 percent greater than it is with married parents, the report stated.

It's possible that today's cohabiting parents were yesterday's married couples who ended up getting divorced. If that is true, then it would seem their children were doomed to higher risks of social and developmental problems anyway. But Wilcox argued that marriage can be stabilizing even for at-risk couples. He cited research showing that parents -- fathers in particular -- work harder, drink at bars less and take other steps toward responsibility after they are married.

"Marriage is a stabilizing institution," Wilcox said.

(Doherty, the U of M coauthor, was unavailable Tuesday morning. He is a U of M professor in the Department of Family Social Science and author of several books on marriage and family relationships).

An executive summary of the report is here. The full report is for sale on the institute's web site.