One finding in the Knot Yet marriage report in particular seems to hit a nerve with readers, and that is that raising a child in a cohabiting relationship is harder on a child than raising him or her in a marriage.

Before people start firing off an email or tweet, let me just say that the report—and ample research before it—is talking about averages. There are always exceptions to the rule in which a clearly committed couple has opted not to marry and is raising a child in a long-term, loving relationship. That child will no doubt thrive. In fact, long-term cohabitation in a good relationship shows few serious consequences for children, according to recent research by scholars at the Center for Family and Demographic Research at Bowling Green University.

But unfortunately, the typical cohabiting couple in the United States doesn’t make it past year five. For a variety of reasons—money problems, the stress of raising kids—the relationship begins to fray. As Heather, a single mom who has been on her own since she had her son at age 18 and recently married at age 31, told me, “Kids are very hard on any relationship. If you’re not with someone willing to make a commitment, then the chances of that relationship being able to withstand the stress is pretty slim. If a guy can just get up and walk away and go somewhere without the stress, he probably will.”

So maybe the guy moves out, or the mom decides it’s just not worth the fighting anymore and leaves. (And it should be noted, living through a volatile relationship is not good for children either). She and her child might live on their own for awhile, but now on a single income, which increases the risk for poverty. Approximately 32% of single-mother families were in poverty in 2010. And poverty has a long reach in a child’s life. (As a comparison, the poverty rate nationally was 15.1%).

Mom might fall in love again, as human beings are wont to do. She might move in together, hoping that this time will be different, that this one might last. And it might. But it also might not.

Cohabiting relationships in the U.S. are still a shaky prospect. Unlike in Europe, where cohabitation is, in some countries, replacing marriage as an equally stable relationship, we’re not yet there. Andrew Cherlin, an authority on American family life, finds in his book, The Marriage Go-Round, that family life in the U.S. churns considerably more than elsewhere, either through divorce, remarriage, stints of living alone, or cohabitation. And for cohabitation, the United States has the shortest duration of any Western country.

That churn can leave a lasting impact on children. Evidence shows that children in cohabiting relationships have more behavioral problems and cognitive problems than those in married-couple families. Here’s a good overview of the research on cohabitation and child well-being.

Ronald Bulanda and Wendy Manning in a 2008 journal article looked down the road to the child’s teen years and found that “that children born to cohabiting parents initiate sex at an earlier age and are more likely to have a teenage birth than children born to married parents.” However, in this case, it was not instability that contributed to the poorer outcomes. The authors offers some other possibilities, including less monitoring by parents.  The working paper version of that article is here.

Indeed, the big question in the research community is whether these poorer outcomes are because of the instability of the relationships or their typically poorer quality.

Another big issue further complicating matters is what researchers call “selection bias.” It could very well be that couples who marry might be more put together than couples who don’t and these qualities might be the reasons for the child outcomes, not the family structure itself.

That is, children fare best in married families, on average, because as researcher Susan Brown, in reviewing the research, notes, “adults who form and maintain such families are the most stable, well-adjusted, resource-rich individuals.” Likewise for marriage itself: Marriage may not really make people happier, healthier, and more financially secure. Instead, happy, healthy, secure individuals are more likely to marry in the first place.

But for now, what the research seems to point to is that cohabitation is as yet an unstable relationship on average. And kids like stability, whether in living arrangements or waking up to the same safe, familiar faces each day.

As Heather, the former single mom said, “I truly believe people should live together before being married. You have to get used to the other person’s quirks and differences. People who have lived together understand each other better before taking the leap. But, I don’t agree with people bringing kids into the situation because there’s always the possibility that it’s not going to work. I think for children, marriage is better,” she says. “It’s the security and stability that it brings. If someone is willing to make that commitment it tells the woman that it’s security in that situation.”

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