Here are two important, largely uncontested facts:

Family stability is important for childhood outcomes. All else equal, children raised in stable families are healthier, better educated, and more likely to avoid poverty than those who experience transitions in family structure.1

Married parents are more likely to stay together than cohabiting ones. In fact, two-thirds of cohabiting parents split up before their child reaches age 12, compared with one quarter of married parents:

Recent work by Brad Wilcox and Laurie DeRose, summarized here, shows that the stability gap between married and cohabiting parents can be seen in every country (even if the overall levels of stability differ quite considerably). It seems as if the old phrase “tying the knot” remains an appropriate one.

The real question now is not whether married parents are more likely to stay together, but why. Is it something about marriage per se, as Wilcox and DeRose suggest? Or is that the factors leading couples to stay together also lead to them to marry? This is not a semantic point. Understanding cause and effect is likely to be important when it comes to designing policy.

To understand what lies behind the “stability gap” between married and cohabiting parents, it is therefore useful to look at the other ways in which married and cohabiting couples differ, aside from marital status. In this paper, we examine three factors in particular—intendedness of childbearing, levels of education, and earnings—and show stark differences between cohabiting and married parents. Most married parents planned their pregnancy; most cohabiting couples did not. Married parents are also, on average, much better educated and earn much more than cohabiting parents.

Difference 1: Planning the baby

It is generally better for children if their parents intended to have them and plan to have them with their current partner. For one thing, parents are more likely to stay the course if they embark on it together deliberately: unintended parenthood is associated with a higher risk of union dissolution. Controlling for a variety of socioeconomic factors, Guzzo and Hayford find that, “relative to an intended birth, having an unintended or disagreed-upon birth increases the risk of dissolution.” Further, they find that “cohabiting unions are strongest and most likely to transition to marriage when the pregnancy was intended.”2

There are a number of reasons why an unintended pregnancy might be a prelude to a relationship breakdown. Following an unplanned birth, parents report greater conflict, lower levels of relationship happiness, and higher rates of depression compared with parents following the birth of a planned child. This is not a surprising finding; the very fact that a mother and father enter parenthood unintentionally might reflect poor communication or disagreement as well as a lack of foresight and self-efficacy.

Given the relationship between intended births and stable unions (no doubt with the causal arrow pointing both ways), it matters that rates of unintended childbearing among married and cohabiting parents are starkly different:

The rate of unintended births to cohabiting mothers is lower than for single parents, but still much higher than for those who are married. One in four births to married mothers are unintended, compared to one in two of those who are cohabiting. The definition of “unintended” here includes births that are described by the mother as either “unwanted” or “mistimed.” Within the “mistimed” category, a further distinction is made between births mistimed by more than two years, and those by less than two years.

There are then varying degrees to which a birth might be considered unintended. A baby coming a year earlier or later than planned is one thing; a baby being unwanted, or many years too early or late may be something else altogether. Compared to cohabiting mothers, wives reporting their birth as unintended are much more likely to say that it was mistimed, rather than unwanted; and if mistimed, to say that the mistiming was by less than two years:

It seems likely that the “unwanted” births to married couples (31 percent) are those that come too late, rather than too early, but we do not address this question in our analysis. What is clear is that not only are unintended births much less likely for married couples, but also that when they do occur, they are much more likely to be slightly mistimed (i.e., 2 years or less) than for cohabiting couples (43 percent vs. 17 percent).

The stark differences in the manner in which married and cohabiting couples become parents in the first place seems likely to explain a good deal of the stability gap between them. What Isabel Sawhill describes as “drifting” into parenthood does not set the stage for family stability. In his book, Our Kids: The American Dream in Crisis, Robert D. Putnam provides a rich descriptive portrait of these differences in the way in which many young Americans become parents, especially along class lines. Darleen, for instance, gets pregnant just months into a relationship with her boss at Pizza Hut. As reported in Putnam’s book, “It didn’t mean to happen. It just did. It was planned and kind of not planned.” David, after becoming a father at 18, acknowledges that, “It wasn’t planned. It just kind of happened.”

We don’t know whether Darleen and David succeeded in sustaining a relationship with the other parent of their child and creating a stable family environment. But given the nature of the start to their parenting journeys, it would be surprising.

Planning matters. Unplanned births lead to unstable families, planned births to more stable ones. Of course, marriage may still matter here. An unintended birth, even to the extent of being described as unwanted, may have less chance of derailing a couple who have made a lifelong commitment to each other. And for many couples, the decision to marry amounts to a decision about who they want to bear and raise children with. Cause and effect are, as always, hard to tease out here. But it is hard to imagine that the very large gaps in rates of unintended births are not related to the lower subsequent stability.

Difference 2: Most married parents have been to college, most cohabiting parents have not

There is a wide class gap in marriage in America. Marriage is more prevalent and more durable among better educated, higher income Americans. It should come as no surprise, then, to find an education gap between married and cohabiting parents. Married mothers and fathers are over four times more likely to hold a bachelor’s or advanced degree than cohabiting biological parents:

At the other end of the educational scale, most cohabiting biological parents have just a high school diploma or less, compared to a minority of married parents. The gaps are wider among fathers than mothers; two in three fathers cohabiting with the mother of their biological child have a high school diploma or less.

Some of this difference in educational attainment is likely to be explained by the age differences between married and cohabiting parents: the latter tend to be much younger than the former (this age gap is of course partly the mechanical result of the different rates of dissolution). Nonetheless, the gaps are striking, and relevant to the stability gap because education is an important, independent predictor of family stability.

Difference 3: Married parents earn more

Given that married parents better educated and older, it should come as no shock to learn that they are higher earners, too. Mothers and fathers who are married earn substantially more than all other types of family structures, with cohabiting biological parents earning the least:

The figure above depicts the median personal earnings of the individual mothers and fathers in each type of family structure. One of the advantages of both marriage and cohabitation is that two incomes can be pooled. But cohabiting couples have less income to pool. The earnings gap between fathers in different family types stands out particularly strongly. While married fathers earn $55,000 a year, men living with the mother of their child or children earn just $29,000. In fact, married fathers earn more on their own than the average cohabiting couple with a joint biological child earns between both parents ($51,000). Again, a big part of the story here is the age gap—married parents are older and thus more likely to be higher earners. But the earnings gap also reflects the education gap discussed above.

A higher family income predicts greater family stability, in part perhaps because of reduced financial stress. As Jessica Hardie and Amy Lucas note, “economic factors are an important predictor of conflict for both married and cohabiting couples…Economic hardship was associated with more conflict among married and cohabiting couples.” So, a final reason married parents are more likely to stay together may be their greater financial resources.

How, then, to promote stability?

There are stark differences between cohabiting and married parents in the degree to which they intend to become parents, as well as in their levels of education and earnings. In some ways, the fact that married couples are more likely to stay together must rank as one of the less surprising findings in social science.

Promoting marriage will not necessarily promote stability, though, even if such promotion is possible. Previous efforts at marriage promotion have been largely unsuccessful, as our colleague Ron Haskins shows. Perhaps other pro-marriage approaches would be more effective. Stronger messaging from political and civic leaders—“preaching what we practice,” to borrow Charles Murray’s phrase—might help. This kind of public advocacy was one of the recommendations in the recent Brookings/AEI report, Opportunity, Responsibility, and Security. Maybe more aggressive financial incentives to marry would raise marriage rates: the scholar Scott Winship has suggested a tax bonus for married parents of up to $4,000 per child, at a cost to the Federal government of between $60-$70 billion a year. Nobody knows.

Far better, then, to promote the ingredients of family stability, many of which are associated with marriage, and in particular intended childbearing, more education, and higher family incomes, rather than marriage itself. Boosting educational attainment, especially among young women, has a direct influence on their ability to start their families more successfully. Higher tax credits and higher minimum wages would boost incomes among cohabiting and single-parent homes.

Most importantly, reducing rates of unintended pregnancies and births would ensure that more parents were prepared for the responsibilities and rigors of parenthood. Only one in ten of the women using contraceptives used Long-Acting Reversible Contraceptives (LARCs) in 2012, and over half of unintended pregnancies result from women not using contraception at all.

The policy priority here is to improve access to and use of contraception, and especially the most effective form, LARCs. A number of approaches have been shown to work here, including lowering costs through health insurance reform (including the Affordable Care Act), improving training among providers, and running public information campaigns. At the national level, there is a danger that family planning policy is about to go into reverse, which would almost certainly mean more unintended pregnancies and more unplanned births, and therefore less family stability.

Stability: The end that matters

None of this is to say that marriage doesn’t matter, but simply that those factors beyond marriage need to be taken into account when crafting appropriate interventions to support stability and childhood outcomes. The message that stability matters is one that applies to families of all shapes and sizes, especially when marriage has failed to deliver it.

In his bestselling memoir Hillbilly Elegy, JD Vance recounts years of instability during his years of living with (and without) the different partners and husbands of his drug-addicted mother, with constant changes in his home and school. JD eventually found stability with his grandmother (Mamaw):

Now consider the sum of my life after I moved in with Mamaw permanently. At the end of tenth grade, I lived with Mamaw, in her house, with no one else. At the end of eleventh grade, I lived with Mamaw, in her house, with no one else. At the end of twelfth grade, I lived with Mamaw, in her house, with no one else…What I remember most is that I was happy—I no longer feared the school bell at the end of the day, I knew where I’d be living the next month, and no one’s romantic decisions affected my life. And out of that came the opportunities I’ve had for the past twelve years.

Finding this stability in his grandmother’s home, JD started to do better at school and in life—and was then able to move up the economic ladder through the U.S. Marine Corps and college. Critically, what provided the stability was the fact that “no one’s romantic decisions affected my life.” That’s also the hope and commitment of couples who marry before having children: they’ve made their lifetime romantic decision, and so can now provide a stable home for their children.

The greater stability of married parents compared to cohabiting parents likely results from a wide range of differences described in this paper—all of which may certainly improve the likelihood of marriage, be expressed through marriage, and even assisted by marriage—but which have little to do with marital status itself. If family stability is the end, getting cohabiting couples to marry is not the right means. Instead, we should foster the ingredients of stability—especially better family planning, more education, and higher incomes. It seems likely that these will turn out to encourage marriage too, since most Americans still want to raise their children within a marital union. But marriage here will be a byproduct of stability, rather than the other way around.

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