Selection Explains Some—But Not All—of the Benefits of Marriage For Young Adults

There is abundant evidence that marriage is associated with a plethora of positive outcomes for both partners, including better physical and mental health. But skeptics rightfully question whether it is marriage itself that creates these positive outcomes, or whether it is simply that people who are selected as marriage partners are physically and mentally healthier than people who are not selected. 

One way to find out if the positive effects of marriage are causal is as follows: imagine you have two identical twins who are alike in all possible ways—genetically (they share 100 percent of their genes) and in terms of their family environment growing up. This means that the twins are likely to be physically and mentally very similar. One twin marries, the other does not, and the married twin is found to be in better physical and mental health than the single twin. This would be powerful evidence that it is marriage in and of itself that causes better physical and mental health, and it is not just the case that physically and mentally healthy individuals are more likely to marry.

A 2013 study by Erin Horn and coauthors performs an analysis analogous to this with a sample of identical twins and non-identical siblings (with an average age of 29) from the National Longitudinal Survey of Adolescent Health (Add Health). This study is the first genetically-informed study of the benefits of marriage in young adults. In their analysis, the authors were able to separate the effects of the siblings’ genetic makeup, their shared family environment, and their non-shared environment (in this case marriage or cohabitation) on various measures of physical and mental health. They also adjusted for factors such as age, sex, and ethnicity. They were thus able to estimate the effect of marriage/cohabitation alone on outcomes, separate of the effects of an individual’s genetic predispositions and family background on selection into marriage/cohabitation. To further isolate the specific effects of marriage as distinct from the effects of cohabitation, they also examined outcomes for married individuals versus outcomes for cohabiting individuals.

The researchers found that the relationship between physical health and cohabitation or marriage for young adults was completely explained by selection effects—that is, physically healthier people are more likely to cohabit or marry than less physically healthy people. But they found direct positive effects of marriage or cohabitation on mental health outcomes independent of selection on genes and shared family environment. Both marriage and cohabitation were associated with better mental health outcomes: less self-reported depression, lower risk of suicide ideation, and less alcohol use, and this is not just because people with less depression, lower risk of suicide ideation, and less alcohol use are more likely to cohabit or marry. For example, the single twin in a pair of identical twins had average depression scores that were 0.13 standard deviations higher than their married or cohabiting co-twin, and they were four times likely to report suicidal ideation. After taking into account the effects of genetic predispositions and family background on outcomes, they found no difference between singles and cohabiting and married young adults on the risk of antisocial behavior (participation in criminal acts) and cigarettes smoked per day. That is, any difference between singles and cohabiting/married individuals on these factors is due to differential selection into marriage/cohabitation (i.e., people who engage in antisocial behavior and/or smoke are less likely to cohabit or marry).

To separate out the effects of marriage as distinct from cohabitation, the researchers compared outcomes for those who were married versus those who were cohabitating. They found no physical health differences between the married and the cohabiting. Compared to cohabiting individuals, when the effects of genes and shared family environment were adjusted, marriage was associated with a lower risk of antisocial behaviors. For example, the cohabiting twin in a pair of identical twins was 67% more likely to report engaging in antisocial or criminal behavior than their married co-twin. Thus, selection effects can fully explain the lower rates of antisocial behavior for cohabitors but not for the married, and marriage itself encourages lower rates of antisocial behavior.

In sum, Horn and coauthors found the physical health benefits of married and cohabiting among young adults can be explained by the fact that physically healthier people are more likely to marry or cohabit. They also found separate causal, positive effects of both marriage and cohabitation on mental health outcomes independent of selection into marriage and cohabitation. That is, married people and cohabitors have better mental health outcomes not simply because people with better mental health are more likely to marry or cohabit. The authors also found a direct causal effect of marriage in lowering the risk of antisocial behavior, meaning that married people are less likely than singles or cohabitors to exhibit antisocial behavior and not only because people who are low in risk of antisocial behavior are more likely to marry. This analysis thus provides good evidence that while some of the positive effects of marriage on mental health are due to selection into marriage and others are simply due to the positive effects of having a partner, there are also positive benefits from marriage in and of itself.  

Rosemary L. Hopcroft is Professor Emerita of Sociology at the University of North Carolina at Charlotte. She is the author of Evolution and Gender: Why it matters for contemporary life (Routledge 2016), editor of The Oxford Handbook of Evolution, Biology, & Society (Oxford, 2018), and author (with Martin Fieder and Susanne Huber) of Not So Weird After All: The Changing Relationship Between Status and Fertility (Routledge, 2024).