The most incisive guide to issues facing the American family today . . . An invaluable resource for anyone wishing to stay on the cutting edge of research on family trends.

-W. Bradford Wilcox
Associate Professor of Sociology, University of Virginia 

The Marriage Advantage—The Test of Twins

Social scientists have known for decades that married men and women enjoy a decided advantage over unmarried peers in physical and psychological well-being.  But some analysts have seen nothing in this apparent advantage but a “selection effect.”  That is, these analysts have argued that healthier individuals are simply more likely to marry than are unhealthy individuals.  Hence, they reason, it is a mistake to suppose that wedlock itself confers any physical or psychological benefits.

However, psychologists from the University of Virginia have now used data collected from identical twins and other siblings to test the selection-effect explanation of the marriage advantage.  And these researchers conclude that while the selection effect may account for the marriage advantage in physical health, it cannot account for the marriage advantage in psychological well-being.

The Virginia scholars begin their study acknowledging the difficulty of investigating “the social causation hypothesis [concerning the marriage advantage] . . . because it is practically and ethically impossible to use experimental methods to study the effects of marriage (i.e., we cannot randomly assign couples to marry or divorce).”  However, the researchers believe they can surmount this difficulty by using a “genetically informed research design” based on data collected from twins and siblings.  Such a design gives “traditional correlational studies an additional layer of control for parsing selection from causation.”  “By using sibling pairs of varying degrees of genetic relatedness who have been reared together,” the researchers explain, “it is possible to examine a phenotypic—or observed—relationship after taking into account genetic and shared environmental [variables].”

In this particular study, the researchers reason that “any observed difference between monozygotic (MZ; identical) twins discordant for a life experience (e.g., marital status) cannot be attributable to genetic or shared environmental selection.”  In other words, if an identical twin who is married enjoys an advantage in physical or psychological health over an unmarried twin, that advantage likely reflects the consequences of wedlock itself.

To assess the true character of the marriage advantage, the researchers scrutinize data collected between 2001 and 2009 from 3,226 individuals—1,613 sibling pairs, including 171 pairs of identical twins.  In their initial statistical analysis of these data, the researchers find that “compared with singlehood, marriage or cohabitation was significantly associated with less self-reported depression, less alcohol use, fewer cigarettes smoked per day, lower risk for suicidal ideation, and lower risk for engaging in antisocial behavior.”

In a second round of analysis, the researchers take into account the possible effects of shared genetic and environmental characteristics.  In this secondary analysis, the researchers find that, “compared with being single, being coupled remained statistically significantly associated with fewer depressive symptoms, lower risk of suicidal ideation, and less alcohol use.”  The researchers remark, “The significant direct effect of marital status on these outcomes is consistent with a quasi-causal relation.”  That is, these results are inconsistent with the selection-effect explanation of the marriage advantage.  Rather, such results reinforce the view that marriage itself confers this advantage.

To gauge the magnitude of the marriage advantage, the researchers calculate that “married or cohabiting MZ [identical] twins had mean depression scores that were .13 SD [Standard Deviations] lower than their single co-twins, and were just one fourth as likely to report suicidal ideation.”

What is more, while both marriage and cohabitation apparently offer some psychological advantages over singlehood, a closer look reveals decided relative deficiencies in cohabitation.  “Compared with cohabitation,” the researchers report, “marriage was associated with significantly lower risk for suicidal ideation and antisocial behavior.”  True, the gap in suicidal ideation falls below the threshold for statistical significance when the researchers account for shared genetic or environmental characteristics.  But even after accounting for these shared characteristics, the researchers identify “a quasi-causal protective effect of marriage (compared with cohabitation) on risk for engaging in antisocial behavior.  Indeed, the researchers calculate that “cohabiting MZ [identical] twins were 67% more likely than their married co-twins to have engaged in antisocial behavior.”

Interpreting their findings, the Virginia scholars observe, “Marriage and cohabitation both offer positive supports such as a shared emotional life, companionship, and practical assistance, all of which may lead to greater happiness and lower levels of depression.  Similarly, marriage and cohabitation both protect against negative experiences such as loneliness and social isolation, factors that may increase the risk for anxiety and depression.”

But the researchers regard it as a finding of “considerable interest that marriage offers benefits over cohabitation in relation to externalizing problems.”  These benefits, the researchers plausibly suggest, reflect the fact that “young adults who marry rather than enter or maintain a cohabiting relationship exhibit a greater commitment to one another and perhaps adopt longer time horizons in relation to both their intimate partnerships and other aspects of their lives.”  “It is also possible,” they conjecture, “that husbands and wives have or assume ‘permission’ to monitor their partner’s behavior more closely, fostering greater engagement in prosocial activities and less engagement in antisocial ones.”

The researchers do confess that they were “somewhat surprised to find that selection effects fully accounted for the physical health ‘benefit’ of entry into marriage.”  Indeed, the researchers appear reluctant to endorse this finding strongly.  After all, their data come from relatively young individuals, and “the possible physical health benefits of marriage may accrue over the life course.”  Especially given the finding in this study that married individuals drink less than unmarried peers, the researchers wonder if a different relationship between wedlock and physical health will not show up in studies involving older twins and siblings.  “If single individuals engage in more unhealthy behaviors,” they write, “—such as heavier alcohol use—we may well observe a causal effect of marital status on physical health in future waves of this sample or in other, older adult samples.”

In any case, the researchers see in their findings solid evidence that wedlock delivers psychological and behavioral benefits.  Such evidence they regard as congruent with the thinking behind “legislation supporting marriage promotion initiatives such as the Marriage and Fatherhood Provisions of the Deficit Reduction Act of 2005.”

This new study should indeed embolden policymakers willing to reverse a half-century of anti-marital lawmaking with new measures that reinforce wedlock.

(Erin E. Horn et al., “Accounting for the Physical and Mental Health Benefits of Entry into Marriage: A Genetically Informed Study of Selection and Causation,” Journal of Family Psychology 27.1 [2013]: 30-41.)