Does marriage inhibit antisocial behavior?: An examination of selection vs causation via a longitudinal twin design.

A particularly provocative set of findings within the antisocial behavior literature concerns the role of marriage in inhibiting these behaviors in adult men. Indeed, there is now convincing evidence that the state of marriage is associated with lower crime rates1–4. For example, a recent study2 examined within-individual associations between marriage and antisocial behavior in a sample of 475 high-risk boys followed from adolescence through adulthood and found that the average reduction in crime with marriage was approximately 35%, a rather remarkable decrease. Other research found that living with one’s wife was negatively associated even with month-to-month variation in crime rates4. In short, the “marriage effect” on desistence from antisocial behavior appears to be a robust one. Mechanisms thought to account for the association between marriage and desistence from antisocial behavior2 typically center on social control or social bonding3, decreased affiliation with deviant peers5, and/or direct social control exerted by wives on their husbands2. In other words, extant research has generally interpreted the marriage effect as a causal one, whereby marriage inhibits subsequent antisocial behavior (either directly or indirectly). Should this be true, it could provide valuable leads for enhancing treatment development, as more concrete identification of specific mechanisms may provide a potentially powerful framework for improving interventions. Critically, however, marriage is not a random event, and thus the link between marriage and desistence from antisocial behavior could instead be attributable to selection effects (a possibility that was first discussed more than 20 years ago6). Under this scenario, men who are less inclined towards antisocial behavior (for whatever reason) are more likely to marry. As one example, there is a well-known association between antisocial behavior and low socioeconomic status7, the latter of which is also predicts marriage (in the form of economic potential)8. Selection is thus a key confound in studies suggesting that marriage causally contributes to desistence from antisocial behavior6. This possibility of selection has not gone unnoticed by researchers, many of whom have attempted to address this issue using sophisticated within-person statistical techniques that draw on the counterfactual method2, 4. The counterfactual method9, 10 provides both a definition of a causal effect and an integrative framework for estimating these effects. The average causal effect would be the person’s outcome when married as compared to the same person’s outcome when not married. As simultaneous observations of these two outcomes are clearly impossible, however, the counterfactual method places the problem of causal inference within a general missing data framework. Specifically, researchers would estimate the missing non-married outcomes for those who were married, as well as the missing married outcomes for those who were not (yet) married, to infer causal effects. Although these methods allow for reasonable inferences regarding the effects of marriage on antisocial behavior, we know of no study that has examined this question within a prospective, genetically-informative sample. Such samples are ideally suited for conducting counterfactual studies because monozygotic twins raised in the same family differ only to the extent that they have been exposed to different environmental factors11. Indeed, comparisons of twins discordant for marriage would offer particularly compelling support for or against an environmentally mediated effect of marriage on antisocial behavior, and in this way, would constitute the most powerful test of this association available to date. The current study sought to do just this, examining a sample of male-male twins pairs assessed at ages 17, 20, 24, and 29 years. Data were analyzed using an analog of the idealized counterfactual model of causation, the co-twin control design12, in which the unmarried co-twin of a married twin is used to estimate what the married twin would have looked like had he remained unmarried. The current study thus offers a unique and novel opportunity to evaluate the meaning and origins of desistence from antisocial behavior with marriage.