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

Abstract

Context: Previous studies have indicated that marriage is negatively associated with male antisocial behavior. Although often interpreted as a causal association, marriage is not a random event. As such, the association may stem from selection processes, whereby men less inclined toward antisocial behavior are more likely to marry.

Objective: To evaluate selection vs causation explanations of the association between marriage and desistence from antisocial behavior.

Design: Co-twin control analyses in a prospective twin study provided an analogue of the idealized counterfactual model of causation. The co-twin control design uses the unmarried co-twin of a married twin to estimate what the married twin would have looked like had he remained unmarried. Discordant monozygotic (MZ) twins are particularly informative because they share a common genotype and rearing environment.

Setting: General community study.

Participants: Two hundred eighty-nine male-male twin pairs (65.1% MZ) from the Minnesota Twin Family Study underwent assessment at 17, 20, 24, and 29 years of age. None of the participants were married at 17 years of age, and 2.6% were married at 20 years of age. By 29 years of age, 58.8% of the participants were or had been married.

Main outcome measure: A tally of criterion C symptoms of DSM-III-R antisocial personality disorder, as assessed via structured clinical interview.

Results: Mean differences in antisocial behavior across marital status at age 29 years were present even at 17 and 20 years of age, suggesting a selection process. However, the within-pair effect of marriage was significant for MZ twins, such that the married twin engaged in less antisocial behavior following marriage than his unmarried co-twin. Results were equivalent to those in dizygotic twins and persisted when controlling for prior antisocial behavior.

Conclusions: Results indicate an initial selection effect, whereby men with lower levels of antisocial behavior are more likely to marry. However, this tendency to refrain from antisocial behavior appears to be accentuated by the state of marriage.

Figure 1. Interpretation of Co-Twin Control Results

Should marriage be environmentally or causally linked to reductions in antisocial behavior, we would expect to observe this association at the individual-level, within DZ twin pairs discordant for exposure, and within MZ twin pairs discordant for exposure (scenario A). By contrast, the failure to observe an association within discordant MZ twin pairs would imply that the association of exposure with outcome is solely attributable to selection processes. In particular, if exposure was associated with outcome at the individual-level and in discordant DZ twins (scenario B), we would infer that the selection process was genetic in origin. If the exposure was associated with outcome only at the individual-level (scenario C), we would infer that the selection process was genetic and shared environmental in origin.

Figure 2. AAB by Marital Status, Age 29

Note. To facilitate interpretation of these unstandized fixed effect estimates (as also presented in Table 2), the Adult Antisocial Behavior (AAB) extended symptom count variable was standardized to have a mean of 0 and a standard deviation of 1 prior to analysis. Both marriage and AAB were assessed at age 29. Standard error bars are presented. The between-pair effect estimate, which approximates the individual-level effect, is also presented. Significant within-pair estimates for MZ twins are indicative of a non-shared environmentally-mediated relationship between marriage and AAB, and particularly so when the DZ within-pair estimate is equivalent to the MZ estimate.