There are several important limitations that should be considered in interpreting these results. First, although our multivariate models control for the equal environments assumption with a series of questions regarding the similarity of twins’ environments, it is difficult to know whether this control fully captures these effects. Several studies have tested this assumption, and the results are somewhat mixed (Kessler et al. 2004; Kendler et al. 1993). Further, as the survival models used in this study have been used in only a few behavioral genetic studies, there is no agreed-upon method to reliably adjust parameter estimates for differential environments for MZ and same-sex DZ twins. In ancillary analyses (results not shown), we examined MZ-DZ differences in the equal environment measure and found a monotonic decline over time. That is, differences in MZ and DZ environments were much higher among the earliest birth cohorts and smaller among the most recent cohorts. Therefore, heritability estimates may be exaggerated for the earlier cohorts of the study. However, as this change is consistently decreasing over time, it cannot explain the increasing–decreasing changes that we observed in our study across birth cohorts.
