Some limitations should be noted. First, the sample had limited racial and ethnic diversity, which constrains generalizability to other populations. A second limitation regards restriction of range among adoptive families. Specifically, relative to biological families, mean-levels of externalizing symptoms were lower among the adoptive parents, but not among the adoptive offspring26. However, a thorough analysis of restriction of range in the SIBS sample26 showed that such effects were relatively small and would have little impact on our results. Third, our measure of drug dependence assessed illicit drug problems in general, when there are likely different genetic influences for different drug classes. Fourth, there was a notable age range among the offspring participants, leaving open the possibility that the results could differ slightly for different ages. Additionally, because data collection of the longitudinal data spanned two decades, there may be small cohort effects within the offspring sample. Finally, we used standard biometric models rather than more complex extended twin-family models36 (though they are unlikely to yield substantively different results) or models that incorporate the presence of gene x environment interactions that seem to be present for externalizing disorders37.
