smaller sample of African relative to European ancestry twins, which may have (i) obscured potentially important age interactions in survival models and (ii) contributed to inadequate numbers of intact African American families across the range of risk propensity for parental separation. Lastly, given significant differences by zygosity observed in the likelihood of onset of substance involvement, most likely reflecting the importance of peer influences and the wider social networks of fraternal compared to identical pairs, extension to non-twin samples is critical, including comparison of twins to singletons where data on both are available. Given potential differences by offspring sex, replication of findings in male offspring is also important.
