et al., 2009). The parents of the twins also show high lifetime rates (20‒22%) of alcohol dependence and illicit drug abuse or dependence (Holdcraft & Iacono, 2002, 2004). Other MTFS reports also document significant rates of offspring and parent internalizing and externalizing disorders (Hicks, DiRago, Iacono, & McGue, 2009; Marmorstein, Iacono, & McGue, 2009; Wilson, Vaidyanathan, Miller, McGue, & Iacono, in press). While it is possible that more extreme cases would have helped boost our yield of hits, our results cannot be attributed to our having studied only mentally healthy individuals.
