Ours is a general population sample, so extreme pathology (like schizophrenia or autism) is not represented to any significant extent. Hence, as Cuthbert (2014, this issue) points out, it is possible that we would have more promising results had we overselected for cases at the extremes. We agree, but it is nonetheless the case that common mental disorders are amply represented in the MTFS, with rates suggesting that a thousand or more individuals in these endophenotype studies are affected with disorders like depression and alcoholism. Examining lifetime prevalence of selected disorders in MTFS older cohort twins based on in-person structured psychiatric interviews, Hamdi and Iacono (2014) reported the following rates: antisocial personality disorder (7.7%), cannabis dependence (9.9%), alcohol dependence (21.2%), major depression (27.0%), and nicotine dependence (32.8%). The age-11 prevalence of ADHD among younger-cohort and ES twins is 7% (Keyes 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
