Consistent with past P300 studies from the MTFS (Carlson et al., 2002, 2004; Carlson et al., 1999, 2007; Iacono, Carlson, & Malone, 2000; Iacono et al., 2002, 1999), a disorder was considered present if it was at the definite (i.e., minimum DSM–III–R criteria are met) or probable (i.e., all but one DSM–III–R criterion is met) level of diagnostic certainty and absent otherwise. Collapsing across the definite and probable levels of certainty, an approach for diagnosing lifetime psychopathology that was introduced with the Research Diagnostic Criteria (Spitzer, Endicott, & Robins, 1978), is important for assessing disorders in individuals who are not currently symptomatic and who may have difficulty recalling symptoms experienced years or decades earlier. Earlier onset substance disorder was defined as in Carlson et al. (2007) as the presence of alcohol abuse or dependence, illicit drug abuse or dependence, or nicotine dependence with onset by the first follow-up (e.g., onset by approximately age 20).
