All three studies used an assessment of substance dependence based on the Semi-Structured Assessment for the Genetics of Alcoholism (SSAGA) (13). This shared methodology of interview administration, question format, and queried domains allowed harmonization of phenotypic data across all studies. The Fagerström Test for Nicotine Dependence (FTND) was used to evaluate lifetime history of nicotine dependence, with a score of 4 or more defining “dependence” for this study. Lifetime history of dependence on alcohol, cocaine, and marijuana was determined according to DSM-IV criteria. Though opioid and sedative dependence were assessed, these diagnoses were not included in this analysis because of the small number of subjects dependent on these substances and reduced power to detect association.
