During 2002–2005, the first full-length follow-up interview was completed, which included assessments of childhood experiences, including physical abuse, and mental health, including anhedonia and depression26. All eligible twins, including those who may not have completed a baseline assessment, were invited to participate in the follow-up, provided that they, or their parents, had not previously indicated an unwillingness to participate in future studies. A total of 3,787 twins (14% African-American) aged 18–29 years, completed follow-up interviews. Twins were also invited, during this period, to provide permission for future DNA collection for genomic studies27. In a subsequent effort, of those contacted, 1191 subjects, including one or both members of monozygotic and dizygotic twin pairs provided a sample – collection of samples from remaining subjects is ongoing and this study reports on genotyping efforts completed on 1,188 subjects (with 3 dropped due to genotyping problems). Genotyping was conducted using Golden Gate technology from Illumina Inc with an array of 1536 single nucleotide polymorphisms (SNPs) in genes implicated in the etiology of addictions28 and those for population admixture29. For this study, data from 1041
