The NAG-FIN sample was ascertained from the Older Finnish Twin Cohort consisting of adult twins born in 1938–1957. Based on earlier questionnaires, twin pairs concordant for ever-smoking were recruited along with their family members (mainly siblings) for the Nicotine Addiction Genetics (NAG) consortium [32]. Data collection took place in 2001–2005. A total of 747 families including 2193 subjects were assessed by DNA sample collection, structured psychiatric interview, and additional questionnaires, yielding detailed phenotypes of lifetime smoking behavior (including initiation, quantity, and cessation). For the Genetic Risk Score analyses 2054 subjects from 745 families (including 141 MZ twins (one co-twin from each MZ pair), 856 DZ individuals from 428 full DZ pairs, additional 147 DZ individuals (one co-twin from each DZ pair), 50 individuals of unknown zygosity, and 860 other family members (mainly siblings)) with 1000Genomes imputed genome-wide genotype data were used. Serum samples for cotinine and NMR analyses were not collected.
