FinnTwin12 is a population-based longitudinal study of five consecutive birth cohorts (1983–1987) of Finnish twins, designed to examine genetic and environmental determinants of health-related behaviors [25, 26]. The study has a two-stage sampling design. The first-stage is an epidemiological investigation, with four waves of data collection (at ages 12, 14, 17, and at early adulthood) providing data on approximately 2700 families with twins. The second-stage is an intensive assessment of a sub-sample nested within the epidemiological study. Most of the sub-sample is selected at random, but this random sample is then enriched with twins from families with a history of elevated familial risk for alcoholism. The 1295 subjects of the intensive sample have DNA available while serum was available on 780 subjects. Among this sample, cotinine and 3-hydroxycotinine were measured from all self-identified current smokers. A total of 211 subjects (55 monozygotic (MZ) individuals (one co-twin from each MZ pair), 80 dizygotic (DZ) individuals from 40 full DZ pairs and additional 76 DZ individuals (one co-twin from each DZ pair)) with cotinine above 10ng/ml and 1000Genomes imputed genome-wide genotype data
