A subset of the participants (N=1,866; 11.7%) participated in both studies, in which case we used data from the last assessment. The combined phenotypic sample consisted of 14,087 individuals, of whom 7,172 were genotyped. In both studies, twins were asked the same items about cannabis use: 1)Did you ever use marijuana?, 2) How old were you the very first time you tried marijuana (not counting the times you took it as prescribed)?, and 3) How many times in your life have you used marijuana (don’t count times when you used a drug prescribed for you, and took the prescribed dose). Ever use was measured on a dichotomous scale (ever versus never), whereas age at initiation and frequency of use were open questions. Table 1 shows the prevalence of cannabis use for individuals included in the present study.
