Initiation was assessed by a multiple choice question (i.e., “At which age did you experiment with cannabis for the first time?”) in the NTR surveys 1993, 1995, 2000, and by an open-ended question (“Have you ever tried hashish or cannabis? If yes, at which age?”) in survey 2009. These surveys were sent to all adult twin families and were returned by 23 597 individuals. In addition, data collection in adolescent twins and sibs which took place since 1987 in age-specific surveys (around age 14 and age 16), included a multiple choice question (“Have you ever used soft drugs such as hashish or cannabis?”) assessing frequency of use (on an eight-category scale ranging from ‘never’ to ‘more than 40 times’) in the whole life, in the last 12 months and in the last 4 weeks. This question was completed by 16 556 participants. The phenotypic data obtained from subjects who reported at more than one time point were checked for consistency, and unreliable measures were discarded. Due to inconsistencies, 284 self-reported measures were dropped. Next, the measurements were collapsed into a
