We also collected information on substance use at age 18 years through self administered computer assisted interviews and included measures of harmful drinking, problem cannabis use, regular smoking, and illicit drug use. We used the 10 item alcohol use disorders identification test to assess harmful drinking.30 A cut-off of 16 or more was used to indicate harmful use.31 Problem cannabis use was measured using the six item cannabis abuse screen test,32 which assesses cannabis consumption in the previous 12 months and focuses on difficulties controlling use and associated health and social impairment. All items are answered on a 5 point scale (0 never, 1 rarely, 2 from time to time, 3 fairly often, and 4 very often). A response of fairly often or very often to any of the six items was used to indicate problem cannabis use.33 We derived information on smoking using a series of questions. Participants who responded positively to the questions “having ever smoked” and “having smoked in the past 30 days” were asked to indicate whether they smoked weekly. Response to this item was used
