This study used data from the fourth wave (2004) of data collection of the longitudinal Cardiff Study of All Wales and North-west of England Twins (CaStANET), a population-based twin register, including families with twins born between 1976 and 1991 in the Cardiff area of South Wales and between 1980 and 1991 for the rest of Wales and the North-west of England [26,27]. Previous waves of data collection (1991, 1997, 2000) have focused on indices of child psychological adjustment [including attention deficit hyperactive disprder (ADHD), conduct problems, depression and chronic fatigue] using child and/or parent and teacher reports [26]. In 2004, questionnaires were mailed to families taking part in CaStANET with twins aged 11–19 years, which assessed substance use and associated risk factors, including family functioning, parent health and twin psychological adjustment. The sample is representative of the general UK population in terms of socio-economic status and ethnicity [27,28]. Zygosity was assigned in an earlier wave of the study [23] using a parent-rated twin similarity questionnaire [29,30]. Non-responders were initially sent reminder postcards and were then re-mailed the questionnaire. Of 1755
