Socio-economic status was assessed at T1 using a 5 point scale consisting of five variables: educational

level (father/mother), occupation (father/mother), and family income. The internal consistency of this measure is satisfactory (Cronbach’s alpha 0.84; Veenstra et al., 2006). Parental psychopathology (i.e. depression, anxiety, substance abuse, antisocial behaviour, and psychosis) was measured by means of the Brief TRAILS Family History Interview (Ormel et al., 2005), administered at T1. Each syndrome was introduced by a vignette describing its main symptoms and followed by a series of questions to assess lifetime occurrence, professional treatment, and medication use. The scores for substance abuse and antisocial behaviour were used to construct a familial vulnerability index for externalizing selleck chemicals llc disorder. The scores for depression and anxiety disorder were used to construct an index for internalizing disorder. The construction of a familial vulnerability index was based on Kendler et al. (2003), who performed multivariate twin modelling to investigate shared genetic FK228 risk factors for psychiatric and substance use disorders. More information on the construction of familial vulnerability within TRAILS is described elsewhere (Veenstra et al., 2005). For

both internalizing and externalizing disorder, parents were assigned to one of the following categories: (0) = (probably) not; (1) = (probably) yes, (2) = yes and treatment/medication (substance abuse, depression, and anxiety) or picked up by police (antisocial behaviour). In order

to assess alcohol and tobacco use, participants filled out a questionnaire Histone demethylase at both T2 and T3 on the frequency of use in the past month. For tobacco use reported frequency was recoded into non-weekly (0) versus weekly (1), and for alcohol use, the reported frequency was recoded into non-monthly (0) versus monthly use (1). These categories were similar to those used in other studies investigating cannabis use and mental health in young adolescents (e.g. Monshouwer et al., 2006). It was first examined whether non-responders differed from responders on SES (by means of t-test) and gender (by means of Pearson χ2-test). Next, it was examined whether, among the responders, there were differences between cannabis users and non-users with respect to SES, familial vulnerability for internalizing and externalizing behaviour, use of alcohol and tobacco and gender (using Pearson Chi-square analysis for alcohol, tobacco use and gender and t-tests or GLM univariate analysis of variance for SES and familial vulnerability). These analyses were performed in order to determine which variables should be included in the main analyses as covariates.