Major Depressive Disorder criteria was met if dysphoria or anhedonia were reported as occurring for more than 2 weeks along with DSM-IV criteria for 5 or more major depressive symptoms during the same 2-week period with age of onset defined as age when first met full criteria. Parental characteristics: To account for the impact of socio-economic status, parental education was included in models as a proxy. Individual variables representing mother’s and father’s education levels were created by dummy-coding parental reports on whether each parent had completed high school (with more than high school as the reference variable). As maternal smoking has been strongly associated with young adult cannabis involvement (Hayatbakhsh et al., 2009), history of maternal nicotine dependence was considered and coded from maternal-report of nicotine dependence assessed in the parent interview in accordance with DSM-IV criteria.
