Cox proportional hazards models were set up in STATA (StataCorp, 2003). Twin clustering was accounted for by allowing members of a twin pair to cluster on their family number, which allows standard errors to be adjusted using a robust variance estimator. The event of interest for the survival analysis was one or more symptoms of alcohol dependence and age of onset of the first DSM-IIIR alcohol dependence symptom was the time to event. Individuals who had never used alcohol even once in their lifetime and those who used alcohol but, at the time of interviews, reported no dependence symptoms, were censored and their time to event was set to their age at interview.
