DSM-IV alcohol dependence symptoms were ascertained using items from the SSAGA. These questions were only asked of individuals who reported either having drunk at least once a month for 6 months or longer or drinking to intoxication (exclusion of 354 individuals). Mean number of DSM-IV symptoms (range 0–7) were 0.95 (SD 1.3) and 1.52 (SD 1.6) in women and men respectively. We opted to use AD symptoms (log-transformed) instead of the diagnosis of DSM-IV AD because (a) using AD symptom count allows us to incorporate the severity of the AD diagnosis which may be quantitatively associated with age at first drink (i.e. early onset contributes to more symptoms), (b) the diagnosis of DSM-IV AD is reliant on a threshold (3+ criteria) which is currently under psychometric scrutiny, (c) power to detect G x E is enhanced when using a quantitative measure and (d) flexibility afforded by quantitative measures in twin modeling.
