Both the drinking index and the sum of the alcohol abuse and alcohol dependence symptom counts were each averaged across mothers and fathers, so that parental alcohol use represents maternal and paternal alcohol use in combination. Because their distributions were non-normal, for all participants the drinking index and the sum of alcohol dependence and alcohol abuse symptom counts were each log-transformed. For analysis, for parents and elder siblings separately, the log-transformed drinking index and the log-transformed sum of alcohol abuse and alcohol dependence symptom counts were standardized and averaged to form a combined alcohol problem index, which was itself then standardized. All standardization was calculated relative to the variable values of the total number of available SIBS parents (N=1164) and both adopted and non-adopted offspring (N=1232), including both genotyped and nongenotyped SIBS participants.
