Genetic risk for AUDs was indexed from the history of alcohol abuse and dependence in the subject’s parents and co-twin, based on interviews with the co-twin, and family history reports about parents and co-twins obtained from the index twin (Muffler et al. 1991). The contribution of each measure to the total AUD risk was based on a modified ridit score, which is useful for combining scores from binary and ordinal variables with different numbers of levels and different prevalence rates. Each ordinal level is assigned a score that would be the middle score for that level on a uniform distribution of 0–1. For example, an ordinal variable with prevalence rates of 40, 30, 20 and 10% (0–40, 40–70, 70–90 and 90–100%) would be scored 0.20, 0.55, 0.80 and 0.95 respectively. Scores from the MZ co-twins were unmodified from the 0–1 scale whereas scores from the DZ co-twins and parents were adjusted half way back to the mean score of 0.5 (thus based on a uniform distribution on 0.25–0.75) to reflect the fact that MZ twins are twice as similar genetically
