The effects of maternal maximum drinks paralleled those of paternal maximum drinks in our previous investigation (Malone et al., 2002). This raises the possibility that effects of maternal maximum drinks might actually be due to the father's status on this measure. Maternal and paternal maximum consumption were correlated, r = .29 and r = .34 (p < .001) in the two respective age cohorts. We included both parents' scores as independent variables (in addition to offspring gender) and conduct disorder, ODD in females, any disruptive behavior disorder and all substance-related outcomes as dependent measures. To increase stability of estimates, we combined age cohorts, including cohort as a covariate. Because there was greater variability in age of younger-cohort subjects, we also included age at assessment as a covariate. All analyses were preceded by tests of interactions between parental status and cohort, conducted separately for maternal and paternal measures. Two such tests were significant: effects of paternal maximum consumption on early drug use and on twin maximum consumption differed by age cohort, χ2 = 4.60, p = .032, and χ2 = 7.25,
