Age at 1st drink was only modestly heritable (9–14%) with a large proportion of variance (76–80%) due to shared environmental influences. First, to estimate the extent to which covariation in age at 1st drink and AD symptoms could be attributed to overlapping genetic and environmental influences, a Cholesky/standard bivariate decomposition model, without moderation, was fit, separately to male and female data. The model revealed significant overlap of genetic (RG=0.50–0.95) influences on age at 1st drink and AD symptoms. Shared environmental factors on AD symptoms were completely overlapping with age at 1st drink in women (RC=1.00), which is in agreement with findings from the univariate moderation models described above where no evidence for C was detected. Non-shared environmental factors were important for both age at 1st drink (10–11% of variance) and AD symptoms (54–60% of variance) but were un-correlated (RE=0)
