MZ and DZ correlations were also examined in twin pairs who were stratified by a binary measure representing early vs late age at 1st drink. The advantage of this method is that it allows a quick estimation of whether the magnitude of heritable influences on AD symptoms varies as a function of age at 1st drink. The disadvantage is that this method is highly underpowered to pick up modest to moderate G x E effects as it is reliant on data from complete pairs that are concordant (for early-onset or for late-onset) or discordant for age at 1st drink, which is arbitrarily dichotomized. In our sample, about 22% of the participants reported an age at 1st drink of 14 years or younger – the cut-off was chosen based on the distribution of the data, to allow for adequate number of subjects in the computation of correlations and from inspection of the change in heritability estimates when age of onset was viewed continuously (see Figure 2 and 3). Concordant early-onset twin pairs (both reported age at 1st drink at 14 years
