Given the likelihood of an association between age and the level of alcohol use and friends' alcohol use, an age correction was employed which adjusts the threshold for each twin according to his or her age on the distribution of liability to substance use, using the same method as described for controlling for sharing of best friends [41]. We also tested for quantitative differences in aetiology due to age by assessing whether the genetic and environmental estimates can be equated for individuals from different age groups without a significant drop in fit. For this analysis, the sample was divided into early (12–14 years old, 175 twin pairs), middle (14–16 years old, 386 twin pairs) and late (16–18 years old, 315 twin pairs) adolescent age groups. Although dividing the sample in this way will lead to a reduction of power to detect differences, we can conclude tentatively that there was no evidence for quantitative differences in aetiology due to age (results available from the corresponding author upon request).
