As the alcohol use variables were ordinal, a threshold model was used whereby the underlying liability to each variable is considered as normally, or approximately normally, distributed in the population. This model assumes that behaviour, e.g. problem alcohol use, will occur only if an individual's position on the liability distribution for that behaviour is above a certain liability threshold [36]. In univariate analyses, twin data allowed us to estimate the relative contribution of genetic and environmental influences on quantity of alcohol use and problem use. By conducting similar analyses on twin reports of friends' alcohol use, it is also possible to test whether there are genetic influences on the adolescent that are correlated with friend's alcohol use. The twin method is based on comparisons of monozygotic (MZ) twins, who share 100% of their genes in common, with dizygotic (DZ) twins, who share on average 50% of their genes. For genetically influenced behaviour we would expect greater MZ than DZ similarity. In the basic (ACE) model, variation is assumed to arise from three sources: (1) additive genetic effects (a2); (2) common
