For initiation of alcohol use, common environment was the most important factor (c2 = 65%) while genetic influences contributed 26%, with little evidence for unique environmental influences (e2 = 9%). For quantity of use, there was no evidence of specific common environmental influences, with genetic factors explaining most of the variance (a2 = 64%). For the other indices of alcohol progression (binge drinking, getting drunk and getting into situations regretted due to alcohol), the influence of specific genetic factors onprogression tended to be stronger than the influence of genetic factors on initiation of alcohol use, while estimates of common environmental influences tended to be lower. The beta coefficients between initiation and progression (which is a measure of the degree of relationship between the underlying liabilities) for all alcohol outcomes were moderate in size (α = 0.48–0.65). Between 23% and 42% of the variance of progression was due to factors influencing initiation. These findings suggest partial but not complete overlap between the liabilities for alcohol initiation versus progression to more serious use. Of the variance of progression explained by factors influencing initiation, 65% was due to shared environmental factors, 26% by genetic factors and 9% by non-shared environmental factors.
