For initiation of cigarette use, common environmental and genetic influences were of equal importance (a2 = 41%, c2 = 42%). Genetic factors explained almost the entire variance specific to progression (a2 = 100%), with unique environmental factors explaining very little variation (e2 < 0.1%). The beta coefficient between initiation and progression was high (α = 0.87) and the confidence interval included 1, implying considerable overlap in the initiation and progression liabilities for cigarette use. Of the variance of progression, 76% was explained by factors influencing initiation and of the variance of progression explained by factors influencing initiation, 42% was due to shared environmental factors, 41% by genetic factors and 17% by non-shared environmental factors.
