Phenotypes were selected to represent multiple stages of smoking. These included initiation, heaviness of use among smokers, and a measure of total lifetime exposure to tobacco. For alcohol use only a measure of amount of alcohol use was systematically available across studies. The selected phenotypes are relevant to prior GWAS of smoking and alcohol use; are commonly available in psychological, medical, and epidemiological data sets; and are known to be correlated with measures of substance dependence(4, 39–41).
