Zygosity was significantly (p < 0.0001) associated with both childhood and adult environmental measures for both sexes, controlling for age. The measure of childhood environment significantly predicted twin concordance for cigarette-based ND in males (p = 0.0299) and females (p < 0.0001). However, similarity in this environmental measure accounted for little of the total variance in twin concordance [Δr2 (rescaled) = 0.0032 in males and 0.0106 in females]. Neither measure of adult environment predicted concordance for any phenotype.
