Fourth, our analyses assumed that the exposure to etiologically relevant environmental risk factors is equally correlated in monozygotic and dizygotic twin pairs. Concordance for cigarette-based ND was significantly predicted by our measure of childhood environment. How this environment might influence ND concordance is unclear, particularly given that it was not predictive of concordance for regular use of tobacco which we considered more likely to be influenced by social factors. The measure accounted for only a very small proportion of the total variance, and is within the CIs of our estimates. Therefore, we feel that it is unlikely to have introduced significant biases in our results. Finally, these analyses were not corrected for potentially confounding factors, such as other types of psychopathology, neuroticism or socio-economic status.
