The relationship between regular tobacco use and MD is positive and statistically significant (Table 1) for both sexes, and for both types of tobacco use in males. Odds ratios for MD generally increased with increasing levels of ND, although in some cases the 95% confidence intervals (CIs) span unity. Tetrachoric and polychoric correlations between twins for MD and tobacco use variables are presented in Table 2. Monozygotic twins exhibit higher levels of correlation than do dizygotic twins, suggesting that the covariance between twins is at least partially attributable to genetic factors.
