Table 1 presents concordance rates for regular smoking among the twin pairs in this study. Of the 340 MZ twin pairs, 81.5 percent were concordant for regular smoking status compared to 72.7 percent of same-sex DZ twin pairs. This difference is statistically significant (p < .01), providing evidence for genetic influences on regular smoking for the full sample. The estimates provided in Table 2 quantify the genetic contributions to regular smoking more formally. According to these results, the ACE (additive genetic [A], shared environment [C], and unshared environment [E]) model is the best fitting of the seven models.
