Returning to Table 1, it is important to note that of the four birth cohorts examined MZ pairs have a significantly higher concordance rate, compared to DZ twins, for regular smoking in only two cohorts. These findings are in line with the anticipated changes in heritability described above. In other words, these findings suggest that for those born in the 1940s and 1960s, genetic factors do not significantly contribute to the risk of regular smoking. The same results are evident in Table 3, which summarizes the outcomes from the four methods used to characterize the heritability of regular smoking. The first row of Table 3 presents estimates for those born between 1920 and 1939. Among MZ pairs, the frailty estimate (θ = 1.67) translates into a pairwise .46 correlation. As described earlier, these models control for respondent’s age, gender, and a measure for the equal environments assumption among twin pairs. The bivariate correlation, based on the frailty estimates, for this birth cohort is .58. For DZ pairs, the frailty estimate (θ = .29) is considerably smaller, and it translates into
