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 a pairwise .13 correlation. Using twice the difference of the MZ-DZ correlations as a rough heritability estimate, we calculate the heritability of regular smoking to be in the range of .66 to .76 for this birth cohort. These estimates are in line with the other two methods used to estimate heritability (hDF2 = .71; hVC2 = .58). The heritability estimates for the four birth cohorts are presented for each method in Figure 1. Although there are slight differences in these estimates using the four different methods, it is quite clear from Figure 1 that there are systematic differences in the heritability of regular smoking across the four birth cohorts. As Table 1 shows, only the first and third of these cohorts demonstrate a sizable genetic influence on regular smoking.
