We then calculate heritability estimates for regular smoking across the four birth cohorts. The goal of this analysis is to examine the anticipated highs and lows described above. If changes in the heritability of smoking are consistent with our expectations, then the highest heritability estimates should be among the first (1920s–1930s) and third (1950s) birth cohorts, and the lowest should be among the second (1940s) and fourth (1960s) cohorts. To demonstrate the reliability of the parameter estimates, we present heritability estimates using four different techniques. The first two methods compare the correlations for regular smoking among MZ and DZ pairs. Heritability is estimated using twice the difference of MZ and DZ correlations (Plomin et al. 2008). The first correlation is simply the bivariate correlation among twin pairs. This estimate is calculated separately for MZ and DZ pairs. We also estimate a pairwise correlation using a second, and more complicated, method that takes into account the age of onset for smoking among sibling pairs as well as censored values for those who may later begin smoking but have not by the
