To provide more precise yearly estimates and to document the trend in the heritability of regular smoking, the survival models presented in Table 3 were repeated 47 times for the birth years 1922 to 1968.6 The yearly heritability estimates and the bootstrapped 95 percent confidence intervals are presented in Table 4 and Figure 2. These results correspond with the results presented in Table 3. However, they also provide a more detailed picture regarding temporal changes in the genetic influences on smoking. That is, the averages described in Table 3 are still evident, but important changes occur within each birth cohort, and these changes are theoretically linked to the causal and noncausal forms of G × E described above. The genetic influences on regular smoking appear to be the most pronounced for those born in the early 1930s and the mid-1950s. Sharp changes in the sociogenetic composition of the smoking population occur for those born after 1936 and again for those born after 1954. The first minimum in this figure occurs in 1942, which corresponds with the Surgeon General’s Report; those
