Despite the general acceptance of this perspective, we know of only one other study to consider the influence of birth cohort on the genetic influences on smoking. That study, which used data from twin pairs in Sweden, examined birth cohorts from 1910 to 1958 (Kendler et al. 2000), and it showed an increasing heritability of smoking for women across the cohorts but a relatively stable heritability among men. Kendler and colleagues’ analyses considered those born between 1940 and 1958 as one birth cohort, so it is difficult to compare their results to ours. That is, it is possible that heritability peaked before the end of this cohort and then declined as in our results. But there are also clear differences in the trends in regular tobacco use between Western European nations (Pampel 2003) and the United States (Forey et al. 2002). The United States began a shift to regular smoking earlier than Sweden and peaked much earlier. Importantly, these differences set up an opportunity for future researchers to elaborate upon the temporal variation in genetic factors linked to smoking by comparing these associations across multiple settings and birth cohorts.
