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 born during the early to mid-1940s were at their prime smoking ages (in their early 20s) when the report was released. However, following this first transition there is a persistent and steady increase in the genetic influences on regular smoking until a maximum is reached for those born in 1954. We argue that this increase captures a noncausal form of gene–environment interactions where the sociogenetic composition of smokers is changing over time; those for whom quitting smoking is relatively easy may be the first to quit in light of the evidence about the health risks. Smoking desistance is the most highly heritable smoking phenotype (Vink, Willemsen, and Boomsma 2005). Therefore, those who have the hardest time quitting may also be those who have a stronger physiologic dependence on nicotine. As nondependent individuals are removed from the smoking population, genetic factors responsible for nicotine dependence become relatively more important.
