The findings presented in this paper speak to a large body of work that quantifies genetic and environmental contributions to smoking in the population, with the goal of anticipating dispersion in genetic influences across different social settings (Shanahan and Hofer 2005). The gene–environment interaction perspective anticipates that social environments may either enhance or suppress latent genetic tendencies in a causal manner or they may simply obscure or clarify the influence of genetic factors in a noncausal manner. The context for this dispersion may be discrete social settings like schools (Rowe et al. 1999; Boardman et al. 2008) or neighborhoods (Cleveland 2003); but, as we show here, it may also be a social historical trend or birth cohort. This point is made nicely by Rutter (2006, p. 60):
