Second, there is only one published study that has used the shared frailty technique to examine genetic influences on behaviors (Guo and Tong 2006). Importantly, that article examined the age of first intercourse, and whereas nearly all individuals will have had intercourse in their lifetimes, the proportion of adults who become regular smokers is much smaller and the normative contexts of the two behaviors are very different from one another. Although we believe that the timing of smoking onset better characterizes the genetics of smoking compared to a binary question about lifetime use, and that our heritability estimates are in line with the others, it is important to bear in mind that the extension of this model to smoking onset is relatively novel.
