We chose to examine the genetics underlying a propensity to smoke large quantities of cigarettes at any age, a narrowly defined "extreme" quantitative phenotype that we hoped may have provided improved power [1-3]. Nonetheless, difficulties in assigning phenotypes remained; for example, the inclusion of non-smokers as having trait equal to zero may not have improved power to detect linkage if non-smoking individuals harbored an untriggered propensity for addiction. Because smoking patterns changed dramatically over the last half-century, we also hypothesized that eliminating this source of variance (year of maximum smoking) may affect heritability, however heritability estimates were unchanged and several of the same parametric and VC LOD score peaks were seen.
