Second, these results may not generalize directly to the etiology of nicotine dependence since cigarette pack-years may only reflect average lifetime cigarette use. Further, the magnitude of the contribution of shared environmental influences for pack-years was moderate. The contribution of shared environmental effects was also moderate for the constituent measures of pack-years (number of cigarettes smoked and number of years smoked). The presence of significant shared environmental influence for pack-years is in contrast to many studies of nicotine dependence in adults for which shared environmental influences were not significant. It is likely that some of the variance due to shared environmental influences may result from the use of a 100 or greater cigarette item to identify smokers. Consequently, additional study with validated measures of nicotine dependence such as the Fagerström Test for Nicotine Dependence (Fagerstrom and Schneider 1989) is suggested. Nevertheless, average lifetime cigarette use is expected to contribute to anatomical and chemical changes in the brain (Franklin et al. 2014). Further, the phenotypic associations reported here are similar to others using cigarette pack-years in community-based samples of older adults
