It is important to note that in the studies described here, the researchers only studied habitual smoking—defined as smoking 20 cigarettes (i.e., one pack) or more daily for at least 6 months at some time in life—but did not determine whether the participants were dependent on nicotine. A later study involving a subset of the participants, however, included an interview that assessed nicotine dependence. In that study, investigators determined that 71 percent of the habitual smokers were nicotine dependent (Bierut et al. 2004). The identification of potential genetic differences between habitual smokers with and without nicotine dependence constitutes an interesting topic for future analyses.
