The conceptualization of the different stages of smoking is helpful in understanding the varying environmental and genetic contributions to each step in the development of smoking behaviors. To better understand smoking behavior, it is compartmentalized into (1) initiation (the period during which subjects smoke for the first time and experiment with smoking), (2) regular smoking (defined as subjects having smoked at least 100 cigarettes during their lifetime), and (3) nicotine dependence (a psychiatric disorder defined by symptoms of tolerance, withdrawal, and loss of control).1 The heritability of smoking initiation was estimated to be 44%.29 Conversely, the heritability of nicotine dependence was markedly higher at 75%.29
