initiation and persistence of smoking (Madden et al. 2006; True et al. 1997; True et al. 1999) and on initiation and ND (Fagerstrom 1978) are mostly consistent in that genetic factors for regular smoking are not completely accounted for by the genetic factors contributing to smoking initiation (True et al. 1997; Heath & Madden 1995). We speculate that the variance attributed to genetic factors in the classical twin design are partly associated with sibling substance use and peer substance use which are modeled in the present paper but not accounted for in classical twin studies.
