Individuals who never use cigarettes will never be addicted to them. This simple fact results in a host of complications for the longitudinal measurement of developmentally-specific genetic influences. One complication is that initiation, regular use, and addiction are all moderately to highly heritable, and are also genetically related (Maes, et al., 2004), indicating that some genetic variants are relevant to both initiation of tobacco use as well as eventual addiction. Indeed, using nicotine is also correlated with alcohol dependence, marijuana dependence, and hard drug dependence (Kendler, Jacobson, Prescott, & Neale, 2003; Kendler, Prescott, Myers, & Neale, 2003), and these correlations are highest in adolescence and subside during the transition to adulthood (Vrieze, Hicks, Iacono, & McGue, submitted). A theoretical account of this pattern of relationships (Iacono, Malone, & McGue, 2008) argues that behavioral disinhibition (e.g., impulsivity, risk-taking) is partly responsible for the observed correlations among cigarette initiation, use and dependence and, more broadly, among substance use disorders. Adolescents initiate nicotine, alcohol, and drug use in part because they tend to act impulsively. Those who act more impulsively will be more
