The role of the individual characteristics (and of the genotype) in the variation of addiction risk is particularly eminent and grows with age also due to the ready availability of psychoactive substances. For instance, according to the 2007 National Survey of Drug Use and Health (U.S. Dept. of Health and Human Services, 2008), half of the population aged 18–49 have used marijuana, 70% have used cigarettes, and 90% have used alcohol. Almost a quarter of the population aged 35–49 have used cocaine. Variation in liability to drug use initiation is substantially contributed by shared and nonshared environment (Han et al., 1999; Kendler et al., 2000), in contrast to liability to SUD (abuse and dependence) where the genetic component of variance is predominant and no shared environment effect is detected. This difference suggests that drug use initiation, while obviously a precondition to addiction, does not account for the risk for SUD. Supporting the role of phenotypic propensity in the risk for addiction, the transmissible liability index, TLI (Vanyukov et al., 2009, 2003a,b), a scale based on psychological indicators of CLA prior
