These results suggest that high levels of SUD typically arise through a two-stage process. First, during the critical developmental period, individuals are exposed to an environment that encourages and supports substance use. Second, once exposed to the psychoactive substance, their genotypes confer vulnerability to its rewarding properties and/or insensitivity to its negative effects so that heavy use can lead to the development of dependence. In Western societies, the critical period for drug experimentation is adolescence to young adulthood, although the particular age for each substance varies as a function of drug availability and the associated societal regulations and sanctions.
