Abundant research in psychology, human development, and other fields has shown that events and circumstances early in peoples’ lives influence future decisions, life events, and life circumstances—or what is called the life course trajectory. People who use drugs typically begin doing so during adolescence or young adulthood (see “Adolescent Drug Use”), but the ground may be prepared for drug use much earlier, by circumstances and events that affect the child during the first several years of life and even before birth.