Publication Type: Conference Paper/Unpublished Manuscript
Abstract: Because marijuana use often precedes the use of other psychoactive substances, it is often thought of as a contributing cause of, or "gateway" to, later use of more serious, so-called "hard" drugs. We examined this hypothesized "gateway effect" with data from both monozygotic (MZ) and dizygotic (DZ) twin pairs drawn from the National Longitudinal Study of Adolescent Youth (Add Health). Difference score analyses reveal that within-pair differences in early marijuana use, controlling for differences in early hard drug use and peer marijuana use, predicted later within-pair hard drug use differences for DZ twin pairs. Among MZ pairs, in contrast, early differences in marijuana use did not predict later hard drug use differences, as they should have under the gateway hypothesis. Rather than supporting the interpretation that early marijuana use triggers later hard drug use, these results suggest that the pattern of drug use escalation that has been labeled the "gateway effect" might be better conceptualized as a genetically-influenced developmental trajectory, and that marijuana is often used earlier than other drugs simply because it is more widely available and socially acceptable, and is perceived of as less dangerous, than hard drugs.