This rather complicated study looks at the “gateway theory” through data collected from 510 pairs of twins who participated in a very large, long-term health study — and the gateway theory doesn’t emerge with much backing. The authors write, “… although the gateway hypothesis implies that there is something about marijuana use that, by itself, potentiates the risk of hard drug use (Koob, 2002), our results imply that there is something genetic that renders the use of both marijuana and other drugs more likely for some people than others(see Kandel & Yamaguchi, 2002).”

H. Harrington Cleveland and Richard P. Wiebe, “Understanding the Association Between
Adolescent Marijuana Use and Later Serious Drug Use: Gateway Effect or Developmental Trajectory?” Development and Psychopathology 20, issue 2 (Spring 2008): 615-632
Free abstract and full text for a fee available at