a1 Pennsylvania State University
a2 Fitchburg State College
Because marijuana use often precedes the use of other psychoactive substances, it has been characterized as a gateway to these other substances. The present study used data from both monozygotic (MZ) and dizygotic (DZ) twin pairs drawn from the National Longitudinal Study of Adolescent Youth (Add Health) to examine the “gateway effect” role of earlier marijuana on later hard drug use. Difference score analyses reveal that within-pair differences in earlier marijuana use, controlling for differences in earlier hard drug use, and peer marijuana use predicted later within-pair hard drug use differences for DZ twin pairs. In contrast, earlier differences in marijuana use among MZ twin pairs did not predict later hard drug use differences. Rather than supporting the interpretation that earlier marijuana use “triggers” later hard drug use, these results suggest that the longitudinal pattern of drug use that has been interpreted as the “gateway effect” might be better conceptualized as a genetically influenced developmental trajectory.
c1 Address correspondence and reprint requests to: H. Harrington Cleveland, Department of Human Development and Family Studies, S113-C Henderson, Pennsylvania State University, University Park, PA 16802; E-mail: [email protected]
This research uses data from Add Health, a program project designed by J. Richard Udry, Peter S. Bearman, and Kathleen Mullan Harris, and funded by Grant P01-HD31921 from the National Institute of Child Health and Human Development, with cooperative funding from 17 other agencies. Special acknowledgment to Ronald R. Rindfuss and Barbara Entwisle for assistance in the original design. We also acknowledge the assistance of Jacquelyn D. Wiersma and Guang Guo and thank the anonymous reviewers of this manuscript for their perspicacity. Persons interested in obtaining data files from Add Health should contact Add Health, Carolina Population Center, 123 W. Franklin Street, Chapel Hill, NC 27516-2524 ([email protected]).