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.

Aspergillus niger and A. nidulans were grown separately in glucose-limited chemostat cultures on modified Vogel's medium. Periodic selection (the appearance of new mutant populations) in A. niger was determined by monitoring oscillations in the concentration of cycloheximide-resistant mycelial fragments in samples from the fermenter vessel. Using these data, the interval between the periodic selection of each new mutant population was 32±6 generations (mean±S.E.M.). Periodic selection in cycloheximide resistance in three morphologically distinct sub-populations of the A. niger culture averaged 36±2, 31±11 and 28±5 generations and these values were not significantly different from that of the whole population (32±6 generations). Also, it was possible to estimate the rate of evolution taking place in these cultures by monitoring oscillations in the frequency of morphological mutants. The intervals between the periodic selection of morphological mutant populations in A. niger and A. nidulans were 28±3 and 22±2 generations respectively.