"Overall, supply reduction—that is, reducing the availability of drugs—does not appear to have played as major a role as many had assumed in four of the five most important downturns in illicit drug use that have occurred to date, namely, those for marijuana, cocaine, crack, and ecstasy (see, for example, Figures 8-4, 8-5, and 8-6). The case of cocaine is particularly striking, as perceived availability actually rose during much of the period of downturn in use that began in the mid- 1980s. (These data are corroborated by data from the Drug Enforcement Administration on trends in the price and purity of cocaine on the streets.⁸) For marijuana, perceived availability has remained very high for 12th graders since 1976, while use dropped substantially from 1979 through 1992 and has fluctuated considerably thereafter. Perceived availability for ecstasy did increase in parallel with increasing use in the 1990s, but the decline phase for use appears to have been driven much more by changing beliefs about the dangers of ecstasy than by any sharp downturn in availability. Similarly, amphetamine use declined appreciably from 1981 to 1992, with only a modest corresponding change in perceived availability. Finally, until 1995, heroin use had not risen among 12th graders even though availability had increased substantially.

"• What did change dramatically were young peoples’ beliefs about the dangers of using marijuana, cocaine, crack, and ecstasy. We believe that increases in perceived risk led to a decrease in use directly through their impact on young people’s demand for these drugs and indirectly through their impact on personal disapproval and, subsequently, peer norms. Because the perceived risk of amphetamine use was changing little when amphetamine use was declining substantially (1981–1986), other factors must have helped to account for the decline in demand for that class of drugs—quite conceivably some displacement by cocaine. Because three classes of drugs (marijuana, cocaine, and amphetamines) have shown different patterns of change, it is highly unlikely that a general factor (e.g., a broad shift in attitudes about drug use) can explain their various trends.

"• The increase in marijuana use in the 1990s among 12th graders added more compelling evidence to this interpretation. It was both preceded and accompanied by a decrease in perceived risk. (Between 1991 and 1997, the perceived risk of regular marijuana use declined 21 percentage points.) Perceived peer disapproval dropped sharply from 1993 through 1997, after perceived risk began to change, consistent with our interpretation that perceived risk can be an important determinant of disapproval as well as of use. Perceived availability remained fairly constant from 1991 to 1993 and then increased seven percentage points through 1998.⁹

"• We do think that the expansion in the world supply of heroin, particularly in the 1990s, had the effect of dramatically raising the purity of heroin available on the streets, thus allowing for new means of ingestion, such as snorting and smoking. The advent of new forms of heroin, rather than any change in respondents’ beliefs about the dangers associated with injecting heroin, very likely contributed to the fairly sharp increase in heroin use in the 1990s. Evidence from this study, showing that a significant portion of the self-reported heroin users in recent years are using by means other than injection, lends credibility to this interpretation. The dramatic decline in LSD use in the early to mid 2000s is also not explainable by means of concurrent changes in perceived risk or disapproval; but availability did decline sharply during this period and very likely played a key role in reducing the use of that drug.

"We should also note that other factors, such as price, could play an important role for some drugs. Analyses of MTF data have shown, for example, that price probably played an important role in the decline of marijuana use in the 1980s, and in changes in cigarette use in the 1990s.^10,11 However, price does not appear to have the same influence in all periods for all drugs, as the dramatic reduction in cocaine prevalence during the late 1980s took place at the same time that the price of cocaine decreased,¹² contrary to the supply/demand model."