By the 1950s, Federal Bureau of Narcotics Commissioner Harry Anslinger had backed away from his claim that marijuana turns people into murderers. Instead he began arguing that it turns them into heroin addicts. "Over 50 percent of those young addicts started on marijuana smoking," Anslinger told a congressional committee in 1951. "They started there and graduated to heroin; they took the needle when the thrill of marijuana was gone."
Half a century later, this idea, known as the "gateway" or "stepping stone" theory, remains a bulwark of marijuana prohibition. Its durability is largely due to its ambiguity: Because it's rarely clear what people mean when they say that pot smoking leads to the use of "harder" drugs, the claim is difficult to disprove.
Survey data indicate that heroin and cocaine users generally use marijuana first, and that people who try pot are much more likely than people who don't to try other drugs. But there are several ways of interpreting these facts. A recent study by the RAND Corporation's Drug Policy Research Center, for example, found that a general predisposition to use drugs, combined with a four-year lag between access to marijuana and access to other illegal intoxicants, was enough to account for the patterns observed in the government's surveys.
"The people who are predisposed to use drugs and have the opportunity to use drugs are more likely than others to use both marijuana and other drugs," said Andrew Morral, the lead author of the study, which appeared in the December issue of the journal Addiction. "Marijuana typically comes first because it is more available. Once we incorporated these facts into our mathematical model of adolescent drug use, we could explain all of the drug use associations that have been cited as evidence of marijuana's gateway effect."
Case closed? Not quite. A study reported in this week's Journal of the American Medical Association surveyed 311 pairs of Australian twins in which one used marijuana by age 17 and one did not. The researchers found that the early cannabis users were more likely than their twins to use other drugs. They were four times as likely to use psychedelics, three times as likely to use cocaine or other stimulants, and more than twice as likely to use opioids.
These relative probabilities may sound impressive, but they're quite modest compared to the numbers usually cited by defenders of the war on drugs. The prohibitionist propaganda mill known as the Center on Addiction and Substance Abuse, for example, trumpets the fact that "12-to-17-year-olds who smoke marijuana are 85 times more likely to use cocaine than those who do not." The results of the twin study suggest that almost all of this difference is due to environmental and personality factors, as indicated by RAND's analysis.
Even with twins, of course, there are differences in environment and personality. The study's results were similar for monozygotic ("identical") and dizygotic ("fraternal") twins, which suggests that genetic differences of the magnitude seen in siblings are not important in determining who uses the "harder" drugs. But both kinds of twins clearly differed in significant respects; otherwise, it would not have been the case that one from each pair used marijuana early while the other did not. If one twin happens to be less risk-averse or more rebellious, or if he happens to have friends who know where to get pot, that factor could explain both his early marijuana use and his subsequent use of other drugs.
The researchers, for their part, speculated that the link between early pot smoking and later drug use "may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs."
To expand on that point a bit, the government's decision to put marijuana in the same category as cocaine and heroin may contribute to a gateway effect in three ways:
1) Once teenagers break the law to try pot, they are less reluctant to break the law to try other drugs.
2) Once they discover that the government has been lying about marijuana, they are less inclined to believe official warnings about other drugs.
3) Once they buy marijuana on the black market, they are more likely to have the opportunity to buy other drugs.
A more obvious explanation for the connection between pot smoking and other drug use is that people who discover that they like marijuana may be more inclined to try other psychoactive substances, in the same way that people who discover that they like bungee jumping may be more inclined to try sky diving. You could say that bungee jumping is a gateway to sky diving.
Notice that none of these interpretations involves a specific pharmacological effect of the sort drug warriors seem to have in mind when they suggest that pot smoking primes the brain for cocaine or heroin. As a National Academy of Sciences panel observed in a 1999 report, "There is no evidence that marijuana serves as a stepping stone on the basis of its particular drug effect." Last year the Canadian Senate's Special Committee on Illegal Drugs likewise concluded that "cannabis itself is not a cause of other drug use. In this sense, we reject the gateway theory."
Of course, it all depends on which "sense" you have in mind. A few years ago in the Drug Policy Analysis Bulletin, the social psychologist Robert MacCoun laid out seven—count 'em, seven—different versions of the gateway theory. "Given our current state of knowledge," he concluded, "one can coherently argue that (a) the gateway is a myth—it doesn't exist; (b) the gateway is very real and it shows why we must sustain or strengthen our ban on marijuana, or (c) the gateway is very real and it shows why we should depenalize or even legalize marijuana."
A theory that versatile will never die.