Marijuana, Gateways and Circuses

Picture a circus act: A giant clown leaps into the air and lands beside a poodle. As the clown lands, the poodle turns a flip -- as if they were on a trampoline and the clown's landing had launched him into the air. The crowd laughs. It knows both clown and poodle are on solid ground and the clown's landing did not propel the dog's flip, but because they occurred one after the other, it looks like that's what happened. We laugh because we know that just because one event precedes another, it doesn't mean that the first causes the second.

Nobody laughs at the "gateway theory," but perhaps they should. The mistaken notion that cannabis (commonly called marijuana) creates an urge for hard drugs the way eating salt makes people thirsty has about as much research support as the clown flipping the poodle. Unfortunately, a recent study in the Journal of the American Medical Association has been misinterpreted to suggest that cannabis causes hard drug use. A close look tells a very different story.

Australian researchers found 311 pairs of same-sex twins where one had tried cannabis before age 17 but the other had not. Whether identical or fraternal, the twins who were early cannabis users were more likely to try hard drugs later in life. And, just like the poodle and the clown, some have leapt to the conclusion that early marijuana use caused the later hard drug use.

If we believe that, we're sadly mistaken. The study's authors even say as much.

In this study, early tobacco and alcohol use were also significant predictors of drug problems. Undoubtedly, most people use these legal substances before cannabis because they're more available. But the possibility that these legal drugs are the gateway never appears in the study.

Worse, anyone who used a hard drug before using cannabis was dropped from the analysis. That's right: People who took downers or snorted cocaine before trying cannabis, or did anything else counter to the gateway theory, were omitted from the study, stacking the deck.

Still, something must propel these early users to try hard drugs. If not cannabis, then what?

The study does not reveal what would make teens try cannabis when their identical twins did not, but whatever this factor is, it undoubtedly also contributed to the other results. It's not genetics, since identical twins have the same genes. Nevertheless, anyone who knows identical twins knows they are not two copies of the same person.

One difference might be a personality trait called "sensation seeking." Sensation seekers like to ride in the front of the roller coaster, leap from airplanes and experiment with drugs. The trait is not completely heritable, and may account for why one person might smoke cannabis before his or her identical twin. It could also explain why those same twins might try speed or cocaine.

We all know troubled souls who use a lot of drugs, drive without seatbelts, have unsafe sex and engage in other risky behaviors. They use cannabis first because it's the most available substance, but the cannabis does not cause their use of other drugs. In fact, in neighborhoods where crack is more available than cannabis, people use crack first, but no one claims crack is a gateway drug to cannabis.

But one gateway effect probably is real: When kids try cannabis, they realize that the propaganda they've been fed is untrue. They don't take an axe to their siblings or flunk out of high school. They don't get terrorists showing up at their doors for cash. And they start to suspect that what they've been told about other drugs is equally false.

That's not just a theory. I teach a university class about the science of drugs, and every year my students describe similar experiences.

Cannabis does not cause kids to try harder drugs, but lying to them about it does.

Mitch Earleywine, Ph.D., is associate professor of psychology at the University of Southern California, and author of "Understanding Marijuana" (Oxford University Press, 2002).


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