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Of all the arguments that have been used to demonize marijuana, few have been more powerful than that of the “gateway effect”: the notion that while marijuana itself may not be especially dangerous, it ineluctably leads to harder drugs like heroin and cocaine. Even Nick Kristof — in a column favoring marijuana legalization — alluded to it this week in the New York Times. In what is known as the “to be sure” paragraph, where op-ed writers cite the arguments of opponents, he wrote:

I have no illusions about drugs. One of my childhood friends in Yamhill, Ore., pretty much squandered his life by dabbling with marijuana in ninth grade and then moving on to stronger stuff. And yes, there’s some risk that legalization would make such dabbling more common.

The idea that marijuana may be the first step in a longer career of drug use seems plausible at first: when addicts tell their histories, many begin with a story about marijuana. And there’s a strong correlation between marijuana use and other drug use: a person who smokes marijuana is more than 104 times more likely to use cocaine than a person who never tries pot, according to the National Institute on Drug Abuse. (More on Time.com: 7 Tips for California: How to Make Legalizing Marijuana Smart)

The problem here is that correlation isn’t cause. Hell’s Angels motorcycle gang members are probably more 104 times more likely to have ridden a bicycle as a kid than those who don’t become Hell’s Angels, but that doesn’t mean that riding a two-wheeler is a “gateway” to joining a motorcycle gang. It simply means that most people ride bikes and the kind of people who don’t are highly unlikely to ever ride a motorcycle.

Scientists long ago abandoned the idea that marijuana causes users to try other drugs: as far back as 1999, in a report commissioned by Congress to look at the possible dangers of medical marijuana, the Institute of Medicine of the National Academy of Sciences wrote:

Patterns in progression of drug use from adolescence to adulthood are strikingly regular. Because it is the most widely used illicit drug, marijuana is predictably the first illicit drug most people encounter. Not surprisingly, most users of other illicit drugs have used marijuana first. In fact, most drug users begin with alcohol and nicotine before marijuana — usually before they are of legal age. In the sense that marijuana use typically precedes rather than follows initiation of other illicit drug use, it is indeed a “gateway” drug. But because underage smoking and alcohol use typically precede marijuana use, marijuana is not the most common, and is rarely the first, “gateway” to illicit drug use. There is no conclusive evidence that the drug effects of marijuana are causally linked to the subsequent abuse of other illicit drugs.

Since then, numerous other studies have failed to support the gateway idea. Every year, the federal government funds two huge surveys on drug use in the population. Over and over they find that the number of people who try marijuana dwarfs that for cocaine or heroin. For example, in 2009, 2.3 million people reported trying pot — compared with 617,000 who tried cocaine and 180,000 who tried heroin. (More on Time.com: See photos of cannabis conventions)

So what accounts for the massive correlation between marijuana use and use of other drugs? One key factor is taste. People who are extremely interested in altering their consciousness are likely to want to try more than one way of doing it. If you are a true music fan, you probably won’t stick to listening to just one band or even a single genre — this doesn’t make lullabies a gateway to the Grateful Dead, it means that people who really like music probably like many different songs and groups.

Second is marijuana’s illegality: you aren’t likely to be able to find a heroin dealer if you can’t even score weed. Compared with pot dealers, sellers of hard drugs tend to be even less trusting of customers they don’t know, in part because they face greater penalties. But if you’ve proved yourself by regularly purchasing marijuana, dealers will happily introduce to you to their harder product lines if you express interest, or help you find a friend of theirs who can.

Holland began liberalizing its marijuana laws in part to close this particular gateway — and indeed now the country has slightly fewer young pot-smokers who move on to harder drugs compared with other nations, including the U.S. A 2010 Rand Institute report titled “What Can We Learn from the Dutch Cannabis Coffeeshop Experience?” found that there was “some evidence” for a “weakened gateway” in The Netherlands, and concluded that the data “clearly challenge any claim that the Dutch have strengthened the gateway to hard drug use.” (More on Time.com: Is Marijuana Addictive? It Depends How You Define Addiction)

Of course, that’s not the gateway argument favored by supporters of our current drug policy — but it is the one supported by science.

More on Time.com:

Best of ‘Stoner Cinema’

The Marijuana Number That Was Too Good to Check

Is Drug Use Really on the Rise?