Alcohol, meanwhile, is most users' first drug, with 88 percent having never tried another substance before they started drinking. And while the number of people who stop at booze is slightly higher (50 percent) than the number who go no further than marijuana (40 percent), alcohol serves as its own sort of gateway, with a third of drinkers moving on to pot as their next drug.

It's fun data to play around with (who are these insane hipsters who snorted cocaine before they ever drank?), but the creators caution that it's important not to read too much into it: "These are snapshots of people of different ages, and some of them may go on to try other drugs in the future," they note.

What's more, these lines aren't gateways, per se. They're more like pathways, revealing the trajectories drug users tend to take as they bounce from one substance to another (or, in many cases, simply stick with their drug of choice).

And that brings up an important flaw of the gateway theory in general. Science writers and readers are fond of saying that correlation does not imply causation, and this is a perfect example. Let's say 11 percent of pot smokers start using cocaine, as this graphic shows. That doesn't mean one drug led to the other. As Miriam Boeri, an association professor of sociology at Bentley University points out, poverty, mental illness, and friend groups are all much stronger predictors of drug use. Marijuana isn't a "gateway" to harder drugs in the same way that ordering an appetizer isn't a "gateway" to an entree: One comes before the other, but you're eating both because you're already at the restaurant.

Mark Kleiman, the University of California, Los Angeles, marijuana-policy expert and Washington's "pot czar," once told PBS that marijuana might be a type of gateway because it introduces kids to illegal behavior. "It can get them into illicit drugs because it gets them to know people who sell illicit drugs, who might be prepared to sell them things other than cannabis," he said. But, as Kleiman notes, proponents of legalization see this as another reason why buying and selling pot shouldn't be against the law. Somehow I don't think that's quite what D.A.R.E. had in mind.

What's more, the scientist who coined the "gateway" term recently came out with a new paper showing that it's actually nicotine that is, biologically, the most potent of gateway of all. When rodents were primed with nicotine, then given cocaine, they liked the cocaine much more. In that case, the fact that e-cigarette use has tripled among teens in the past year should be the more worrisome trend.