"Gateway Effect" -- Is It Just Genetics?

Okay, I'm a bit behind in my reading, but a study published last month in the journal Addiction casts an interesting light on the so-called "gateway effect" -- the idea that use of one drug, usually marijuana, somehow leads to use of others.

Gateway associations have regularly been found between tobacco and marijuana: Young people who use one are pretty consistently more likely to use the other as well. But does tobacco cause kids to smoke marijuana, marijuana cause kids to use tobacco, or are both tendencies the result of other factors entirely?

The new study, by researchers in Queensland, Australia, and St. Louis, suggests that genetics, not the effects of any particular drug, are at the heart of these associations. The researchers studied over 500 pairs of twins, some identical and some fraternal, and did a lot of advanced number crunching to tease out connections between "early cannabis use" (use before age 17), later nicotine dependence, and the factors that may contribute to these outcomes.

Teens who had used marijuana early were indeed more likely to later become dependent on nicotine than those who did not. But, the researchers wrote, that increased risk "can be attributed largely to the effects of common genetic factors. ... [T]here remains no compelling evidence for causal processes linking EC [early cannabis use] to ND [nicotine dependence]."

This study only examined the relationships between marijuana and tobacco, not other drugs, but it's clearly another hit to claims that marijuana use somehow causes people to use other substances.