The myth that smoking marijuana makes you less intelligent has just been effectively debunked. After analyzing studies of twins in the U.S., researchers at the University of Southern California and Loyola Marymount University found no evidence to support claims that cannabis use during adolescence was associated with a lower IQ.

The study, published by the Proceedings of the National Academy of Sciences on Monday, challenges earlier research that seemed to link teenage marijuana use to IQ drops. If anything, smoking pot in your early years appears to be a symptom of something else that causes drops in test scores, the researchers concluded.

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Twin studies are especially useful in this field of research. By looking at data from more than 3,300 test participants, the team was able to compare intelligence tests the twins had taken over the course of 10 years and identify environmental factors that might have influenced their results. Marijuana didn't seem to be one of them.

The data revealed no significant differences in the test results of marijuana users and non users. The assumption — that the people who smoked more pot (daily use or more than 30 times a month) would do worse — proved unfounded. While marijuana users experienced a four-point IQ drop over 10 years, their twin siblings who didn't smoke had similar results. So something else appears to be at the root of the decline.

"Our findings lead us to believe that this 'something else' is related to something about the shared environment of the twins, which would include home, school, and peers," said statistician Nicholas Jackson, the lead author of the study.

Valerie Curran, a psychopharmacologist at the University College London, called the study "very well-conducted" and "a welcome addition to the literature." Earlier this month, Curran and a team of researchers published a similar study on the effect of marijuana on the IQ of 2,000 (non-twin) British teenagers. They had reached "broadly the same conclusions," she told Science Magazine.

The idea that marijuana makes you dumber is one of the most common claims made by legalization opponents who argue that pot "fries your brain." But as researchers continue to explore the relationship between marijuana use and cognitive development, that argument appears to be falling apart.

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While the best way to conduct studies on marijuana's effect on the brain would be to give the drug to people and analyze the effects of duration, frequency, and dose, those types of studies "are nearly impossible due to federal restrictions,” Jackson said.

"I'm mostly concerned about what's going on in the child's environment, that a 14-year-old is seeking refuge in drugs," he added.