The 'marijuana will rot your brain' debate: neurological differences don't necessarily mean impairment

There's a new study out that's being touted as proof that marijuana makes you dumb. But, while the results do show differences in the brains of people who smoked pot, the conclusion about what that means is seriously flawed, writes Maia Szalavitz at The Daily Beast.

Most of the time, it's difficult to explain why scientific research or a conclusion about research results is flawed. That's not the case here. You only have to understand two concepts: "normal" and "healthy".

The 20 marijuana-smoking participants, who took the drug at least once a week, were deliberately selected to be healthy. If they had any marijuana-related problems—or any psychiatric problems or other issues—they were excluded from participating.

Are you beginning to see what’s wrong? Although the pot-smoking participants showed brain differences in comparison to the controls who were also selected to be normal— both groups were normal! If the smokers had any marijuana-related problems or any type of impairment, they would not have been included in the first place. Therefore, the brain changes that the researchers found were—by definition—not associated with any cognitive, emotional, or mental problems or differences.