Stereotypes do come true. Sometimes.

In the tell-us-something-we-don't-know department, a new study suggests that teenage marijuana use could damage the developing brain. They call it getting stupid for a reason.

Brazilian researchers found that subjects who started smoking weed before age 15 were less likely to be able to perform tasks related to attention, impulse control and “executive functioning.”

The study was published this month in the British Journal of Psychiatry.

According to the research abstract, the lab coats conclude that …

Early-onset chronic cannabis users exhibited poorer cognitive performance than controls and late-onset users in executive functioning. Chronic cannabis use, when started before age 15, may have more deleterious effects on neurocognitive functioning.

In other words — and no offense to the developmentally challenged here — the Black Eyed Peas' Let's Get Retarded was indeed an accurate description of teen cannabis smoking.

The researchers compared 104 heavy weed smokers — 49 started smoking before age 15 and 55 started after — and compared their skills at “neuropsychological tasks” to 44 non-smokers.

The good news: Those who started toking after 15 aren't so damaged, apparently. But yeah, smoking weed, while medicinal for some, for kids — not so good.

Like we said, tell us something we don't know.

[@dennisjromero/djromero@laweekly.com]

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