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Twins Study Finds No Evidence That Marijuana Lowers IQ In Teens (sciencemag.org)
sciencehabit writes: Roughly half of Americans use marijuana at some point in their lives, and many start as teenagers. Although some studies suggest the drug could harm the maturing adolescent brain, the true risk is controversial. Now, in the first study of its kind (abstract), scientists have analyzed long-term marijuana use in teens, comparing IQ changes in twin siblings who either used or abstained from marijuana for 10 years. After taking environmental factors into account, the scientists found no measurable link between marijuana use and lower IQ.
That isn't how twin studies usually work. In this case, one twin decided to smoke marijuana, while the other did not. They weren't assigned roles in an experiment by researchers. Their parents either didn't have a problem with the drug, or they weren't involved in the decision.
You didn't read TFA. The study is of teenage twins who previously had/hadn't smoked marijuana (ie they were given questionaires, and twins were then selected for IQ testing if one had smoked and the other hadn't) not of twins who signed up to take part in a controlled experiment.
Parents either had nothing to do with it, or simply failed to prevent one of their twins taking the drug.
The study is probably, to a certain extent, flawed because of that methodology, but without commiting ethics violations of the type you describe, there's probably no way to get a more accurate result.
You are not alone. This is not normal. None of this is normal.
You are correct
From the study
Standardized measures of intelligence were administered at ages 9–12 y, before marijuana involvement, and again at ages 17–20 y. Marijuana use was self-reported at the time of each cognitive assessment as well as during the intervening period. Marijuana users had lower test scores relative to nonusers and showed a significant decline in crystallized intelligence between preadolescence and late adolescence. However, there was no evidence of a dose–response relationship between frequency of use and intelligence quotient (IQ) change.
You posted an comment so vague it is meaningless. You did not provide anything agreeing with or refuting what was written.
There's nothing to agree with or refute. If you take even a cursory look at the story, you will see that this was not the kind of study that the GP assumed it was.
Teenagers were not subjected to drug use for this study. Adults who had used marijuana as teens were studied.
The first hint was right there in the summary:
If we're studying the effects of marijuana use on teens, and ten fucking years have gone by, then what exactly is the probability that these subjects are still teens at the time of the study?
Go ahead, get out your calculator and I'll wait here for your answer.
You are welcome on my lawn.