And measures of so-called inherent intelligence, like problem-solving, didnвЂ™t fall in users on some measures, like puzzle-solving, scores actually rose. But more frequent marijuana use wasnвЂ™t associated with greater IQ decline, as youвЂ™d expect if marijuana were toxic to brain function. Overall, usersвЂ™ IQ droppedВ 4 points relative to non-users over the study period. In hundredsВ of cases, one twin smoked and the other didnвЂ™t. The twins underwent standard intelligence tests at ages 9 to 12 and again at 17 to 20, and reported whether or not they smoked marijuana (60 percent in California and 36 percent in Minnesota did). Other studies, but fewer, find no long-term association between marijuana and IQ, like this one from Canada in 2005 and thisВ 2004 researchВ on twins by scientists at Boston University.ДЇirst Take:В The new PNAS study is also observational, but with an important twist: It zeroes in on that вЂњthird factorвЂќ possibility by studying twins, 789 in California and 2,277В in Minnesota. Given those limitations, it almost doesnвЂ™t matter what the studies have found, but hereвЂ™s a sampling: marijuana use is associated with decreased intelligence, a 2007 studyВ from New Zealand found В or with poorerВ memory, according to a 1996 study of Costa Rican men and a 2002 US study of long-term users В or with poorer attention and verbal skills, a 2010 studyВ reported.

And all the studies have been observational -В assigning some people to smoke marijuana and others to abstain would yield cleaner results but is obviously unethical. MostВ studies have compared marijuana users to non-users at a single point in time, and so canвЂ™t assess how marijuana changes brain function over the years any observedВ differences might have always existed, marijuana or not. According to 2015 government data,В 15 percent of 10thВ graders and 21 percent of 12thВ graders have used marijuana in the past month. The Backstory:В Research on how marijuana affects the developing brain goes back decades, but has heated up as more US states decriminalize or legalize the drug for adults. The Claim: Smoking marijuana does not make teenagersВ stupid, concludes a study in the currentВ Proceedings of the National Academy of Sciences, contrary to other researchВ that linkedВ getting stoned to impaired cognitive function in adolescents. Gut Check is a periodic look at health claims made by studies, newsmakers, or conventional wisdom. Exclusive analysis of biotech, pharma, and the life sciences Learn More

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