Pubdate: Mon, 25 Jan 2016
Source: Boston Globe (MA)
Copyright: 2016 Globe Newspaper Company
Contact: http://services.bostonglobe.com/news/opeds/letter.aspx?id=6340
Website: http://bostonglobe.com/
Details: http://www.mapinc.org/media/52
Author: Sharon Begley

DOES SMOKING POT ACTUALLY MAKE YOU STUPID?

Gut Check is a periodic look at health claims made by studies, 
newsmakers, or conventional wisdom. We ask: Should you believe this?

The Claim: Smoking marijuana does not make teenagers stupid, 
concludes a study in the current Proceedings of the National Academy 
of Sciences, contrary to other research that linked getting stoned to 
impaired cognitive function in adolescents.

The Backstory: 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. According to 2015 government data, 
15 percent of 10th graders and 21 percent of 12th graders have used 
marijuana in the past month.

Most studies have compared marijuana users to non-users at a single 
point in time, and so can't assess how marijuana changes brain 
function over the years; any observed differences might have always 
existed, marijuana or not. And all the studies have been 
observational - assigning some people to smoke marijuana and others 
to abstain would yield cleaner results but is obviously unethical. 
Given those limitations, it almost doesn't matter what the studies 
have found, but here's a sampling: marijuana use is associated with 
decreased intelligence, a 2007 study from New Zealand found; or with 
poorer memory, according to a 1996 study of Costa Rican men and a 
2002 US study of long-term users; or with poorer attention and verbal 
skills, a 2010 study reported. Other studies, but fewer, find no 
long-term association between marijuana and IQ, like this one from 
Canada in 2005 and this 2004 research on twins by scientists at 
Boston University.

In a key paper from 2012 - it was the largest and longest-running 
study to date, testing 1,037 people at age 13 and 38 - scientists at 
Duke University found a dramatic drop in intelligence among long-term 
marijuana users "suggestive of a neurotoxic effect of cannabis on the 
adolescent brain," they wrote in PNAS.

As is often the case, even reputable researchers publishing in 
reputable journals run roughshod over the difference between 
correlation (two phenomena go together) and causation (one causes the 
other). One 2015 paper declared that "regular cannabis use in 
adolescence approximately doubles the risks of ... cognitive 
impairment." No: At most, such use is associated with impairment. 
Observational research cannot rule out the possibility that a third 
factor causes both cognitive decline and marijuana use.

First Take: The new PNAS study is also observational, but with an 
important twist: It zeroes in on that "third factor" possibility by 
studying twins, 789 in California and 2,277 in Minnesota. 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). In hundreds of cases, 
one twin smoked and the other didn't.

Overall, users' IQ dropped 4 points relative to non-users over the 
study period. But more frequent marijuana use wasn't associated with 
greater IQ decline, as you'd expect if marijuana were toxic to brain 
function. And measures of so-called inherent intelligence, like 
problem-solving, didn't fall in users; on some measures, like 
puzzle-solving, scores actually rose. That also undermines the idea 
that marijuana impairs cognition. Instead, said co-author Joshua Isen 
of the University of Minnesota, it suggests that something was going 
on "in how much information they were absorbing," which can reflect 
truancy, conscientiousness, and other factors apart from inherent 
cognitive ability and brain function.

The strongest evidence against the idea that marijuana is neurotoxic 
was that marijuana-using twins showed no greater IQ decline than 
their non-using siblings. "This fails to support the implication of 
the [2012] study that adolescent use of marijuana causes 
neurocognitive declines," the researchers wrote in PNAS. Instead, it 
suggests that any drop in brain function is not from the drug but 
from "factors that underlie both marijuana initiation and low 
intellectual attainment" and are present in both twins' lives.

That is, whatever spurred teenagers to start using also hurt their IQ.

Second Take: Although the findings challenge the idea that marijuana 
use hurts cognitive ability, it's not the final word. If the drug has 
only a minor effect, this study might have been too small to find it. 
And since the researchers didn't measure working memory or executive 
function (judgment, decision-making ability, and other higher 
cognitive skills), they couldn't rule out marijuana's effect on 
those. Also, some twins dropped out of the research; if they were 
greater users than those who stayed, the study would have missed a 
possible effect of heavy marijuana use.

The Takeaway: This study can't rule out the possibility that 
marijuana hurts teenagers' brains, especially in ways that show up 
only after decades. But it undermines claims that it does.
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MAP posted-by: Jay Bergstrom