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Associated Press

Scientists have linked teenage marijuana use with a host of undesirable outcomes: difficulty in paying attention, weaker memory and lower verbal ability and intelligence.
But is the drug itself to blame?
Two long-term studies of twins published Monday suggest that other factors are at fault, at least as far as vocabulary skills are concerned.
In one study, children who went on to become marijuana users were not as bright to begin with as their abstinent peers. And in both studies, drug-using teens fared no worse on IQ tests than their non-using twins in the same household, suggesting that some other factor was to blame, the authors wrote.
Something in the family environment perhaps, or simply the fact that kids who gravitate toward pot use may be less motivated to try hard in school.
Results of the two studies, described in a paper in Proceedings of the National Academy of Sciences, surely will not settle the debate.
In interviews, two of the authors cautioned that even if marijuana does not directly impair intelligence in the developing brain, there are plenty of other reasons that teenagers should steer clear of weed.
The studies did not measure executive function — an umbrella term that covers the ability to focus and organise, among other skills — and previous research suggests that pot use can weaken it.
And despite the push to legalise marijuana in some states, it remains illegal for teens, said the University of Pennsylvania’s Adrian Raine, who helped design the studies with the University of Southern California’s Laura A. Baker, the paper’s senior author.
“We’re only talking about verbal ability,” said Raine, a professor of criminology, psychiatry and psychology at Penn. “There’s more to life than verbal ability.”
Joshua D. Isen, a co-lead author of the paper, cautioned that teen use of the drug also is associated with poorer performance in school.
“I’d think marijuana use could be a symptom of a larger problem,” said Isen, a University of Minnesota post-doctoral fellow and a lecturer at Loyola Marymount University in Los Angeles.
It is difficult to study how pot affects kids because it would be unethical and illegal to undertake the gold-standard study, a randomised controlled trial in which one group would be given an illegal substance and compared to another group. The paper’s authors acknowledged as much, calling their work “quasi-experimental.”
One of the studies started with 2,277 Minnesota youths age 11 to 12, 93 per cent of whom were white. The other tracked 789 adolescents from the Los Angeles area, age 9 to 10, three-quarters of whom were racial minorities or of mixed ethnicity.
By the time they reached their late teens, 36 per cent of the Minnesotans said they used marijuana. A subset of them was included in the study because they had shown disruptive behaviour and “academic disengagement.”
In the California group, 60 per cent of the teens reported they had used the drug, but they were 20 when surveyed — about two years older than their Minnesota counterparts.
In Minnesota, the future users started out two points lower on the vocabulary portion of an IQ test, and six years later the gap had widened by 4 points. In California, the future users and non-users started out with similar scores as young adolescents, but the users scored 3.4 points lower when tested at age 20.
The researchers did not find similar before-and-after disparities in some other IQ subtests, such as one called “block design,” which involves rearranging coloured blocks to match a pattern.
The researchers cited two results in concluding that the drug does not directly impair intelligence.
Not good enough, said Madeline H. Meier, an Arizona State assistant professor of psychology.
Meier led a previous study in which participants were followed up to age 38, in which marijuana use did appear to lead to cognitive decline.
“The problem is that for some teens,” she said, “short-term low-level cannabis use leads onward to long-term dependence on cannabis when they become adults.”
Raine, the Penn professor, agreed that the drug is not benign.
“I’m not convinced that there are no effects at all of taking marijuana,” he said.


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FILE - In this Dec. 6, 2012, file photo, a person holds a freshly-rolled marijuana joint just after midnight at the Space Needle in Seattle. A new analysis released Monday, Jan. 18, 2016, is challenging the idea that smoking marijuana during adolescence can lead to declines in intelligence. Instead, the new study says, pot smoking may be merely a symptom of some other problem that is really responsible for a brainpower effect seen in some previous research. (AP Photo/Ted S. Warren, File)

This Friday, Dec. 18, 2015, photograph, shows the logo on the front of jars of marijuana buds marketed by rapper Snopp Dogg in one of the LivWell marijuana chain's outlets south of downtown Denver. LivWell grows the Snoop pot alongside many other strains on its menu. (AP Photo/David Zalubowski)

A person holds a freshly-rolled marijuana joint just after midnight at the Space Needle in Seattle. A new analysis released Monday is challenging the idea that smoking marijuana during adolescence can lead to declines in intelligence. Photo: AP

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