Dumb and dumber? Teen marijuana use linked to lower IQ in later life
Those who regularly used marijuana as teens, the study found, lost significant IQ points between their 13th and 38th years. Friends and associates also reported more cognitive problems among regular pot users, and even those who quit did not entirely regain all the ground they had lost in their youth.
The data is so rich that Meier and her colleagues were able to deflect two challenges to their conclusions, and so far no more have surfaced.
One critic from Norway published a critique suggesting that socioeconomic differences may have been the real culprit. Perhaps, he posited, people with duller careers and less stimulating associates had their IQs artificially boosted during their schooling years, and then failed to maintain mental growth as they aged.
Meier and her colleagues went back to the original data and broke it down again, this time focusing strictly on middle class kids, and found that the results held firm.
Another challenger suggested that kids who used pot were naturally lacking in self-control, and thus likely to see their IQ slip as they aged.
Again re-analyzing the Dunedin data, the researchers demonstrated that marijuana’s impact on IQ took place regardless of how much self-control the subject had in childhood.
Meier readily admits the limits of this kind of observational study, which — unlike controlled, clinical studies — cannot determine causation and is often confounded by unseen variables.
“With observational data there could always be an alternative explanation,” Meier said. “However, we did rule out some of the best and most plausible explanations.”
One of the key findings was that the IQ reduction does not occur if the user began smoking marijuana after adulthood. This, Meier says, has led some to see validation in the study for legalization among adults.
“The association seems clear but causal mechanisms not fully understood,” Wilson Compton said. “What we need is additional work.”
The NEJM article by Volkow, Compton and their co-authors cites research showing “impaired neural connectivity” among users who began smoking pot in their teens — including areas of the brain that affect alertness, learning and memory. They also cite studies showing reduced function in the prefrontal networks, which manage conscience and self-control.
All of this is not really surprising, they argue, since the developing brain is peculiarly vulnerable to damage in adolescence and early adulthood.
The NIDA team is currently planning (and arranging funding for) a study that will follow 10,000 children from age 10 through adulthood, looking at the impact of numerous substances and behaviors on the brain. The team will do biological tests and interviews, as well as functional magnetic resonance imaging to see what the brain is doing in real time.
The key to such an ambitious study, Compton said, will be follow-up rates. Many studies struggle to keep track of people over many years, he said, but there are models for what works. Persistence is critical, he said, because tracking down people who move is tough.
“The science and the art is to not make it too burdensome,” he says. “You have to make it interesting and important enough for them that they will be willing to continue.”
By the time the new study is funded, launched and completed 20 years later, an entire generation will have grown up under shifting attitudes toward and usage of marijuana.
And, if Meier and her colleagues are right, many of these newly minted adults will be carrying permanent mental handicaps acquired in the experiment.
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