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Teen pot smoking lowers adult IQ? A second look says maybe not

January 15, 2013|By Melissa Healy
  • A study suggesting that early and frequent use of marijuana erodes intelligence into adulthood may have missed a potent causal factor, says a new analysis. The real cause might be social and economic disadvantage.
A study suggesting that early and frequent use of marijuana erodes intelligence… (Ted S. Warren / Associated…)

In late August, baby boomers (and others whose teen years were spent in a haze of marijuana smoke) seemed to get the comeuppance they had long feared: A study suggested that early and frequent pot smoking resulted in depressed intelligence scores well into adulthood. But a new analysis suggests that in assigning blame for the lower IQ scores they found, the authors of that study may themselves have gotten caught in a haze of confusion.

Social and economic disadvantage in youth -- a factor that predisposes kids to early marijuana use as well as to adult lives that suppress intelligence scores -- may explain the earlier findings, asserts a Norwegian economist writing this week in the Proceedings of the National Academy of Sciences.

The original study, led by Duke University researcher Madeline Meier, is described here in a Booster Shots blog post (and here's the abstract). It found that in a large group of New Zealanders tracked from birth to age 38, those who initiated marijuana use before 18 and who smoked marijuana most frequently in adolescence had IQs that were five to six points lower by age 38. In adulthood, they performed, on average, more poorly on tests of reasoning, attention and working memory than did those who smoked little to no pot before the age of 18. The latter group picked up on average a point or two on IQ tests and fared better on tests of "executive function."

"Collectively, these findings are consistent with speculations that cannibis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects," the writers of the August PNAS study concluded.

But while Colorado and Washington were passing laws making recreational marijuana possession legal for those over 18, Ole Rogeberg of the Ragnar Frisch Centre for Economic Research in Oslo, Norway, was poring over the PNAS study in search of an alternative explanation for the pot smokers' loss of intelligence in adulthood. Rogeberg reviewed research on socioeconomic factors, self-control, educational attainment, occupational history and IQ and lined those findings up against the average trajectories shown in the Dunedin cohort -- the 1,037 New Zealand children studied in the earlier research.

"Simulation results suggest that SES-related cognitive decline is sufficient to reproduce" the results found in the early research, he writes. 

Those relationships are complicated and murky, and tend to reinforce each other in circular ways: As kids with higher IQs grow up, they will more often move into educational and career opportunities that provide greater cognitive challenges, with resulting IQ rewards in adulthood. In a group of kids with similar IQs but socioeconomically diverse backgrounds, those from more affluent homes with better-educated parents are more likely to follow the same path. And neither group -- those with higher IQs and those with social and economic advantages -- is likely to start smoking pot early and often, said Rogeberg.

By contrast, the socioeconomically disadvantaged and those who start with lower IQs are more likely to follow a less cognitively challenging life path -- including lower educational attainment and a higher prospect of brain-sapping time in jail. And research finds that those who start life with lower IQs are cognitively more harmed by environments devoid of stimulation and challenge than children who start life with higher IQs are helped by richer mental environments.

Rogeberg cites a 2006 study that found that between early childhood and young adulthood, the IQs of African American children decline an average 10 points.

"Adolescent-onset users are a group that (independently of cannabis use) would tend to be self-selected or sorted away from cognitively-demanding environments and consequently experience IQ-declines after childhood," Rogeberg suggests. Considering the socioeconomic mix of the kids studied, the range of IQs they started with, and their likely life trajectories, wrote Rogeberg, "the true effect" of adolescent marijuana use "could be zero."

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