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Instead, the analysis indicated that the earlier findings could have been thrown off by another factor -- the effect of poverty on IQ.
The author of the new analysis, Ole Rogeberg, cautioned that his theory may not hold much water. "Or, it may turn out that it explains a lot," said Rogeberg, a research economist at the Ragnar Frisch Center for Economic Research in Oslo, Norway.
The authors of the initial study responded to a request for comment with a joint statement saying they stand by their findings. "While Dr. Rogeberg's ideas are interesting, they are not supported by our data," wrote researchers Terrie Moffitt, Avshalom Caspi and Madeline Meier. Moffitt and Caspi are psychology professors at Duke University, while Meier is a postdoctoral associate there.
Their study, published in August in the Proceedings of the National Academy of Sciences, attracted media attention because it suggested that smoking pot has more than short-term effects on how people think.
Based on an analysis of mental tests given to more than 1,000 New Zealanders when they were 13 and 38, the Duke researchers found that those who heavily used marijuana as teens lost an average of eight IQ points over that time period. It didn't seem to matter if the teens later cut back on smoking pot or stopped using it entirely.
In the short term, people who use marijuana have memory problems and trouble focusing, research has shown. So, why wouldn't users have problems for years?
"The question reminds me of something adults say when kids make weird faces: 'Careful, or your face will stay that way,'" Rogeberg said. "It is certainly possible that in the long term, heavy cannabis use has permanent or persistent effects on the brain. But to find out what these changes are and what they mean is not easy. We can't just look at the short-term effects and assume that these gradually become fixed and permanent over time."
In his report, Rogeberg used simulation computer modeling to argue that the initial study was possibly flawed because of the effects of poverty on IQ.
"Recent research indicates that IQ and brainpower are kind of like muscular strength: strengthened if it is regularly challenged. IQ is strengthened or sustained by taking education, studying hard, spending time with smart, challenging people, doing demanding work in our jobs," he said. "Some kids, unfortunately, are burdened with a poor home environment, poor self-control and conduct problems. These kids are likely to gradually shift away from the kinds of activities and environments that would exercise their IQs."
Rogeberg, whose report appears in this week's online issue of the Proceedings of the National Academy of Sciences, suggests that the initial study didn't properly take this into account. "Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature," he wrote.
In their response, the Duke researchers said that only 23 percent of the people they studied were from poor families, making it unlikely that these participants threw off the overall results. And, they added, their results were the same when they only focused on people from middle-class families.
The Duke team also noted that another group shows similar results from marijuana exposure: rats. And, as they pointed out, rats don't go to school or fall into rich, middle-class or poor categories.
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