A longitudinal study that found teens who smoked marijuana had lower IQ scores in adulthood has been challenged by a researcher at the Frisch Centre for Economic Research in Norway.

In an analysis published online by the Proceedings of the National Academy of Sciences, Ole Røgeberg said the original study of almost 1,000 participants in New Zealand over nearly four decades was “flawed and the causal inference drawn from the results premature.”

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The original study found those who started smoking marijuana before 18 years of age tended to score lower on IQ tests in adulthood. There was no decline in IQ for those who began smoking after 18 years of age, suggesting the teenage brain was particularly vulnerable to the drug.

However, Røgeberg found the participants’ socioeconomic status — their income, education, occupation, and family history — could explain the apparent correlation between marijuana use and decline in IQ scores.

Though his analysis does not entirely debunk the original study, it does suggest that factors besides marijuana could be the real driving force behind the IQ decline. Due to the confounding variables, the true effect of marijuana on IQ scores could potentially be zero, Røgeberg wrote in his abstract.

The authors of the original study disagree. They told the Associated Press the results of their study remained even after their data was re-examined in light of Røgeberg’s criticisms.