“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,” Ole Røgeberg, the sole author of the new study and an economist at The Ragnar Frisch Centre for Economic Research in Oslo, wrote in the paper.
Both papers used data from the Dunedin Study, which followed more than 1,000 New Zealanders from birth to until age 38 and interviewed them periodically about cannabis use starting at age 18. Previous analyses of the Dunedin Study showed that starting to use marijuana young and becoming dependent on it were both associated with low socioeconomic status. Røgeberg said that low socioeconomic status, which may come with lower educational attainment and less challenging work, is associated with IQ declines.
But Madeline Meier, a psychologist at Duke University and author of the original PNAS study, said that her study controlled for socioeconomic status and found that, among participants who did not use marijuana heavily in their youths, IQ remained the same throughout the study regardless of socioeconomic status.
Other research has shown that in the short-term, marijuana use is related to reversible problems with memory and focus.