Cannabis IQ impact defended
In August of last year New Zealand researchers, in collaboration US colleagues, published a study showing heavy and persistent teenage cannabis use led to a decline in IQ.
At the time, the study was lauded as one of the best efforts at untangling the myriad of factors which could influence IQ - thanks mainly to the fact the Dunedin-born participants in the study had been periodically monitored since birth.
This week however, the methods of the study were brought into question by one Ole Rogeberg, a Norwegian economist who published a critique of the study in the journal PNAS. Rogeberg claimed that certain socio-economic factors had not been taken into account and the link between cannabis and IQ - if there was any link at all - might be explained by these factors.
Rogeberg was very direct in his conclusions, stating "It would be too strong to say that the results have been discredited, but fair to say that the methodology is flawed and the causal inference drawn from the results premature."
The authors of the original study were blind-sided by the critique, confessing that neither the journal nor Rogeberg had contacted them about the concerns raised prior to publishing the article.
Nevertheless, they performed further checks on the data to test Rogeberg's assumptions, and found his concerns about the methods unfounded. In a response, the authors stated:
"The paper published today by Rogeberg has raised a seemingly plausible alternative explanation for the finding of an IQ decline in early and heavy cannabis users - but analyses of the Dunedin study data do not support it.
"The most plausible explanation for the data remains that using cannabis from adolescence and into young adulthood contributes to a decline in cognitive ability, as indicated by performance on IQ tests."
While the extra analyses have to some extent rebutted Rogebers claims, there remains some uncertainty over the basis for the causal link.
Thomas Lumley, Professor of Biostatistics at The University of Auckland, commented on the research, saying, "Even if the association is real and causal there could still be explanations that don't involve brain toxicity." He suggested a scenario in which cannabis users may gravitate to less cognitively demanding situations which in turn lead to lower IQ.
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