Young teens who smoke cannabis may not be stunting their intellect, says a new study challenging a claim by New Zealand scientists last year.
The earlier, and widely-praised, Dunedin study found people who started smoking cannabis in their teenage years, and kept using it for years afterwards, showed an average decline in IQ of eight points.
It sampled more than 1000 people, comparing their IQs from when they were 13 to 38 years old.
The study also found those who started using cannabis after the age of 18 did not show the same IQ decline.
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But the new study, by the Ragnar Frisch Centre for Economic Research in Norway, said there were hidden factors in the Dunedin research that were not taken into account.
Ole Rogeberg, the scientist behind the Norwegian study, said socioeconomic status was one of those factors.
In the simulation model Rogeberg created, poverty was able to reproduce the same IQ change as cannabis exposure.
"This model, too, would predict reduced IQ in so far as heavy, persistent, adolescent-onset cannabis use involves a culture and norms that raise the risk of dropping out of school, getting entangled with crime, and other such behaviours," he said.
Because early cannabis use was more common for those with poor self-control, behavioural problems, and those from poor families, Maori participants would be over-represented.
He suggested it was too early to connect cannabis smoking and declining IQ, and further analysis of the Dunedin study might resolve the competing results.
Rogeberg said the political approach to cannabis could change depending on whether there was a change in young smokers' IQ because of the drug itself, or because of living conditions.
He said his study did not mean the results of the Dunedin study were discredited, but it was fair to say the New Zealand study's methodology was flawed and the results premature.
But Professor Richie Poulton, co-author of the Dunedin research, said Rogeberg's data was not taken from real people.
"Rogeberg's challenge is based on simulations, but we used actual data on 1000 people to carry out the analyses he suggested," he said.
"If many cannabis users were former low-SES [socio-economic status] children, Rogeberg says this coincidence could create the false impression that cannabis was responsible for their adult IQ drop."
Poulton said looking only at the children sampled from middle-class homes, the Dunedin findings remained the same.
He also said although many of the cannabis users in the Dunedin study were from slightly poorer homes, not all of them were.
An Australian scientist said it was significant the Dunedin scientists were specialists in psychiatry and psychology, and Rogeberg was an economist.