A study published today in the scholarly journal PNAS questions the conclusion of a paper from last year that was widely seen as greatly strengthening the evidence that regular cannabis use beginning in adolescence and continuing throughout young adulthood causes a decline in IQ by the late 30s.
In the original study, co-author of this article Madeline Meier and her colleagues assessed changes in IQ and specific cognitive abilities between adolescence and the age of 38 in 1,037 New Zealanders. All the subjects were born in Dunedin in 1972 or 1973.
The researchers assessed IQ and other mental abilities at age 13 (before cannabis was first used) and again at age 38, and asked participants about their cannabis use throughout adolescence and young adulthood. They also collected other related data.
They found that early and persistent cannabis users showed an eight-point decline in IQ compared to those who hadn’t used cannabis in this way. More detailed analyses pointed to cannabis use being the most plausible explanation for the decline for a number of reasons.
First, the decline in IQ was largest in those who began using cannabis in adolescence and used it regularly throughout adulthood. This relationship persisted after the researchers statistically adjusted for other factors that may affect IQ (such recent cannabis use, alcohol, tobacco and other drug use, and schizophrenia).
Second, the IQ decline was not explained by the lower educational achievement among cannabis users. The same effects were found in cannabis users who finished high school, and the decline persisted after statistically controlling for educational level attained.
Third, there was some recovery in IQ if users quit. But it was limited in participants who started smoking cannabis in adolescence and had only stopped using for a year or more. There was no IQ decline in cannabis users who started in young adulthood and ceased 12 or more months before.
Fourth, key informants (friends and family who knew the study participants well) were much more likely to report that heavy and persistent cannabis users had problems with memory and attention than were key informants of those who had not used cannabis in this way.
But the article published today suggests an alternative explanation for these findings.
Its author, Ole Rogeberg, argues that the apparent impact of cannabis on IQ could be attributed to reasons unrelated to cannabis use if low socioeconomic status (SES) participants were more likely to start early and become persistent cannabis users, and if their IQ declined more rapidly, especially after they left school.
Rogeberg argues that the same decline in IQ would occur if the two above conditions were true, and if cannabis use had no effect on IQ. He suggests his hypothesis could be tested by conducting additional statistical analyses of the Dunedin data.
The authors of the original paper have done these analyses and their results don’t support Rogeberg’s hypothesis. First, they eliminated the effects of SES on IQ by only examining the relationship between cannabis use and IQ decline in children who came from middle-class homes. They found the same IQ decline in cannabis users who started in adolescence and persisted using into young adulthood within middle-class cannabis users.
And, they didn’t find any support for Rogeberg’s hypothesis that the IQ of low SES participants would be boosted by schooling and decline faster after they left school. Rather, Meier and her colleagues found that average IQs were unchanged in low SES participants between beginning school and adolescence. Most critically, low SES was not related to IQ decline between adolescence and young adulthood.
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.