Marijuana Doesn’t Negatively Effect IQ? Not So Fast

A newly released long term, longitudinal twin study is being touted as a refutation of the long stated assertion that marijuana use has a detrimental effect on a person’s IQ.

This story from AlterNet, begins with the title,  “Twin Study: Marijuana Use Has No Direct Effect on IQ“.

In the report, the author writes,

Investigators at the University of California, Los Angeles and the University of Minnesota evaluated whether marijuana use was associated with changes in intellectual performance in two longitudinal cohorts of adolescent twins. Participants were assessed for intelligence at ages 9 to 12, before marijuana involvement, and again at ages 17 to 20.

Researchers reported no dose-response relationship between cannabis use and IQ decline. They also found no significant differences in performance among marijuana using subjects when compared to their non-using twins.

The study’s abstract does make that contention,  but there’s also this concession,

Marijuana users had lower test scores relative to nonusers and showed a significant decline in crystallized intelligence between preadolescence and late adolescence.

Just what is “crystallized intelligence “? According to this article,

Crystallized intelligence involves knowledge that comes from prior learning and past experiences. Situations that require crystallized intelligence include reading comprehension and vocabulary exams.

This is offered in contention to “fluid intelligence”, which is,

Fluid intelligence involves being able to think and reason abstractly and solve problems. This ability is considered independent of learning, experience, and education.

Whether or not marijuana use has an effect on fluid intelligence is not ascertained,  but its effect on crystallized intelligence is measurable since we use the later throughout our lives.

Let’s keep this in mind, the oldest tested were in their 20s, when fluid intelligence can offset deficiency in crystallized intelligence.  The problem is that the former declines and the latter increases, but since marijuana users are already a disadvantage they will never be able to catch up.

The lesson here, always read the results of any scientific study on what they don’t say, as opposed to what they do.



  1. It’s also poor scholarship to rely on a single study to support a hypothesis. It’s an ordinary habit for news story generation, but single studies don’t prove anything in research, and are quickly disregarded unless many others are able to replicate identical results.

    • I was particularly focused on the telling statement in the abstract that the author seemed to overlook about the effects of marijauna usage on crystallized intelligence, which is what is used and continues to accumulate throughout adulthood. But you are absolutely correct, there is an unwise dependence on single studies by news sources that are pushing a particular agenda.

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