Dr. Tomas Paus, professor of psychology and psychiatry at the University of Toronto, has co-authored a study linking the use of marijuana among schizophrenia-prone adolescents to structural changes in the brain.

The study examines almost 1,600 subjects, some of whom did, and some of whom did not use marijuana before the age of 16. Some subjects had a high risk of schizophrenia, and some had a low risk, based on polygenic testing (testing numerous genetic locations for traits associated with schizophrenia).

The goal was to find the effect of cannabis use on the development of the cerebral cortex — the outer layer of brain tissue, which plays an important role in perception, memory, language, and consciousness in general.

Among male subjects who had a high risk of developing schizophrenia, those who used cannabis during adolescence developed significantly thinner cerebral cortices than those who didn’t, and the more often a subject used cannabis, the more pronounced the effect was. But males with a low risk of schizophrenia saw no change to their cerebral development as a result of cannabis use, and females at either high or low risk were likewise unaffected.

Paus is quick to point out that the study doesn’t prove any sort of cause-and-effect relationship — there may be some other factor affecting cortical thickness that also creates conditions in which a youth would be more likely to use marijuana. He does note, however, that the thinning is most pronounced in areas of the cortex with more cannabinoid receptors, so there is some evidence that cannabis itself is at fault.

The ultimate answers to questions of causation, says Paus, lie in experimental work, not in work that studies health trends across populations like his. But a good first step, he thinks, would be longer-term studies with larger samples, to more concretely establish the link between cannabis use, cortical development, and schizophrenia.

The current study is nonetheless an important step in exploring the development of mental disorders in adolescence, and identifying the role of cannabis use in brain development.

Paus is aware that the study has been released into a contentious political climate, with three dominant political parties and as many positions on marijuana legalization. However, he cautions those who would attach any political significance to his results, saying “This kind of research does not contribute to policy decisions” — at least until it proves more concrete connections between marijuana use and mental illness.

This is not to diminish the importance of the research. Paus thinks that even if it shouldn’t inform policy, this evidence, in conjunction with other research showing that “adolescent cannabis use increases probability of schizophrenia later in life,” should be taken seriously by the academic community and the general public, especially by those who have significant family histories of mental illness, or anything else increasing the risk of such illness, making them more susceptible to the dangers the study highlights.