January 28th, 2019

Cannabis Research in Twins – The Contribution of Research to Understanding the Complexities Cannabis/Marijuana

Cannabis researchDespite the difficulties encountered in this country by cannabis researchers, some cannabis research in twins is getting done and the results are enlightening. In recent years I have been promulgating the results of a large prospective study that concluded that heavy cannabis had a long-lasting and perhaps permanent negative effect on the cognitive development people who began their use before the age of 18. New research now throws doubt on that conclusion.

Studying similarities and differences between pairs of twins allows researchers to look more deeply into the effects of biology versus environment. William Iacono, a research psychologist at the University of Minnesota, studied the IQ of sets of twins in which one twin had used cannabis regularly as an adolescent and the other had not. What he found was:

  • The using twin had a lower IQ than the non-using twin even before use began
  • The IQ decrease in the using twin was not greater if the cannabis use was higher
  • The non-using twin also showed a decline in IQ

His conclusion from the study on cannabis research in twins was that the IQ decline was significant but was due to other factors than cannabis use. He could not identify what these factors were but speculated that poor education or lack of supportive parenting and supervision would be examples of what could have contributed to the decline.

Iacono also reviewed a new prospective study by the authors of the study that had first shown the decline in IQ. The new one, which included a twin component, failed to replicate the first study – heavy cannabis use was not associated with a decline in IQ. He pointed out that IQ is a limited measurement of cognitive function and cannabis could affect the developing brain in a variety of ways that could be revealed by other tests.

My own take on these inconsistent findings is the importance of being cautious and thoughtful in interpreting research results. Pressure from ideologically driven proponents on both sides of the cannabis safety question can lead to premature judgments. More research is badly needed and fortunately will be forthcoming. The Adolescent Brain Cognitive Development (ABCD) study being conducted by NIDA has now recruited its full complement of almost 12,000 children and will include a twin component as well as repeated brain imaging studies. It promises to provide significant new information on the effect of cannabis on the developing brain, as well as a wealth of other issues. 

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