The importance of twin studies to debunk the prohibitionist stigmas attached to cannabis

In all matters relating to cannabis, different study models have been used to investigate how this plant affects our health, both in the short and long term. However, studies on twins, particularly those over long periods of time, are among the most useful and effective when it comes to understanding cannabis at a deeper level.

These studies have numerous advantages in research, especially because they greatly increase the statistical impact: Twins have the same genes; and, although their environments may often differ as they age, using them as test subjects helps to understand to what extent genetics influences behaviour, intelligence, personality, and the risk of psychological disorders. What's more, by studying twins over a lengthy period of time, scientists can observe how genetic and environmental factors interact at different stages of life, which is critical to many areas of cannabis research.

This is demonstrated by a recent article published by Cambridge University Press and entitled 'Limited psychological and social effects of lifetime cannabis use frequency: Evidence from a 30-year community study of 4,078 twins'. The article states, using twins as the case-control study subjects, that legalisation is not associated with an increase in disorders caused by the use of other substances, or with other psychological, cognitive, social, relational, or financial problems and vulnerabilities. This all helps to debunk one of the biggest prohibitionist arguments against cannabis: that marijuana use has negative effects on mental health; and the belief that it is the gateway to other drugs.

Analysing the study in question

Researchers from both the University of Colorado and the University of Minnesota collected data from as many as 4078 twins, where only one of each pair was using cannabis. These twins, who currently live in states with different marijuana policies, were first evaluated in adolescence and then up to 24 to 49 years of age (i.e. for as long as 30 years). Participants were recruited when they were teenagers, using birth records from 1972 to 1994, and beginning before 2014, when recreational cannabis stores opened in Colorado.

In addition, researchers collected data from longitudinal studies of twins in two opposite states: one where marijuana is legal (Colorado) and another one where it isn't (Minnesota). By specifically comparing twins in pairs, when one of them lives in a state where cannabis has been legalised and the other in a state where it hasn't, the researchers sought to determine what changes, if any, are caused by cannabis legalisation.

"This study suggests that lifetime exposure to cannabis has few persistent effects on mental health and other psychosocial outcomes", said the researchers. "We do not identify differences in cognitive ability between each pair of twins", they added. In fact, lifetime marijuana use didn't produce any negative mental health outcomes among the study subjects.

"In general terms, our results don't support a causal relationship between the average frequency of lifetime cannabis use and most of the evaluated psychiatric and psychosocial outcomes of substance use. Genetic and family confounding factors are more likely to explain the links between cannabis use and the negative results associated with it", concluded the authors.

Other twin studies on cannabis

Other studies on twins share similar results in terms of the assessment of other stigmas that haunt cannabis users, such as the common argument that cannabis use makes you stupid (although there are no studies or evidence to support these claims).

In this regard, a 2016 twin study conducted by researchers at the University of California and the University of Minnesota, sought to understand whether cannabis was related to changes in IQ levels. Participants were assessed using various intelligence benchmarks between the ages of 9 and 12, before they started using marijuana, and then again between the ages of 17 and 20.

According to the researchers, there were no dose-specific relationships associated with decreased IQ and cannabis use. "In the broader longitudinal examination on marijuana use and changes in IQ, we found little evidence to suggest that marijuana use in adolescents has a direct effect on intellectual decline", concluded the scientists.

But there are also other prohibitionist claims that clearly state that cannabis use alone causes psychosis. But here you could say that the twins 'strike twice'.

Researchers from the Institute of Child Development at the University of Minnesota conducted another twin study in September 2021. They assessed the relationship between adult-onset psychosis and cannabis use in adolescence, using a longitudinal twin study; and concluded that exposure to cannabis was not related to a future risk of schizophrenia.

"The results suggest that this association can probably be attributed to familiar confounding factors rather than to a causal effect of cannabis exposure", said the researchers. "Our results imply that the threat of potential harm to teens through significant increases in the risk of psychotic disease in the long term may be exaggerated", they concluded.

Having said that, we must keep in mind that someone with schizophrenic tendencies should refrain from using cannabis or other psychoactive drugs, unless these are taken under the supervision of a medical professional. There are certainly other factors at play and, if someone is already predisposed to suffering from a specific mental illness, it is not recommended for them to self-medicate with any type of drug.

Conclusion

Twin studies add credibility to other studies evaluating similar cannabis-related issues in addition to other health problems. Identical twin comparisons are a powerful tool for this type of analysis because their genetic makeup is almost identical, and their early family environment is consistent. This automatically controls many of the confounding factors that can make it difficult to determine causation. And they can help us obtain valuable amounts of information that can change our understanding of how cannabis affects our bodies.