Which plays a bigger role in your desire for cannabis?
Ever since affordable consumer genetic testing products came on the market, curious citizen scientists have become increasingly interested in the secrets that their DNA holds. Companies like 23andMe™extract DNA from saliva and ‘spit’out a report detailing information on how genetics contribute to the likelihood of acquiring certain diseases. While this methodology and level of data sharing has faced its share of controversies, it nonetheless represents the future of personalized medicine, where you can expect tailored medications and treatment plans based on your unique genetic profile.
Aside from the medical and health applications, consumer genetics tests also can reveal some interesting quirks about personality – for example, you may learn that you are more likely to have dimples or are more likely to drink a lot of coffee. But, what about preferences for other substances like cannabis?
Last year, 23andMe™ researchers published a report based on genome-wide association studies (GWAS) they performed on data from over 180,000 people from combined datasets.  They found a link between cannabis use and schizophrenia (and other psychiatric behaviors) – a trend identified by many other studies. However, their data indicated that, rather than cannabis use causing schizophrenia, having schizophrenia may lead to cannabis use – meaning that, as symptoms begin to emerge before diagnosis, people may use cannabis to treat their condition.While this study shows a correlation (not a direct causation), it is a landmark finding, as the link between cannabis and psychosis is often cited as evidence against .
Their findings on the genetic contribution to cannabis use is consistent with previous studies, which have reported that cannabis initiation is approximately 40% heritable. [2, 3] GWAS studies are a strong methodology to evaluate correlations due to the large databases they pull from. However, twin studies employ a classic and useful method to assess genetic correlations with certain behaviors.
Identical twins have matching DNA profiles – but they are not identical in every way, as even twins do not share every aspect of their lives together.While both environment and biology play important roles in shaping personality, it is very difficult to tease these two apart. With twin studies, genetics are consistent while the environment is variable, allowing study of environmental differences in isolation – providing an opportunity to study the key debate in genetics of nature or nurture.
Several studies have been conducted on cannabis use in twins. [4, 5] One looked at cannabis use in both monozygotic and dizygotic female twins – here, both variables of environment and genetics were evaluated, as dizygotic twins only share 50% of their DNA. They found that the prevalence of lifetime cannabis use was 46% in monozygotic twins and nearly 53% in dizygotic twins; in addition, similarity in environments significantly predicted similarity in cannabis use across twins. 
A meta-analysis (or review and evaluation of data from multiple reports) of 28 twin studies evaluated the influence of genes, shared environment, and unshared environment on cannabis use. This analysis found similar rates of cannabis use initiation(males: 48%; females: 40%) linked to genetics as previous studies across same-sex (female-female; male-male) and mixed-sex twins (male-female). However, the remaining 52-60% of variance in use was attributed to the environment (both shared which includes family environment and socio-economic status, and unshared, which includes interactions with friends and other unique life experiences) – in fact, across females, the shared environment was nearly as important as genetics in predicting cannabis use (39%). 
While each study type has its strengths and weaknesses, taken together, these data show that genetics play a large role in predicting whether someone will start using cannabis. However, more than half of the variance is due to environmental factors, meaning that, like most things in life, where you are and who you are with greatly affects your behavior. So don’t blame it all on your genes.
- Pasman, J.A., Verweij, K.J.H., Gerring, Z, et al., “GWAS of Lifetime Cannabis Use Reveals New Risk Loci, Genetic Overlap with Psychiatric Traits, and a Causal Influence of Schizophrenia”, Nature Neurosci, 2018, Volume 21, pg. 1161-1170. (impact factor:19.912; cited by: 7)
- Stringer, S., Minică, C., Verweij, K.J.H., et al., “Genome-wide Association Study of Lifetime Cannabis Use Based on a Large Meta-analytic Sample of 32,330 Subjects from theInternational Cannabis Consortium”, Transl Psychiatry, 2016, Volume 6, pg. 1-9. (impact factor:5.620; cited by: 1)
- Verweij, K.J.H., Vinkhuyzen, A.A.E., Benyamin, B., et al., “The Genetic Etiology of Cannabis Use Initiation: A Meta-analysis of Genome-wide Association Studies and a SNP-based Heritability Estimation”, Addict Biol, 2013, Volume 18, pg. 846-850. (impact factor:5.929; cited by: 42)
- Kendler, K.S., Prescott, C.A., “Cannabis Use, Abuse, and Dependencein a Population-Based Sample of Female Twins”, Am J Psychiatry, 1998, Volume 155, pg. 1016-1022. (impact factor:13.391; cited by: 308)
- Verweij, K.J.H.,Zietsch, B.P., Lynskey, M.T., et al., “Genetic and Environmental Influences on Cannabis Use Initiation and Problematic Use: A Meta-analysis of Twin Studies”, Addiction, Volume 105, pg. 417-430. (impact factor:5.789; cited by: 161)