Is the relationship between early-onset cannabis use and educational attainment causal or due to common liability?

Abstract

Background Several studies have shown that early cannabis use is correlated with poor educational performance including high school drop-out. The predominant explanation for this relationship is that cannabis use causes disengagement from education. Another explanation is that the association between early cannabis use and educational attainment is not causal, but the result of overlapping risk factors that increase the likelihood of both early cannabis use and disengagement from education. These confounding factors could be of genetic and/or environmental origin. Methods Here we use data from a large community-based sample of adult twins (N=3337) who completed a comprehensive semi-structured telephone interview. We first apply the classical twin-design to determine whether genetic and/or environmental influences underlie the relationship between early-onset cannabis use (prior to age 18) and early school leaving. Next, with a co-twin control design we investigate whether the relationship between the two variables is more likely due to direct causality or overlapping risk factors. Results We find a significant phenotypic correlation between early-onset cannabis use and early school leaving (r=0.26), which could be explained by familial influences (of genetic and/or shared environmental origin). The pattern of odds ratios found in the co-twin control design is not consistent with direct causation, but rather suggests that the association is due to shared environmental factors influencing both variables. Conclusion Our findings suggest that the relationship between early-onset cannabis use and school leaving is due to shared environmental risk factors influencing both the risk of early-onset cannabis use and early school leaving.

Adolescent cannabis and tobacco use and educational outcomes at age 16: birth cohort study

“…Evidence that this association is causal, such that preventing cannabis use among young people would increase their educational attainment, would have important implications for policy. A recent co‐twin control study found that cannabis does not cause adverse education outcomes, but both traits are influenced by the same family environmental factors 5. The available population‐based evidence is exclusively observational, reflecting the practical and ethical difficulties inherent in an experimental approach.…”

Can we make cannabis safer?

“…Similarly, a 1-year follow-up study of 1155 adolescents found that weekly cannabis use was related to poorer performance in GCSE Maths and English at age 16 (albeit less than tobacco), after controlling for confounders 51 . However, a large twin study found that early school leaving was explained by shared environmental risk factors which increases the likelihood for both cannabis use and early school leaving 52 . These findings suggest that although there is a link between cannabis use and poorer educational outcomes, these effects may be explained by other factors.…”

Cannabis Involvement and Nonsuicidal Self-Injury: A Discordant Twin Approach

“…For example, early initiation of cannabis has been robustly linked to the use of other illicit drugs, as well as drug abuse and dependence (Agrawal et al, 2004;Grant et al, 2010;Lessem et al, 2006;Lynskey et al, 2003Lynskey et al, , 2006. Early cannabis use has also been linked to lower educational attainment (Homel et al, 2014;Verweij et al, 2013) and to a variety of mental health problems, including anxiety and depression (Lynskey et al, 2004;Scholes-Balog et al, 2013), and psychosis (Bagot et al, 2015).…”