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. 2013 Dec 1;133(2):580-6.
doi: 10.1016/j.drugalcdep.2013.07.034. Epub 2013 Aug 11.

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

Karin J H Verweij  1 , Anja C Huizink, Arpana Agrawal, Nicholas G Martin, Michael T Lynskey
Affiliations

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

Karin J H Verweij et al. Drug Alcohol Depend. .

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.

Keywords: Cannabis; Education; Environment; Genetics; School drop-out; Twin research.

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Conflict of interest statement

Conflict of interest: No conflict declared

Figures

Figure 1
Figure 1
Expected pattern of odds ratios when the relationship between early onset cannabis use and school leaving is causal, or due to overlapping genetic or shared environmental influences. Results are shown for the general population, and discordant dizygotic (DZ) and monozygotic (MZ) twins.
Figure 2
Figure 2
Correlated factor model. Graphical presentation of the parameter estimates and proportions of variance in early-onset cannabis use and early school leaving accounted for by additive genetic (A), shared environmental (C) and residual influences (E). h2 (heritability) is the percentage of variance accounted for by genetic factors. The double-headed arrows represent the genetic, shared environmental and residual correlations, indicating the degree to which the same genetic or environmental factors are influencing the two traits. Broken lines represent non-significant paths (p>=0.05).
Figure 3
Figure 3
Observed odds ratios for and early school leaving as compared between individuals who used cannabis before the age of 18 and those who did not. Bars show results for unrelated individuals, and DZ and MZ twins discordant for adolescent cannabis use. Error bars denote the 95% confidence intervals.
See this image and copyright information in PMC

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