Twin studies disentangle familial resemblance into genetic and shared environmental factors by comparing the similarity of identical (monozygotic; MZ) and non-identical (dizygotic; DZ) twins. There have been numerous twin studies into cannabis use, but due to considerable variation in the results it is difficult to draw clear conclusions regarding the relative magnitude of genetic and environmental influences. Estimates of the proportion of variance in cannabis use accounted for by genetic influences (i.e., heritability) range from close to zero (e.g., studies 27, 28) to over 60% (e.g., studies 29, 30, 31). Similarly, estimates of the proportion of variance accounted for by shared environmental factors range from zero (e.g., 30, 32) to 68% (28). Inconsistent results have also been found for problematic cannabis use ((symptoms of) abuse and dependence). Various explanations could be proposed for these inconsistent results, including differences in measurement scales, sample size, and demographic differences (age, sex, nationality, socioeconomic status). In particular, very large sample sizes are required to accurately estimate genetic and shared environmental influences when using dichotomous variables (which is the case in most cannabis use studies).
