There are a few important methodological limitations to be considered for this study. Firstly, there are some shortcomings in relation to the data used for this study. We were unable to estimate the heritability of the proposed DSM-5 definition of cannabis use disorders. This was because the twin study had completed data collection before the DSM-5 proposal was announced and, consequently, we did not collect data on the craving criterion. Furthermore, withdrawal was measured using the proposed symptoms for cannabis withdrawal Criterion B of the DSM-5 but, in the interest of sample size, heavy or prolonged use (criterion A) and impairment (criterion C) were not used. For cannabis abuse/dependence, we did not require that the criteria cluster within the same 12 month period – however, only 19 individuals did not satisfy the clustering requirement. This potentially makes our findings less applicable for clinicians – for example, prevalence rates of the full diagnosis of withdrawal may be lower in other populations. However, this would not have biased the variance component estimates in any particular direction. Additionally, our measurements of cannabis withdrawal,
