Selection of relevant twin studies on cannabis use and problematic use for this study started with a search of the electronic databases PubMed (http://www.ncbi.nlm.nih.gov/entrez) and ISI web of Knowledge (http://apps.isiknowledge.com) by using the following keywords: heritability/heredity/twin and cannabis/marijuana/hashish. No restrictions regarding date range were specified. Abstracts of these search results (N = 122) were examined and relevant articles were retrieved for review. Three additional studies identified from reference lists from these studies and one manuscript in press (44, results obtained by personal communication) were also added.
