Over the last decade or so, twin studies have firmly established that additive genetic influences contribute to the etiology of cannabis use disorders [5]. These heritable factors (h2) explain in the region of between 30−80% of the total variance in risk of cannabis use disorders. These estimates have also been largely consistent across samples of adult Australians (h2= 45%) [6], U.S. Virginia (h2 = 58−76%)[7,8], US Vietnam Era Veteran men (h2= 33%) [9] and adult Norwegian twins (h2=77%) [10].
