Our findings demonstrated that additive genetic factors explained approximately half the variance in DSM-5 cannabis withdrawal and the remaining variance was mainly due to unshared environmental factors. The shared environmental influences on cannabis withdrawal were not significant, partly due to a lack of power (see below). The estimate for genetic variance was slightly higher for abuse/dependence, while the C estimate was somewhat lower. Our estimates are in accordance with the results from a meta-analysis of twin studies by Verweij et al. (2010), which showed that A, C, and E estimates of problematic cannabis use are, respectively, 51%, 20% and 29% for males and 59%, 15% and 26% for females. Large variation across studies in A, C, and E estimates of cannabis abuse and dependence exists (Verweij et al. 2010), due to the different (aged) populations from which research samples are gathered and different measures used. A low or absent shared environmental influence, as estimated in the present study, has also been found in several previous studies (Kendler & Prescott, 1998; van der Bree et al. 1998; Kendler et al. 2006;
