Our findings must be interpreted in the context of at least three potential limitations. First, our data were collected from white males born in Virginia. Previous analyses using the same data (6) suggested that this sample does not differ from the general population in rates of psychopathologic conditions, including illicit substance use, and that it is likely to be broadly representative of US men. With regard to sex differences, rates of cannabis and other substance abuse are lower in females. Although this may be attributable to greater male exposure opportunities rather than greater chances of progressing from opportunity to use (53), the genetic and environmental pathways to abuse and dependence across a range of illicit substances still appear to be the same for men and women (3). Nevertheless, it remains to be seen whether similar results will be obtained for females if sex differences in self reported drug availability exist. Second, our data were collected retrospectively. Twins were asked to recollect drug availability, which introduces the potential for recall bias and telescoping effects (54). In order to improve the quality
