intercept measures for consumption. In addition, their drug use measure was a composite of eight illicit drugs; our drug use analyses focused on cannabis, although we did include a measure of any illicit drug dependence (which was not significant in our cotwin-control analyses). Also, our sample was Vietnam-era veteran male twins with a mean age of roughly 37 years when educational attainment was assessed, whereas King et al. analyzed data from a case-control sample focused on offspring of alcoholics with a mean age of 25 years at the final assessment. Thus age, gender, and military history could also contribute to the different results in our study compared to King et al. (2006).
