Our use of a veteran sample potentially limits the generalizability of our results. First, the near universal requirement of completion of high school (or a GED) for service in the military means that while our analyses are able to address the effects of substance use history on educational attainment among high school graduates, they can say nothing about the effects of high school dropout on early substance use and vice versa. Second, because the Registry is based on twin pairs in which both individuals were in the military during the Vietnam-era, the sample may not generalize to twin pairs that were excluded from the Registry (because one or both members were not in the military, e.g., because one or both were in school still, married, parents, deemed medically ineligible, or because one/both found other ways of avoiding the draft). Third, Henderson et al. (1990) found that non-responders in the Survey of Health were less educated at military enlistment than responders, so the educational attainment in this sample may not be representative of all veterans. Finally, the present sample of male
