To our knowledge, this is the first study to examine the role of genetic and environmental influences on cannabis use, and on covariation between cannabis use and cigarette use, separately in EA and AA women. Our study also includes the largest number of AA twins currently available for the study of substance use. We broadly replicated existing racial/ethnic trends in cannabis and cigarette use with one exception in that rates of cannabis use in our sample were comparable, if not marginally higher in AA than EA women and this difference was more pronounced in MOAFTS, which is a general population twin sample. As we relied on multiple longitudinal reports of cannabis use, it is possible that our study design allowed participants greater opportunity to admit to a potentially illicit behavior, particularly during adulthood.
