Findings from the study should be interpreted with certain limitations in mind. First, the proportion of women endorsing CSA in our study was somewhat lower than prevalence estimates of CSA in women found in the larger literature (1–3). The lower degree of specificity in our CSA assessment (e.g., we did not query vaginal, oral, and anal sexual abuse acts separately) and relatively young age cut-off compared to other studies may have contributed to this difference. Second, in discordant pairs, the unaffected twin may be indirectly influenced by her co-twin’s CSA history, such that the affected twin seeks out a substance at a young age and uses with her unaffected co-twin (43, 44). Co-twin early use status is therefore not a very conservative indicator of familial influences. Third, the greater degree of overlap between early use and CSA in concordant than discordant twins results in a more modest drop in the estimated HR for CSA for a sexually abused twin whose co-twin was also abused, thus leading to an overestimate of CSA-associated risk in these twins. Fourth, using a time-varying covariate
