We performed additional analyses that used unaffected full siblings of patients as controls to account for possible familial confounding. In these analyses, we identified as cases those persons with OCD who also had full siblings without OCD, and we compared those persons with their unaffected full siblings using logistic regression for matched design. Comparisons of siblings raised in the same family will automatically match on many unmeasured shared factors, including cultural background, parental characteristics, child-rearing practices, and— particularly for monozygotic twins—genetics.13
