Steinhausen et al. [8] investigated a population sample from 1969 until 2009 in Denmark. The data indicated that an early-onset of OCS (N = 2057) significantly increased the risk of having OCD in the FDRs compared with controls (N = 6055) (OR = 34.4, 95% CI = 4.42–262.39 for paternal OCD; OR = 11.21, 95% CI = 3.14–0.04 for maternal OCD; OR = 6.19, 95% CI = 3.65–10.49 when a sibling had OCD; and an OR = 4.54, 95% CI = 1.28–16.13 if one of the children had OCD).
