The first systematic review and meta-analysis of OCD genetic epidemiology were published approximately 20 years ago [19]. This study reported that FDRs of OCD probands had a four-fold higher risk for OCD than the FDRs of non-affected control probands. Since then, more than 20 relevant and high-quality original family studies [5, 7–18, 20–28] including two re-analyses of previous data have been reported [4, 29] and six population-based studies [7–12] have been published. And more than 25 high-quality twin studies [7, 30–56] have also been published.
