Another reason for slow progress in genetic discovery might be the apparent phenotypic heterogeneity in OCD. In adults and youth, OC symptoms generally cluster into four dimensions: symmetry, forbidden thoughts/checking, cleaning, and hoarding13,14, indicating considerable phenotypic heterogeneity. Twin studies in adults indicate that each of these dimensions is heritable with genetic influences that are shared among dimensions and unique to each dimension15,16, suggesting that phenotypic heterogeneity reflects genetic heterogeneity. In youth, the heritability of OC traits and dimensions is unclear17, particularly the degree to which individual dimensions are mediated by unique and shared genetic influences. Hoarding is considered a distinct dimension in adults and youth and is classified as its own disorder in the Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-518). Twin studies in adults suggest that hoarding shares genetic influences with other OC dimensions15,19,20. In youth, it is not known whether hoarding shares genetic risk with other OC dimensions. The present study aimed to uncover the degree of shared and unique genetic effects on OC dimensions in youth.
