Sidorchuk A, et al. - Researchers performed this population-based birth cohort and family study to investigate if obsessive-compulsive disorder (OCD) and suicidal behaviors coaggregate in families and the mechanisms beneath this coaggregation. In this study, individuals born in Sweden in 1967–2003 (n = 3,594,181) were linked to their parents, siblings, and cousins; register-based diagnoses of OCD, suicide attempts, and deaths by suicide were collected. Until December 31, 2013, these individuals were followed. Additive genetic factors (60.7%) and non-shared environment (40.4%) with negligible contribution of shared environment explained familial coaggregation of OCD and suicide attempts. Additive genetics (65.8%) and nonshared environment (34.2%) was related to familial coaggregation with death by suicide. These findings support that genetic factors are largely the cause of coaggregation of OCD and suicidal behaviors in families. They also noted a considerable contribution of unique environment, creating opportunities to target high-risk groups for prevention and treatment.