Association between bullying victimization and obsessive-compulsive disorder: a population-based, genetically informative study.

The extent to which bullying victimization is associated with an increased risk of obsessive-compulsive disorder (OCD) has received little empirical attention. This longitudinal, population-based, genetically informative study examined whether self-reported bullying victimization at age 15 was associated with a clinical diagnosis of OCD in the Swedish National Patient Register and with self-reported obsessive-compulsive symptoms (OCS) at ages 18 and 24 in 16,030 twins from the Child and Adolescent Twin Study in Sweden. Using a discordant twin design, including monozygotic (MZ) and dizygotic (DZ) twins, each twin was compared with their co-twin, allowing a strict control of genetic and environmental confounding. At the population level, adjusting for birth year and sex, each standard deviation (SD) increase in bullying victimization was associated with a 32% increase in the odds of an OCD diagnosis (OR, 1.32; 95% CI, 1.21-1.44), of 0.13 SD in OCS at age 18 (β, 0.13; 95% CI, 0.11-0.16), and of 0.11 SD in OCS at age 24 (β, 0.11; 95% CI, 0.07-0.16). While associations tended to persist in the within DZ-twin comparison models, the estimates attenuated and were no longer statistically significant in the within MZ-twin comparisons. These results suggest that the association between bullying victimization and OCD/OCS is likely due to genetic confounding and therefore incompatible with a strong causal effect. Other mechanisms, such as evocative gene-environment correlations, are more plausible explanations for the observed associations.