Obsessive-compulsive disorder is a chronic psychiatric disorder characterised by obsessions and/or compulsions. Genetic research into obsessive-compulsive disorder is significantly limited by the difficulty in clearly defining the disease. There are only a few good studies and the populations investigated are small. Family and twin studies have provided evidence for a genetic component in obsessive-compulsive disorders. Segregation analyses suggest Mendelian inheritance of a dominant allele. In addition to one gene with a major effect, genes with smaller effects also play a role.