Psychologists have suggested genes to be an explaination of OCD.
Evidence for this tends to come from observations of family and twin studies, and so is largely based on case studies. :( This in itself causes several problems. Case studies are rare and may only represent one example of an occurance, so cannot be soley relied upon to explain a subject and therefore cannot be generalised.
Nestadt et al (2000) compared 80 OCD patients and their relatives with 73 control patientrs without the disorder and their relatives.
Results showed a strong familial link, people with first degree relatives with OCD had a 5 times greater risk of developing the disorder at some point in their lives, compared to the 'general population'.
This is supported by Billet at al's (1998) meta analysis of 14 twin studies of OCD. They found that Mz (identical) twins were more than twice as likely to develop OCD if their co-twin had the disorder, than DZ (non-identical) twins. This shows that the siblings who share the most genes are most likely to develop the same disorder, supporting the claim that OCD is caused by genetics.