AO3

• Nestadt reviewed previous twin studies of OCD and found that 68% of identical twins, compared to 31% of non-identical twins, share OCD. This suggests that there is a genetic basis for this disease as identical twins share 100% of their genes with each other, whilst DZ only share 50% of genes with each other. However, it is important not to be deterministic - must be paired with an environmental stressor to result in OCD, as dictated through the diathesis-stress model.
• Several neuroimaging studies using PET scanners have shown hyperactivity in the OFC and the caudate nucleus in people with OCD both while scanning the brain at rest and when symptoms are stimulated, but there is a problem with this neural evidence, it is correlational; researchers cannot be sure if the hyperactivity in these areas is the cause of OCD or a consequence of having OCD. .
• A meta-analysis by Soomro demonstrated SSRIs are more effective than placebos, suggesting there is a biological aspect to OCD however despite altering levels of serotonin in the synapse within hours, these drugs take weeks to reduce symptoms, and 40% to 60% of patients show no or just partial symptom improvement. These findings suggest low levels of serotonin have a role to play in OCD but not the sole cause