A new study (Science) provides further evidence in the science behind obsessive compulsive disorder. The study showed that there was a decrease in activity in the part of the brain that is associated with decision-making and keeps compulsive behaviors in control (orbitofrontal cortex). This was true for participants who had OCD as well as family member who did not.
The researchers (Chamberlain, et al) note that abnormalities in this area can predispose a person to OCD and this information can, perhaps, lead to brain scans actually diagnosing this disorder, since it tends to run in families, before symptoms emerge.
Reference: Orbitofrontal dysfunction in patients with obsessive-compulsive disorder and their unaffected relatives
Science. 2008 Jul 18;321(5887):421-2.