Brain scans may be able to reveal which people are at genetic risk of developing obsessive compulsive disorder (OCD), researchers say.
The red target shows a brain area that distinguished OCD patients
Individuals with OCD and their close relatives have distinctive patterns in their brain structure, a team at Cambridge University found.
The genes responsible remain unknown, but it appears they change the brain's anatomy, which may aid diagnosis.
The study is published in the latest edition of the journal Brain.
OCD is an anxiety disorder in which the person is compelled by irrational fears and thoughts to repeat seemingly needless actions over and over again.
It can manifest itself in repetitive behaviours, such as excessive hand washing, cleaning or repeated checking, affects 2%-3% of the population and is known to run in families.
Using magnetic resonance imaging, the Cambridge researchers scanned the brains of nearly 100 people, including some with OCD and some who were close relatives of individuals with OCD.
Participants also completed a computerised test that involved pressing a left or right button as quickly as possible when arrows appeared.
When a beep noise sounded, volunteers had to attempt to stop their responses. The aim was to objectively measure ability to stop repetitive behaviours.
Both OCD patients and their close relatives fared worse on the computer task than the control group.
This was associated with decreases of grey matter in brain regions important in suppressing responses and habits - the orbitofrontal and right inferior frontal regions.
Researcher Lara Menzies said: "Impaired brain function in the areas of the brain associated with stopping motor responses may contribute to the compulsive and repetitive behaviours that are characteristic of OCD.
"These brain changes appear to run in families and may represent a genetic risk factor for developing the condition.
"The current diagnosis of OCD available to psychiatrists is subjective and therefore knowledge of the underlying causes may lead to better diagnosis and ultimately improved clinical treatments."
But she said there was a long way to go to identify the genes contributing to the distinctive brain structure found in OCD patients and their relatives.
"We also need to identify other contributing factors for OCD, to understand why close relatives that share similar brain structures don't always develop the disorder."
Ashley Fulwood, of the charity OCD-UK, said although it was a relatively small study, the results were potentially very exciting.
"We hope this new evidence may lead to a larger study which helps conclusively pinpoint a contributory factor into the exact cause of OCD which is so widely debated at the moment."