New research into the brain patterns of compulsive hoarders shows the disorder may have been misclassified and victims could be getting the wrong treatment, U.S. scientists reported on Tuesday.

Hoarding is usually classified as obsessive-compulsive disorder, a catch-all term for a range of symptoms such as constantly repeating actions like handwashing or checking to make sure a stove is turned off.

However, brain scans show the biology of America’s estimated 1 million compulsive hoarders is significantly different to that of other people diagnosed with obsessive-compulsive disorder, the team at the University of California Los Angeles found.

“Our work shows that hoarding and saving compulsions long associated with OCD may spring from unique, previously unrecognized neurobiological malfunctions that standard treatments do not necessarily address,” Dr. Sanjaya Saxena, who led the study, said in a statement.

“In addition, the results emphasize the need to rethink how we categorize psychiatric disorders. Diagnosis and treatment should be driven by biology rather than symptoms,” Saxena added.

Writing in the American Journal of Psychiatry, Saxena and colleagues described tests on 45 adults with obsessive-compulsive disorder, 12 of whom were hoarders, and 17 people without mental health conditions.

None were on medication.

They used positron emission tomography or PET scans to image brain activity in the volunteers.

The hoarders had unique activity, including less activity in brain regions known as the posterior cingulate gyrus and cuneus, they reported.