People who have a compulsive urge to collect and clutter their homes with junk can partly attribute their problem to genes, a new study confirms.

In a twin study, researchers found that genetic predisposition explained a large amount of the risk for compulsive hoarding — a mental health problem in which people have an overwhelming desire to accumulate items normally considered useless, like old newspapers or junk mail.

Of the more than 5,000 twins in the study, roughly 2 percent showed symptoms of compulsive hoarding. And genes appeared to account for half of the variance in risk.

It has long been known that compulsive hoarding tends to run in families, explained researcher Dr. David Mataix-Cols of King's College London, in the UK.

What has not been clear, he told Reuters Health in an email, is whether that pattern is due to genes or to something in the home environment, like parenting practices.

"Twin studies allow us to separate these two sources," Mataix-Cols said.

The study included both identical and fraternal twins; while identical twins share all of their DNA, fraternal twins share roughly half of their genes, making them no more genetically similar than non-twin siblings.

If genes are a more important factor than shared environment in a given disorder, then identical twins would be more similar in their risk of the problem than fraternal twins would be.

Mataix-Cols and his colleagues found that among female identical twins, when one twin showed compulsive hoarding symptoms, the other twin also did 52 percent of the time. Among fraternal twins, that figure was 27 percent.

There was no evidence, however, that environmental factors shared by twins contributed to compulsive hoarding. Instead, "non-shared" environmental factors — those unique to individuals — seemed to be at work.

Past research has shown that many people with hoarding problems have a history of traumatic events, according to Mataix-Cols. In particular, they have elevated rates of sexual abuse and "loss" — of a loved one or a home, for instance.

"What the study suggests," Mataix-Cols said, "is that genes are important, but probably some environmental stressors are needed to cause or trigger the hoarding problem."

Because the study included too few male twins suffering from compulsive hoarding, the researchers were able to analyze only female pairs. However, Mataix-Cols said he and his colleagues "strongly suspect" the findings apply to men as well.

There remains much to be learned about compulsive hoarding. Research is still in its "infancy," Mataix-Cols said, and more studies are needed to pinpoint the specific genes and environmental factors involved.

The hope, he added, is that this work will lead to better therapies for compulsive hoarding. Right now, behavioral therapy and antidepressants are the main forms of treatment, but they have met with limited success.

SOURCE: American Journal of Psychiatry, October 2009.