Hoarding, hand-washing and obsessive checking: Which of these is not like the others?

People with a diagnosis of obsessive-compulsive disorder can often shake their family tree and find a relative who has also contended with obsessive thoughts, hoarding, repetitive hand-washing, behavior in which locks and stove burners are checked over and over again or elaborate rituals must be followed for daily life to proceed.

The disorder seems to have some genetic component. But even related people with obsessive-compulsive disorder often exhibit different behavioral symptoms from one another, suggesting that some of the disorder behaviors are learned. This causes psychiatrists to ask: When people’s dysfunctional behaviors seem related, but spring from different origins, do they represent the same disorder?

It does matter: If they’re all manifestations of the same disorder, hand-washing, checking, obsessive thoughts and rituals, and hoarding behavior might all progress the same way, respond to the same kinds of treatments and maybe even involve the same underlying brain structures and patterns. If not, they need to be treated differently.

A study published Monday in the Archives of General Psychiatry tries to answer that question by exploring the relative contribution that nature and nurture make to obsessive-compulsive disorder symptoms. The researchers tapped one of the largest and longest registries of twins in the United Kingdom--more than 8,000 fraternal and identical twins--and a little more than 5,000 filled out the survey gauging participants’ and their twin’s obsessive-compulsive disorder behaviors, their severity, whether and how long the twins were raised in the same household and whether the twins were near-exact DNA matches (identical twins) or shared the same DNA as any siblings with the same two parents (fraternal twins). All were female.

Twins studies are widely used to gauge the relative role of environment and genetics. If genes are powerfully at work in a given disease, you’ll see that disease very frequently shared by both twins when the twins are near-exact genetic replicas of each other--identical. If environment plays an important role in the development of the disease, fraternal twins are just as likely as identical ones to get the disease if they’ve both been reared together.

When twins are separated at birth, twin studies can be even more revealing because the common factor of a shared household is not there.

The study published Monday found that for most who reported obsessive-compulsive disorder behaviors, genetics was the major factor dictating which behavior would be manifest: The genetic origin of obsessive-compulsive disorder was strongest with hand-washing, and just a little less dominant where obsessing, checking and ordering behaviors were concerned. In all of those cases, environmental cues played only minor roles in determining whether a participant displayed obsessive-compulsive disorder behaviors and which ones came out.

Hoarding behavior seemed to stand out on its own, with far less evidence that a person genetically predisposed to obsessive-compulsive disorder behaviors would end up a hoarder of objects.

The finding supports the notion that while hoarding may look like obsessive-compulsive behavior, it may actually spring from some different origin than classic obsessive-compulsive disorder behaviors: a separate genetic factor or a different environmental cue--or both--may be at work in starting a person on the road to hoarding behavior.

That’s in line with the American Psychiatric Assn.’s current take on hoarding. In the newest draft of psychiatry’s diagnostic manual, called the DSM-5, experts in these behaviors suggest that hoarding should be listed as a mental disorder separate from obsessive-compulsive disorder behaviors. That may lead to different treatments for hoarding than for obsessive-compulsive disorder behaviors, which are widely treated with targeted cognitive behavioral therapy and SSRI antidepressants.