OCD has, as many other psychiatric conditions, a genetic and an environmental component. This is often jotted down in the form of the brief notation G x E.
Now, you may ask: interesting, but in how far is it heritable? What are the chances that, if my dad suffers from it, and both our lives are somewhat comparable, I myself will develop OCD?
I would say: moderately to considerably. With an emphasis on ‘moderately’, simply because that is the assessment that most scientific articles share.
Studying heritability of psychiatric disorders is very difficult. See: if I have a somatic (read: bodily) disorder, like a deficit in the production of a certain substance, tests are often rather simple and deliver a quick ‘yes-or-no’ type of answer. And then proper treatment is a matter of protocol. And frequently we see the presence of such an illness transferring from parents to offspring, or occurring after having ‘skipped’ a generation.
Not so in disorders of the mind, of the psyche. Although most of the experts agree on the idea that our brains are the ‘living room’, the substrate, of our thinking and feeling, that does not mean that there is a simple method to search the brain for what is wrong in OCD. Making pictures of the brain improves on an almost yearly basis, yes. But making the right pictures and interpreting them correctly is specialist work. A central question is for instance: is what we think we are assessing truly the same as that what we are assessing? If we see a difference between brain areas of patients with OCD, and healthy control subjects, what does that difference stand for? Which substance is abnormally present? Which brain area is too active, and if so, what does that activity mean?
So: looking into what is actually operating ‘not normal’ is difficult enough in itself. But moreover: mental disorders are usually diagnosed highly subjectively. The patient reports what he or she feels, experiences, and how that impairs the quality of life. The doctor will duly take note of that self-report. Almost always the patient will fill in some sort of rating scale that is designed to profile the exact features and gravity of the disorder; in OCD the standard index is the so-called YBOCS, the Yale-Brown Obsessive-Compulsive Scale. If one exceeds a certain number of points in the personal scale, the diagnosis will be OCD.
I know, this is a long story. But it is important to know how difficult it is to correctly assess OCD in the first place. I have to stress that the subjective diagnosis by the clinician, and his interpretation of the rating scale, is the dominant procedure: a patient who clearly is troubled by OCD may present with no detectable abnormalities in the brain if subjected to a brain imaging technique and/or a biochemical analysis.
It is not a ‘yes-or-no’ thing. Even the standard rating scale is subjective in the end. Patients may have waxing and waning of the symptoms. One day leaving the house may be simply impossible as a result of inescapable checking compulsions, and another day it may be reasonably attainable.
OK. Back to the beginning. In how far is OCD inheritable? Van Grootheest, an expert, and his group did a truly comprehensive study on all essential studies of OCD occurring in twins. You will understand that identical twins are an ideal object for study here, as these twins share so many characteristics on genetic grounds.
Their conclusion was that the closest relative of an identical twin has a chance in the region of 27-47% for developing OCD, if his/her brother/sister presents with OCD. Now, that is considerably more of a chance than the risk for any average person, which, as we saw, is about 2-3%.
This is an indication for the heritability of OCD. As they say: it runs in the family.
Although the nature of the disorder may vary from one generation to another (e.g. a daughter of a mother with washing compulsions may develop an ordering compulsion), it is more likely that the inherited features of OCD are of the same dimension.