OCD originally was thought to be a very rare disorder (think: before 1970). However, in the past decades more and more surveys trickled in that presented a different picture. At the moment of writing, the occurrence of the disorder is set at a stable 2 to 3% of the general population. Scientists use the term prevalence, which means that roughly speaking one in fourty people will at one or another point in his/her life encounter an episode of serious OCD. The reasons why this rate was underestimated before are that OCD was even more underdiagnosed in earlier days, and modern investigative techniques allow for more precise assessments of ‘pure’ cases of OCD. Probably it is also true that patients experience less fear and awkwardness to address their doctor with problems that are of a mental nature nowadays. After all, before the days of Sigmund Freud and his colleagues, psychiatric disorders were much more burdened with the stigmata of ‘madness’ and ‘unpredictability’. We can also put it this way: centuries ago, OCD symptoms were seen as signs that someone was possessed by the Devil, and treated accordingly: by an exorcist (the author could not find information on the success rates of this type of intervention, alas). Nowadays, the mere idea of ‘being in the hands of Evil Spirits’ is unacceptable, except in circles of fundamentalist religions and isolated sects. The concept of an abnormality in the brain, in nerve cells, is less unsettling: if grasped well enough, it may potentially be not harder to accept than any chronic condition in the rest of the body. I would say that this is one of the greatest merits of the relatively young science of biological psychiatry.
It is remarkable that the average prevalence of OCD is a stable 2,5% across the world’s populations. One would expect that isolated communities, or communities with habits entirely different from those in Western civilizations, would show different rates of patients with the disorder, or perhaps different dimensions, completely unknown to us. That is not the case. What we see is that there are small differences, e.g. in countries where emphasis lies on a very strict upbringing, religious preoccupations of an obsessive nature may be more prominent.
A striking feature of OCD is that the illness announces itself in the form of two ‘peaks’ in the course of life. We call these: Early Onset OCD, and Late, and Adult Onset OCD. Trawling through the literature, we see that these peaks are defined differently by different researchers. A precise statistical calculation was performed by the group of Delorme et al (2005), who put EO OCD at an average of 11.1 years, and AO OCD at a mean of 23.5 years. Both numbers are not absolutes, but the center of a certain span, so variations are possible. EO OCD has as significant co-factors a pronounced family history of the affliction, and also symptoms of tic disorders, notably Tourette’s syndrome*). AO OCD presents with generalized anxiety disorder **) and/or major depressive disorder ***). Note that certainly not all OCD patients have these side symptoms.
Finally, OCD is equally distributed across the sexes. In other words: the so-called male-to-female ratio is 1:1. The only other anxiety disorder that shares this characteristic is social phobia****). All other anxiety disorders show a very significant over-representation in women, ranging from a m/f ratio of 1:2, to even 1:4.