Back in the 1920s, a youngster named “Martin” was plagued by thoughts that he feared would damn him to hell. During the 1950s, his daughter “Lynn” was as well. Although neither Martin nor Lynn was officially diagnosed with obsessive-compulsive disorder (OCD), it is likely that both suffered from it, not just on the basis of their symptoms, but because there is now ample evidence that OCD runs in families.
Yet when the illness clusters in families, how much is actually due to genes? A substantial amount, a study reported May 22 in JAMA Psychiatry suggests. The study was headed by David Mataix-Cols, Ph.D., a professor of clinical psychobiology at the Institute of Psychiatry at King’s College, London.
The first arm of the study, which was conducted in Sweden, included some 25,000 Swedes diagnosed with OCD from 1969 to 2009, as well as all their first- , second- , and third-degree relatives available for study. This arm also included 10 control subjects for each OCD subject, matched by age, gender, migration status, and county of residence, as well as all their first- , second- , and third-degree relatives. The researchers then determined the odds of relatives of the OCD subjects having OCD, as compared with the relatives of the control subjects.
First-degree relatives of the OCD subjects were five times more likely to have OCD than first-degree relatives of the control subjects. Second-degree relatives of the OCD subjects were two times more likely to have OCD than second-degree relatives of the control subjects. And third-degree relatives of the OCD subjects were almost one-and-a-half times more likely to have OCD than third-degree relatives of the control subjects.
Thus, the more closely related one was to an OCD subject, the greater his or her chances of also having the disorder. Therefore, “OCD clusters in families primarily due to genetic factors,” the researchers concluded.
In a second arm of the study, the researchers examined data from a population-based sample of more than 16,000 Swedish twins who had completed a questionnaire about various illnesses they had had, including OCD symptoms. The analysis showed that 47 percent of the familial risk for OCD in that cohort was due to genes, not to a shared environment. Thus, this “analysis confirmed that familial risk for OCD was largely attributable to…genetic factors, with no significant effect of shared environment,” the researchers said.
Yet if about half the familial risk for OCD can be attributed to genes, not to a shared environment, that means that the remaining portion can be attributed to nonshared environmental factors, the researchers pointed out. And this finding has clinical implications, they asserted: “In the future, identification of environmental risk factors for OCD will be at least as important as, if not more important than, finding candidate genes for the disorder, because these risk factors may potentially be amenable to prevention or intervention strategies.”
However, when Psychiatric News asked Mataix-Cols whether any environmental risk factors for OCD are known at this time, he said, “Unfortunately, nothing consistent. There is a lack of high-quality longitudinal research. Some cases may have an infectious/autoimmune origin (PANDAS), but this is controversial.”
Finally, the researchers conducted a third arm of the study in which they examined rates of OCD in spouses or partners of the OCD subjects and of the control subjects.
They found that spouses or partners of the former were three times more likely to have OCD than spouses or partners of the latter, a finding they called “intriguing.” They speculated, for example, that “[i]ndividuals preoccupied with contamination and cleanliness may be more likely to seek partners sharing this characteristic with them” or that “spouses could become more similar the longer they are married.”
The study was funded by the Swedish Council for Working Life and Social Research, the Swedish Research Council, and the Swedish Ministry for Higher Education. ■