Obsessive-compulsive disorder (OCD) is a mental health disorder that affects millions of people worldwide. It’s characterized by intrusive thoughts and repetitive behaviors, which can be distressing and interfere with daily life. Despite the prevalence of OCD, the exact causes of this condition remain unknown. However, research has shown that there are genetic factors contributing to OCD.

What is OCD?

Obsessive-compulsive disorder (OCD) is a mental health disorder that affects approximately 2% of the population. OCD is distinct from other disorders in that its symptoms feature a pattern of unwanted thoughts and fears, known as obsessions, that lead a person to engage in repetitive behaviors and compulsions.  For example, a person might have a strong fear of their apartment being broken into every time they leave the house unless they check the front door is locked 5 times.

These obsessions can interfere with daily activities depending on the number of compulsions a person has and how long each one takes to finish, which can cause significant distress in a person’s life.

The symptoms of OCD generally worsen with time, though the severity of the disorder may vary depending on stress and other factors. While some people may believe having OCD is just preferring items clean and organized or displayed in a certain way, people suffering from OCD experience constant fears that greatly affect their quality of life.

Those people with OCD may use their compulsions to stave off some of their obsession-related anxiety, ultimately this just reinforces the ritual of obsession and compulsion.

The Role of Genetics in OCD

While the exact cause of OCD remains unknown, many papers over the course of the past 50 years have examined the possible role of genetics in generating OCD, though there may be environmental risk factors at play as well.

Genetic vs. Environmental Factors in the Development of OCD

Genetic factors of the disease are variations of genes, known as alleles, which alter a person’s predisposition to certain outcomes. These alleles are passed down through generations, from parent to child, which is why many members of a family will share the same hair or eye color.

Environmental factors, conversely, are not dependent on other family members, but rather on outside influences. For example, the zika virus causes a birth defect known as microcephaly, but this only occurs when a woman, while in the stages of early pregnancy, becomes sick with the zika virus. The baby’s birth defect has nothing to do with either parent’s genetics.

Evidence for genetic inheritance of OCD:

•   Familial clustering of OCD – individuals are anywhere between 6-55% more likely to be diagnosed with OCD when a first-degree family member (sibling, parent, or child) is diagnosed with OCD. This is compared to the general risk in the population, without a diagnosed family member, of 1-3%.
•   Patterns of heritability – certain variations of OCD are more likely to run in families than others. Hoarding and contamination/cleaning symptoms have the highest familial risk.
•   Multiple studies have reported that OCD, ADHD, Tourette syndrome, and autism spectrum disorders overlap in their genetic features.
•   Identical (monozygotic) twins are more than twice as likely to both have OCD than fraternal (dizygotic) twins.

Evidence of Possible Environmental Factors Affecting OCD:

•   Prenatal and perinatal risk factors – low birth weight, preterm birth, and maternal smoking during pregnancy are associated with elevated risks of developing OCD
•   Early immune system impacts – Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections (PANDAS) is a condition in which OCD symptoms will suddenly develop or worsen after a childhood strep infection. This, however, is believed to result from an over-active immune response and not the infection itself.
•   Childhood abuse – childhood sexual assault and emotional abuse are prevalent in individuals with OCD. Emotional abuse has the strongest association with more severe OCD symptoms.

Though there are some environmental factors that are seen more often in patients with OCD, the direct causality of these events to OCD development remains in question. Most of this evidence relies on the credibility of the patient or a family member. Meaning it may be possible that OCD symptoms were present before a potential cause, like emotional abuse.  

Is OCD Genetic?

While there is consensus that OCD has some level of heritability, it is unclear what percentage of cases are inherited as well as which genes are responsible for its transmission. Initial studies of OCD-related genes focused on those related to neurochemical systems, that is, genes that control how serotonin moves throughout the brain. There are some attempts at a genome-wide study for OCD those which are still in their early stages.

Genome-wide studies are investigations that sample the DNA of participants with a condition and without to determine if there are commonalities exclusive to people with the condition. Unfortunately, these studies have found no significant variants associated with OCD.

The problem with these analyses is a lack of adequate sample size. For a study to show a statistically significant difference in findings between two samples, the number of participants must be large enough to allow for genetic differences that can’t be attributed to chance. To combat this typical pitfall, the Psychiatric Genomics Consortium (PGC) is currently working on a larger meta-analysis that will include at least 14,000 patients with OCD and 560,000 controls.

Up until this point, much of the research linking OCD to genetics has focused on twin studies, which, while compelling, may experience confounding with environmental factors as the twins presumably grew up in the same environment, potentially also influencing OCD development in both siblings.

OCD Future Research

While there is consensus there is a genetic explanation for OCD, the specific genes involved remain unclear. Based on the research, it seems that there are several genes that contribute to the development of OCD, and these genes may be influenced by certain environmental factors in fetal or early childhood stages.

Most of this uncertainty results from a lack of adequate sample size, however, current meta-analyses seek to improve the obtainable results from these studies. These larger samples will be critical in identifying genetic factors of OCD and ultimately help other scientists, doctors, and patients target future diagnosis and treatment.