Obsessive Compulsive Disorder (OCD)

Obsessive compulsive disorder (OCD) is a mental illness characterised by uncontrollable, recurring thoughts (obsessions) and repetitive behaviours or routines (compulsions).

Common obsessions include cleanliness, safety, order and unwanted sexual or aggressive thoughts. People with OCD spend a significant amount of time thinking about the obsession (>1 hr/day), to the extent that it interferes with their study, work or relationships. People with OCD are generally aware that their thoughts and behaviours are irrational and excessive, and often feel ashamed of their condition.

What causes OCD?

The exact causes of OCD are unknown, but both genetic and environmental factors are important, and neuroimaging studies show that OCD patients have specific differences in the brains compared to healthy individuals.


Studies in twins reveal a genetic component to obsessive-compulsive disorder. Identical twins (i.e. with same DNA) are more likely to both have OCD than are non-identical twins (who share only ~50% of their DNA). Similarly, if a parent, child or sibling has OCD, an individual has an increased risk of having OCD.

It is likely that variants in many different genes contribute to the risk of OCD. Knowing more about these genes can provide important clues to develop better treatments. One possible genetic variant is linked to the serotonin neurotransmitter. Mutations in the hSERT (human serotonin transporter) gene have been found in a subset of families with OCD history. This causes the transporters to work too effectively, meaning less serotonin is available. Some drugs used to treat OCD aim to reverse this by increasing serotonin levels.

Environmental factors

Various environmental influences can increase the risk of developing OCD. Although they may not directly cause OCD, stressful life events can trigger the onset of OCD symptoms in people with a genetic predisposition. Rarely, in children genetically predisposed to OCD, a strep throat infection can lead to an autoimmune reaction, ultimately raising their risk of developing the disorder.

Brain structure and function

Neuroimaging studies have revealed differences in brain activity between people with OCD and those who are unaffected. In particular, there are differences in a circuit that links a part of the brain called the striatum, thalamus, and parts of the frontal cortex. Studies show that OCD patients have excess activity in frontal regions of the brain, including the orbitofrontal cortex (OFC) and anterior cingulate cortex (ACC), which could explain their intrusive thoughts and high levels of anxiety, respectively.

Diagnosing OCD

A doctor will diagnose OCD based on three criteria:

  • Presence of obsessions
  • Presence of compulsive behaviours
  • Obsessions and compulsions take up a significant fraction of the person’s life, interfering with normal daily living

OCD is typically diagnosed before age 25. Most of these occur in adolescence, although children as young as 6 or 7 can also fit the criteria.

Learn more about OCD

  OCD facts

  OCD signs and symptoms

  OCD treatment

  OCD research at QBI