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Study Notes

Explaining OCD

AS, A-Level
AQA, Edexcel, OCR, IB

Last updated 22 Mar 2021

In this and related study notes we focus on biological explanations and treatments for obsessive compulsive disorder (OCD).

Biological explanations are divided into genetic explanations and neural explanations.

Genetic explanations for OCD

Genetic explanations suggest OCD is inherited and that individuals inherit specific genes which cause OCD

Genetic explanations have focused on identifying particular genes which are implicated in OCD and two genes have been linked to OCD, including the COMT gene and SERT gene.

The COMT gene is associated with the production of , which regulates the neurotransmitter dopamine. One variation of the COMT gene results in higher levels of dopamine and this variation is more common in patients with OCD, in comparison to people without OCD.

A second gene which has been implicated in OCD is the SERT gene (also known as the 5-HTT gene). The SERT gene is linked to the neurotransmitter serotonin and affects the transport of the serotonin (hence SERotonin Transporter), causing lower levels of serotonin which is also associated with OCD (and depression)

Neural explanations for OCD

Neural explanations of OCD focus on neurotransmitters as well as brain structures.

Neural explanations suggest that abnormal levels of neurotransmitters, in particular serotonin and dopamine, are implicated in OCD.

Neural explanations also suggest that particular regions of the brain, in particular the basal ganglia and orbitofrontal cortex, are implicated in OCD.


The neurotransmitter serotonin is believed to play a role in OCD. Serotonin regulates mood and lower levels of serotonin are associated with mood disorders, such as depression. Furthermore, some cases of OCD are also associated with the reduced levels of serotonin, which may be caused by the SERT gene (see above). Further support for the role of serotonin in OCD comes from research examining anti-depressants, which have found that drugs which increase the level of serotonin are effective in treating patients with OCD.

In addition, the neurotransmitter dopamine has also been implicated in OCD, with higher levels of dopamine being associated with some of the symptoms of OCD, in particular the compulsive behaviours.

Brain Structures

Two brain regions have been implicated in OCD, including the basal ganglia and orbitofrontal cortex.

The basal ganglia is a brain structure involved in multiple processes, including the coordination of movement. Patients who suffer head injuries in this region often develop OCD-like symptoms, following their recovery. Furthermore, Max et al. (1994) found that when the basal ganglia is disconnected from the frontal cortex during surgery, OCD-like symptoms are reduced, providing further support for the role of the basal ganglia in OCD.

Another brain region associated with OCD is the orbitofrontal cortex, a region which converts sensory information into thoughts and actions. PET scans have found higher activity in the orbitofrontal cortex in patients with. One suggestion is that the heightened activity in the orbitofrontal cortex increases the conversion of sensory information to actions (behaviours) which results in compulsions. The increased activity also prevents patients from stopping their behaviours.


One strength of the biological explanation of OCD comes from research from family studies. Lewis (1936) examined patients with OCD and found that 37% of the patients with OCD had parents with the disorder and 21% had siblings who suffered. Research from family studies, like Lewis, provide support for a genetic explanation to OCD, although it does not rule out other (environmental) factors playing a role.

Further support for the biological explanation of OCD comes from twin studies which have provided strong evidence for a genetic link. Nestadt et al. (2010) conducted a review of previous twin studies examining OCD. They found that 68% of identical twins and 31% of non-identical twins experience OCD, which suggests a very strong genetic component.

Support for the neural explanations of OCD come from research examining biological treatments including anti-depressants.

Anti-depressants typically work by increasing levels of the neurotransmitter serotonin. These drugs are effective in reducing the symptoms of OCD and provide support for a neural explanation of OCD.

Extension: However, no twin study has found a concordance rate of 100% in identical twins, which means that biological factors are not the only factor contributing to OCD and there must be environmental factors which also contribute to this disorder.

One weakness of the biological explanation for OCD is that it ignores other factors and is reductionist. For example, the biological approach does not take into account cognitions (thinking) and learning. Some psychologists suggest that OCD may be learnt through classical conditioning and maintained through operant conditioning stimulus (for example, dirt) is associated with anxiety and this association is then maintained through operant conditioning, where a person avoids dirt and continually washes their hands. This hand washing reduces their anxiety and negatively reinforces their compulsions.

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