# Monothematic delusion

A monothematic delusion is a delusional state that concerns only one particular topic. This is contrasted by what is sometimes called multi-thematic or polythematic delusions where the person has a range of delusions (typically the case of schizophrenia). These disorders can occur within the context of schizophrenia or dementia or they can occur without any other signs of mental illness. When these disorders are found outside the context of mental illness, they are often caused by organic dysfunction as a result of traumatic brain injury, stroke, or neurological illness.
People who experience these delusions as a result of organic dysfunction often do not have any obvious intellectual deficiency nor do they have any other symptoms. Additionally, a few of these people even have some awareness that their beliefs are bizarre, yet they cannot be persuaded that their beliefs are false.

## Types

Some delusions that fall under this category are:
* Capgras delusion: the belief that (usually) a close relative or spouse has been replaced by an identical-looking impostor.
* Fregoli delusion: the belief that various people whom the believer meets are actually the same person in disguise.
* Intermetamorphosis: the belief that people in one's environment swap identities with each other while maintaining the same appearance.
* Subjective doubles: a person believes there is a doppelgänger or double of themselves carrying out independent actions.
* Cotard delusion: the belief that oneself is dead or does not exist; sometimes coupled with the belief that one is putrefying or missing internal organs.
* Mirrored-self misidentification: the belief that one's reflection in a mirror is some other person.
* Reduplicative paramnesia: the belief that a familiar person, place, object, or body part has been duplicated. For example, a person may believe that they are, in fact, not in the hospital to which they were admitted, but in an identical-looking hospital in a different part of the country.
* Somatoparaphrenia: the delusion where one denies ownership of a limb or an entire side of one's body (often connected with stroke).
Note that some of these delusions are sometimes grouped under the umbrella term of delusional misidentification syndrome.

## Causes

Current cognitive neuropsychology research points toward a two-factor cause of monothematic delusions. The first factor is an anomalous experience, often due to a neurological defect. The second is an impairment of the belief formation cognitive process.
For example, the first factor in Capgras delusion is an impairment in how the brain recognizes a familiar face – specifically, how it cross-references one’s feelings toward the person. A patient with this disorder can recognize a loved one’s face, but seeing it does not elicit the expected emotional reaction. The patient’s mind interprets this as seeing a stranger, who merely looks like their loved one.
The second factor, an impairment in the belief formation cognitive process, remains unclear as the process itself is not well-understood. And while the two-factor hypothesis is currently the dominant model, there are alternatives which do not involve a second factor.
There is evidence that delusional people are more prone to jumping to conclusions, predisposing them to accept anomalous experiences as real and make snap judgments about them. Studies have also shown that such individuals are more prone to making errors due to matching bias, showing an higher than average tendency to seek confirmation for existing beliefs as opposed to testing how well they stand up to being challenged.
These judgment biases help explain how delusion-prone people adopt and hold firm to extreme beliefs, though researchers disagree about whether they are sufficient explanation. Some posit that patients must have an additional neurological defect that affects their belief formation process. This defect is largely speculative, but would likely be located in the right hemisphere of the brain.