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Delusions

Delusions are easier to characterize than to define. A deluded individual has a belief (or beliefs) that is held with a degree of certainty unwarranted by the evidence available to that person, that is strongly resistant to vigorous opposition from others, and that distresses or preoccupies the person or otherwise disrupts his or her everyday functioning. While some aspects of this characterization (e.g., the notion that delusions are beliefs) have been subject to philosophical debate, this entry will focus on issues of classification, etiology, and treatment.

Ordinary versus Bizarre Delusions

Among other distinctions (e.g., the thematic subtype of delusions; the extent to which delusions are circumscribed; the extent to which delusions are congruous with the patient's mood), psychiatrists distinguish between bizarre delusions, concerning events and situations beyond the realm of possibility, and so-called ordinary delusions, involving beliefs that could conceivably be true but that are highly implausible given the evidence at hand. As an example of the former, consider the mirrored-self misidentification delusion. Individuals with this delusion believe that the person they see when looking in the mirror is a stranger—a state of affairs clearly at variance with ordinary reality. An example of the latter is the de Clérambault delusion, the conviction that another person—typically someone important or famous—is in love with the deluded individual. Although such a belief may well be implausible, it is nevertheless possible—it could be true.

The ordinary or bizarre distinction has important implications for clinical diagnosis, as the occurrence of bizarre delusions is sufficient for a diagnosis of schizophrenia, while ruling out a diagnosis of delusional disorder. Note in this connection that delusions manifest in a range of both psychiatric and neurological conditions. While they are a hallmark of psychotic disorders such as schizophrenia and schizoaffective disorder, they are also observed in cases of dementia, epilepsy, Parkinson's disease, and traumatic brain injury, among other so-called organic conditions.

Theoretical Accounts

A variety of theoretical accounts of delusions have been proposed. At the cognitive level a general distinction can be made between defense explanations, which conceptualize delusions as purposive constructions serving emotional functions and deficit explanations, which view delusions as stemming from functionless disturbances in ordinary cognitive processes, underpinned by neuroanatomical or neurophysiological abnormalities.

Defense Explanations

Consider the Capgras delusion. Individuals with this delusion believe that a person close to them has been replaced by an impostor. In an early formulation of this condition, Capgras patients were viewed as beset by ambivalent feelings toward the replaced individual—perhaps appropriate feelings of familial love in conflict with unacceptable incestuous desires. The suggestion was that in coming to believe that the object of their desire was actually a fraudulent impersonator of their family member, such individuals had found a way of resolving the tension between their incongruous feelings (sexual desire is not so troubling if the desired individual is merely impersonating a family member).

Richard Bentall, Peter Kinderman, and colleagues are more rigorous contemporary advocates of delusion-as-defense explanations. These authors focus on persecutory delusions, which are the most common delusional subtype. An example would be an individual who believes that his neighbors are trying to harm him by pumping poisonous gas into his house. Bentall, Kinderman, and colleagues have suggested that such delusions serve the defensive function of preserving overt self-esteem. Their formulation is complex, but in essence, the idea is that in attributing unpleasant events to the malicious intent of others, persecutory deluded individuals evade responsibility for their misfortunes and thus avoid activating negative beliefs about themselves.

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