Although non-specific concepts of madness have been around for several thousand years, the psychiatrist and philosopher Karl Jaspers was the first to define the three main criteria for a belief to be considered delusional in his 1913 book General Psychopathology. These criteria are:
- certainty (held with absolute conviction)
- incorrigibility (not changeable by compelling counterargument or proof to the contrary)
- impossibility or falsity of content (implausible, bizarre or patently untrue)
Furthermore, when a false belief involves a value judgment, it is only considered as a delusion if it is so extreme that it cannot be or never can be proven true. For example: a man claiming that he flew into the sun and flew back home. This would be considered a delusion, unless he was speaking figuratively.
Delusions are categorized into four different groups:
- Bizarre delusion: A delusion that is very strange and completely implausible; an example of a bizarre delusion would be that aliens have removed the reporting person’s brain.
- Non-bizarre delusion: A delusion that, though false, is at least possible, e.g., the affected person mistakenly believes that he is under constant police surveillance.
- Mood-congruent delusion: Any delusion with content consistent with either a depressive or manic state, e.g., a depressed person believes that news anchors on television highly disapprove of him, or a person in a manic state might believe she is a powerful deity.
- Mood-neutral delusion: A delusion that does not relate to the sufferer’s emotional state; for example, a belief that an extra limb is growing out of the back of one’s head is neutral to either depression or mania.
In addition to these categories, delusions often manifest according to a consistent theme. Although delusions can have any theme, certain themes are more common. Some of the more common delusion themes are:
- Delusion of control: This is a false belief that another person, group of people, or external force controls one’s general thoughts, feelings, impulses, or behavior.
- Cotard delusion: This is a false belief that one does not exist or has died.
- Delusional jealousy: A person with this delusion falsely believes that a spouse or lover is having an affair, with no proof to back up their claim.
- Delusion of guilt or sin (or delusion of self-accusation): This is an ungrounded feeling of remorse or guilt of delusional intensity.
- Delusion of mind being read: The false belief that other people can know one’s thoughts.
- Delusion of thought insertion: The belief that another thinks through the mind of the person.
- Delusion of reference: The person falsely believes that insignificant remarks, events, or objects in one’s environment have personal meaning or significance.
- Erotomania A delusion in which someone falsely believes another person is in love with them.
- Grandiose religious delusion: The belief that the affected person is a god, or chosen to act as a god.
- Somatic delusion: A delusion whose content pertains to bodily functioning, bodily sensations, or physical appearance. Usually the false belief is that the body is somehow diseased, abnormal, or changed. A specific example of this delusion is delusional parasitosis: a delusion in which one feels infested with insects, bacteria, mites, spiders,lice, fleas, worms, or other organisms. Affected individuals may also report being repeatedly bitten. In some cases, entomologists are asked to investigate cases of mysterious bites. Sometimes physical manifestations may occur including skin lesions.
- Delusion of poverty: The person strongly believes that he is financially incapacitated. Although this type of delusion is less common now, it was particularly widespread in the days before state support.
The modern definition and Jaspers’ original criteria have been criticised, as counter-examples can be shown for every defining feature.
Studies on psychiatric patients show that delusions vary in intensity and conviction over time, which suggests that certainty and incorrigibility are not necessary components of a delusional belief.
Delusions do not necessarily have to be false or ‘incorrect inferences about external reality’. Some religious or spiritual beliefs by their nature may not be falsifiable, and hence cannot be described as false or incorrect, no matter whether the person holding these beliefs was diagnosed as delusional or not.
In other situations the delusion may turn out to be true belief. For example, delusional jealousy, where a person believes that their partner is being unfaithful (and may even follow them into the bathroom believing them to be seeing their lover even during the briefest of partings) may result in the faithful partner being driven to infidelity by the constant and unreasonable strain put on them by their delusional spouse. In this case the delusion does not cease to be a delusion because the content later turns out to be true.
In other cases, the delusion may be assumed to be false by a doctor or psychiatrist assessing the belief, because it seems to be unlikely, bizarre or held with excessive conviction. Psychiatrists rarely have the time or resources to check the validity of a person’s claims leading to some true beliefs to be erroneously classified as delusional. This is known as the Martha Mitchell effect, after the wife of the attorney general who alleged that illegal activity was taking place in the White House. At the time her claims were thought to be signs of mental illness, and only after the Watergate scandal broke was she proved right (and hence sane).
Similar factors have led to criticisms of Jaspers’ definition of true delusions as being ultimately ‘un-understandable’. Critics (such as R. D. Laing) have argued that this leads to the diagnosis of delusions being based on the subjective understanding of a particular psychiatrist, who may not have access to all the information that might make a belief otherwise interpretable. R.D. Laing’s hypothesis has been applied to some forms of projective therapy to “fix” a delusional system so that it cannot be altered by the patient. Psychiatric researchers at Yale University, Ohio State University and the Community Mental Health Center of Middle Georgia have used novels and motion picture films as the focus. Texts, plots and cinematography are discussed and the delusions approached tangentially. This use of fiction to decrease the malleability of a delusion was employed in a joint project by science-fiction author Philip Jose Farmer and Yale psychiatrist A. James Giannini. They wrote the novel Red Orc’s Rage, which, recursively, deals with delusional adolescents who are treated with a form of projective therapy. In this novel’s fictional setting other novels written by Farmer are discussed and the characters are symbolically integrated into the delusions of fictional patients. This particular novel was then applied to real-life clinical settings.
Another difficulty with the diagnosis of delusions is that almost all of these features can be found in “normal” beliefs. Many religious beliefs hold exactly the same features, yet are not universally considered delusional. These factors have led the psychiatrist Anthony David to note that “there is no acceptable (rather than accepted) definition of a delusion.”In practice, psychiatrists tend to diagnose a belief as delusional if it is either patently bizarre, causing significant distress, or excessively pre-occupying the patient, especially if the person is subsequently unswayed in belief by counter-evidence or reasonable arguments.
It is important to distinguish true delusions from other symptoms such as anxiety, fear, or paranoia. To diagnose delusions a mental state examination may be used. This test includes appearance, mood, affect, behavior, rate and continuity of speech, evidence of hallucinations or abnormal beliefs, thought content, orientation to time, place and person, attention and concentration, insight and judgment, as well as short-term memory.
Johnson-Laird suggests that delusions may be viewed as the natural consequence of failure to distinguish conceptual relevance. That is, the person takes irrelevant information and puts it in the form of disconnected experiences, then it is taken to be relevant in a manner that suggests false causal connections. Furthermore, the person takes the relevant information, in the form of counterexamples, and ignores it.