The Truman Show delusion, informally known as Truman syndrome, is a type of delusion in which the person believes that their lives are staged reality shows, or that they are being watched on cameras. The term was coined in 2008 by brothers Joel Gold and Ian Gold, a psychiatrist and a neurophilosopher, respectively, after the film "The Truman Show".

The Truman Show delusion is not officially recognized nor listed in the "Diagnostic and Statistical Manual of the American Psychiatric Association".

Boanthropy is a psychological disorder in which a human believes himself or herself to be a bovine.

Folie à deux (; ; French for "madness of two"), or shared psychosis, is a psychiatric syndrome in which symptoms of a delusional belief and sometimes hallucinations are transmitted from one individual to another. The same syndrome shared by more than two people may be called "folie à trois", "folie à quatre", "folie en famille" ("family madness"), or even "folie à plusieurs" ("madness of many").

Recent psychiatric classifications refer to the syndrome as shared psychotic disorder (DSM-IV – 297.3) and induced delusional disorder (F24) in the ICD-10, although the research literature largely uses the original name. This disorder is not in the current DSM (DSM-5). The disorder was first conceptualized in 19th-century French psychiatry by Charles Lasègue and Jean-Pierre Falret and is also known as Lasègue-Falret syndrome.

Ideas of reference and delusions of reference describe the phenomenon of an individual's experiencing innocuous events or mere coincidences and believing they have strong personal significance. It is "the notion that everything one perceives in the world relates to one's own destiny".

In psychiatry, delusions of reference form part of the diagnostic criteria for psychotic illnesses such as schizophrenia, delusional disorder, bipolar disorder (during the elevated stages of mania), as well as schizotypal personality disorder. To a lesser extent, it can be a hallmark of paranoid personality disorder. Such symptoms can also be caused by intoxication, especially with hallucinogens or stimulants like methamphetamine.

This syndrome is most commonly diagnosed when the two or more individuals concerned live in proximity and may be socially or physically isolated and have little interaction with other people. Various sub-classifications of "folie à deux" have been proposed to describe how the delusional belief comes to be held by more than one person :

- Folie imposée is where a dominant person (known as the 'primary', 'inducer' or 'principal') initially forms a delusional belief during a psychotic episode and imposes it on another person or persons (known as the 'secondary', 'acceptor' or 'associate') with the assumption that the secondary person might not have become deluded if left to his or her own devices. If the parties are admitted to hospital separately, then the delusions in the person with the induced beliefs usually resolve without the need of medication.

- Folie simultanée describes either the situation where two people considered to suffer independently from psychosis influence the content of each other's delusions so they become identical or strikingly similar, or one in which two people "morbidly predisposed" to delusional psychosis mutually trigger symptoms in each other.

Folie à deux and its more populous cousins are in many ways a psychiatric curiosity. The current Diagnostic and Statistical Manual of Mental Disorders states that a person cannot be diagnosed as being delusional if the belief in question is one "ordinarily accepted by other members of the person's culture or subculture" (see entry for delusion). It is not clear at what point a belief considered to be delusional escapes from the "folie à..." diagnostic category and becomes legitimate because of the number of people holding it. When a large number of people may come to believe obviously false and potentially distressing things based purely on hearsay, these beliefs are not considered to be clinical delusions by the psychiatric profession and are labelled instead as mass hysteria.

The glass delusion is an external manifestation of a psychiatric disorder recorded in Europe mainly in the late Middle Ages and early modern period (15th to 17th centuries). People feared that they were made of glass "and therefore likely to shatter into pieces". One famous early sufferer was King Charles VI of France who refused to allow people to touch him, and wore reinforced clothing to protect himself from accidental "shattering".

The article "Cotard's syndrome: A Review" (2010) reports successful pharmacological treatments (mono-therapeutic and multi-therapeutic) using antidepressant, antipsychotic, and mood stabilizing drugs; likewise, with the depressed patient, electroconvulsive therapy (ECT) is more effective than pharmacotherapy. Cotard syndrome resulting from an adverse drug reaction to valacyclovir is attributed to elevated serum concentration of one of valacyclovir's metabolites, 9-carboxymethoxymethylguanine (CMMG). Successful treatment warrants cessation of the drug, valacyclovir. Hemodialysis was associated with timely clearance of CMMG and resolution of symptoms.

Concentration of the glass delusion among the wealthy and educated classes allowed modern scholars to associate it with a wider and better described disorder of scholar's melancholy.

Psychotherapies that may be helpful in delusional disorder include individual psychotherapy, cognitive-behavioral therapy (CBT), and family therapy.

In rare cases, individuals may believe that other people have transformed into animals. This has been termed "lycanthropic intermetamorphosis" and "lycanthropy spectrum". A 2009 study reported that, after the consumption of the drug MDMA (Ecstasy), a man displayed symptoms of paranoid psychosis by claiming that his relatives had changed into various animals such as a boar, a donkey and a horse.

A religious delusion is any delusion involving religious themes or subject matter. Though a small minority of psychologists have characterized all or nearly all religion as delusion, others focus solely on a denial of any spiritual cause of symptoms exhibited by a patient and look for other answers relating to a chemical imbalance in the brain, although there is actually no evidence of pathology in any psychiatric illness which means a diagnosis is made purely on opinions of professionals based on symptoms the person exhibits.

Paranoia is an instinct or thought process believed to be heavily influenced by anxiety or fear, often to the point of delusion and irrationality. Paranoid thinking typically includes persecutory, or beliefs of concerning a perceived threat towards oneself (e.g. ""Everyone is out to get me"", which is an American parochial phrase). Paranoia is distinct from phobias, which also involve irrational fear, but usually no blame. Making false accusations and the general distrust of others also frequently accompany paranoia. For example, an incident most people would view as an accident or coincidence, a paranoid person might believe was intentional. Paranoia is a central symptom of psychosis. It is also a matter of personal tolerance for the individual that might be in conflict with psychiatric diagnoses.

One important factor may be differences or changes in parts of the brain known to be involved in representing body shape (e.g., see proprioception and body image). A neuroimaging study of two people diagnosed with clinical lycanthropy showed that these areas display unusual activation, suggesting that when people report their bodies are changing shape, they may be genuinely perceiving those feelings.

Examples from a 295-subject study in Lithuania showed that the most common religious delusions were being a saint (in women) and being God (in men).

In one study of 193 people who had previously been admitted to hospital and subsequently diagnosed with schizophrenia, 24% were found to have religious delusions.

A 1999 study identified that religious delusions were often present or expressed in persons with forensic committal to a psychiatric unit.

A fixed fantasy — also known as a "dysfunctional schema" — is a belief or system of beliefs held by a single individual to be genuine, but that cannot be verified in reality. The term is typically applied to individuals suffering from some type of psychiatric dysregulation, most often a personality disorder.

The term is also used in the different context of psychoanalysis to distinguish between 'a normal transitory one and a fixed fantasy' with respect to the phantasised 'fulfilment in conscious or unconscious thought of the sexualised wish'.

An idée fixe is a preoccupation of mind believed to be firmly resistant to any attempt to modify it, a fixation. The name originates from the French "idée", "idea" and "fixe", "fixed."

A delusion is a mistaken belief that is held with strong conviction even when presented with superior evidence to the contrary. As a pathology, it is distinct from a belief based on false or incomplete information, confabulation, dogma, illusion, or some other misleading effects of perception.

Delusions typically occur in the context of neurological or psychiatric disease, although they are not tied to any particular disorder and have been found to occur in the context of many pathological states (both physical and mental). However, they are of particular diagnostic importance in psychotic disorders including schizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.

A popular symptom of paranoia is the attribution bias. These individuals typically have a biased perception of reality, often exhibiting more hostile beliefs. A paranoid person may view someone else's accidental behavior as though it is with intent or threatening.

An investigation of a non-clinical paranoid population found that feeling powerless and depressed, isolating oneself, and relinquishing activities are characteristics that could be associated with those exhibiting more frequent paranoia.

Some scientists have created different subtypes for the various symptoms of paranoia including erotic, persecutory, litigious, and exalted.

Due to the suspicious and troublesome personality traits of paranoia, it is unlikely that someone with paranoia will thrive in interpersonal relationships. Most commonly paranoid individuals tend to be of a single status. According to some research there is a hierarchy for paranoia. The least common types of paranoia at the very top of the hierarchy would be those involving more serious threats. Social anxiety is at the bottom of this hierarchy as the most frequently exhibited level of paranoia.

The most famous sufferer of this disorder is claimed to be Nebuchadnezzar II, who in the Book of Daniel "was driven from men, and did eat grass as oxen". Most probably, the authors were making fun of his aversion to meat or violence.

Carl Jung would subsequently instance 'Nebuchadnezzar...[as] a complete regressive degeneration of a man who has overreached himself'.

According to Persian traditions, the Buyid prince Majd al-Dawla was suffering from an illusion that he is a cow, making the sound of a cow and asking that to be killed so that his flesh could be consumed. He was cured by Avicenna.

Individual therapy may be best suited to treat the individual's delusions. Persistence is needed in establishing a therapeutic empathy without validating the patient’s delusional system or overtly confronting the system. Cognitive techniques that include reality testing and reframing can be used. Antipsychotics and other therapeutic drugs have been used with relative success.

Sluggish schizophrenia or slow progressive schizophrenia (, "vyalotekushchaya shizofreniya") was a diagnostic category used in Soviet Union to describe what they claimed was a form of schizophrenia characterized by a slowly progressive course; it was diagnosed even in a patient who showed no symptoms of schizophrenia or other psychotic disorders, on the assumption that these symptoms would appear later. It was developed in the 1960s by Soviet psychiatrist Andrei Snezhnevsky and his colleagues, and was used exclusively in the USSR and several Eastern Bloc countries, until the fall of Communism starting in 1989. The diagnosis has long been discredited because of its scientific inadequacy and its use as a means of confining dissenters. It has never been used or recognized outside of Soviet Union, or by international organizations such as the World Health Organization. It is considered a prime example of the political abuse of psychiatry in the Soviet Union.

Sluggish schizophrenia was the most infamous of diagnoses used by Soviet psychiatrists, due to its usage against political dissidents. After being discharged from a hospital, persons diagnosed with sluggish schizophrenia were deprived of their civic rights, credibility and employability. The usage of this diagnosis has been internationally condemned.

In the Russian version of the 10th revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10), which has long been used throughout present-day Russia, sluggish schizophrenia is no longer listed as a form of schizophrenia, but it is still included as a schizotypal disorder in section F21 of chapter V.

According to Sergei Jargin, the same Russian term "vyalotekushchaya" for sluggish schizophrenia continues to be used and is now translated in English summaries of articles not as "sluggish" but as "slow progressive".

A challenge in the treatment of delusional disorders is that most patients have limited insight, and do not acknowledge that there is a problem. Most patients are treated as out-patients, although hospitalization may be required in some cases if there is a risk of harm to self or others. Individual psychotherapy is recommended rather than group psychotherapy, as patients are often quite suspicious and sensitive. Antipsychotics are not well tested in delusional disorder, but they do not seem to work very well, and often have no effect on the core delusional belief. Antipsychotics may be more useful in managing agitation that can accompany delusional disorder. Until further evidence is found, it seems reasonable to offer treatments which have efficacy in other psychotic disorders.

Psychotherapy for patients with delusional disorder can include cognitive therapy which is conducted with the use of empathy. During the process, the therapist can ask hypothetical questions in a form of therapeutic Socratic questioning. This therapy has been mostly studied in patients with the persecutory type. The combination of pharmacotherapy with cognitive therapy integrates treating the possible underlying biological problems and decreasing the symptoms with psychotherapy as well. Psychotherapy has been said to be the most useful form of treatment because of the trust formed in a patient and therapist relationship.

Supportive therapy has also been shown to be helpful. Its goal is to facilitate treatment adherence and provide education about the illness and its treatment.

Furthermore, providing social skills training has helped many persons. It can promote interpersonal competence as well as confidence and comfort when interacting with those individuals perceived as a threat.

Insight-oriented therapy is rarely indicated or contraindicated; yet there are reports of successful treatment. Its goals are to develop therapeutic alliance, containment of projected feelings of hatred, impotence, and badness; measured interpretation as well as the development of a sense of creative doubt in the internal perception of the world. The latter requires empathy with the patient's defensive position.

Erotomania is a type of delusional disorder where the affected person believes that another person is in love with him or her. This belief is usually applied to someone with higher status or a famous person, but can also be applied to a complete stranger. Erotomanic delusions often occur in patients with schizophrenia and other psychotic disorders, but can also occur during a manic episode in the context of bipolar I disorder. During an erotomanic delusion, the patient believes that a secret admirer is declaring his or her affection for the patient, often by special glances, signals, telepathy, or messages through the media. Usually the patient then returns the perceived affection by means of letters, phone calls, gifts, and visits to the unwitting recipient. Even though these advances are unexpected and often unwanted, any denial of affection by the object of this delusional love is dismissed by the patient as a ploy to conceal the forbidden love from the rest of the world.

Erotomania is also called de Clérambault's syndrome, after French psychiatrist Gaëtan Gatian de Clérambault (1872–1934), who published a comprehensive review paper on the subject ("Les Psychoses Passionnelles") in 1921. Erotomania should not be confused with obsessive love or obsession with unrequited love, neither of which involves delusion.

Current cognitive neuropsychology research points toward a two-factor approach to the cause of monothematic delusions. The first factor being the anomalous experience—often a neurological defect—which leads to the delusion, and the second factor being an impairment of the belief formation cognitive process.

As an example of one of these first factors, several studies point toward Capgras delusion being the result of a disorder of the affect component of face perception. As a result, while the person can recognize their spouse (or other close relation) they do not feel the typical emotional reaction, and thus the spouse does not seem like the person they once knew.

As studies have shown, these neurological defects are not enough on their own to cause delusional thinking. An additional second factor—a bias or impairment of the belief formation cognitive process—is required to solidify and maintain the delusion. Since we do not currently have a solid cognitive model of the belief formation process, this second factor is still somewhat of an unknown.

Some research has shown that delusional people are more prone to jumping to conclusions, and thus they would be more likely to take their anomalous experience as veridical and make snap judgments based on these experiences. Additionally, studies have shown that they are more prone to making errors due to matching bias—indicative of a tendency to try and confirm the rule. These two judgment biases help explain how delusion-prone people could grasp onto extreme delusions and be very resistant to change.

Researchers claim this is enough to explain the delusional thinking. However, other researchers still argue that these biases are not enough to explain why they remain completely impervious to evidence over time. They believe that there must be some additional unknown neurological defect in the patient's belief system (probably in the right hemisphere).

Grandiose delusions (GD), delusions of grandeur, expansive delusions also known as megalomania are a subtype of delusion that occur in patients suffering from a wide range of psychiatric diseases, including two-thirds of patients in manic state of bipolar disorder, half of those with schizophrenia, patients with the grandiose subtype of delusional disorder, and a substantial portion of those with substance abuse disorders. GDs are characterized by fantastical beliefs that one is famous, omnipotent, wealthy, or otherwise very powerful. The delusions are generally fantastic and typically have a religious, science fictional, or supernatural theme. There is a relative lack of research into GD, in contrast to persecutory delusions and auditory hallucinations. About 10% of healthy people experience grandiose thoughts but do not meet full criteria for a diagnosis of GD.