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.

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".

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.

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.

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

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.

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.

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.

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.

Affected individuals believe that they are in the process of transforming into an animal or have already transformed into an animal. It has been associated with the altered states of mind that accompany psychosis (the that typically involves delusions and hallucinations) with the transformation only seeming to happen in the mind and behavior of the affected person.

A study on lycanthropy from the McLean Hospital reported on a series of cases and proposed some diagnostic criteria by which lycanthropy could be recognised:

- A patient reports in a moment of lucidity or reminiscence that they sometimes feel as an animal or have felt like one.

- A patient behaves in a manner that resembles animal behavior, for example howling, growling, or crawling.

According to this criteria, either a delusional belief in current or past transformation or behavior that suggests a person thinks of themselves as transformed is considered evidence of clinical lycanthropy. The authors note that, although the condition seems to be an expression of psychosis, there is no specific diagnosis of mental or neurological illness associated with its behavioral consequences.

DSM-IV Criteria

Clinical Lycanthropy is thought to be a cultural manifestation of schizophrenia due to the first 4 symptomatic criteria. The first criteria are delusions, and this fits clinical lycanthropy because a person believing that he or she turns into an animal is a delusion. The second symptom is hallucination, and people with clinical lycanthropy have vivid hallucinations of being an animal, and having traits that animal has, whether it be claws, fur, fangs, or whatever that particular animal has. The next symptom is disorganized speech, from a certain human, cultural perspective. The people who have the diagnosis of clinical lycanthropy often emit the sounds of the animal which they believe they become. So, if a person believes that he or she is a werewolf, they may begin to howl under the moon or sometimes even in the daylight. The last symptom that matches schizophrenia is grossly disorganized behavior. This is appropriate because individuals with clinical lycanthropy often act like the animal they believe they have become, including living outside and changing their diet.

It also seems that lycanthropy is not specific to an experience of human-to-wolf transformation; a wide variety of creatures have been reported as part of the shape-shifting experience. A review of the medical literature from early 2004 lists over thirty published cases of lycanthropy, only the minority of which have wolf or dog themes. Canines are certainly not uncommon, although the experience of being transformed into a hyena, cat, horse, bird or tiger has been reported on more than one occasion. Transformation into frogs, and even bees, has been reported in some instances. In Japan, transformation into foxes and dogs was usual (, ). A 1989 case study described how one individual reported a serial transformation, experiencing a change from human to dog, to horse, and then finally cat, before returning to the reality of human existence after treatment. There are also reports of people who experienced transformation into an animal only listed as "unspecified".

There is a case study of a psychiatric patient who had both clinical lycanthropy and Cotard delusion. The term "ophidianthropy" refers to the delusion that one has been transformed into a snake. Two case studies have been reported.

Individuals experiencing religious delusions are preoccupied with religious subjects that are not within the expected beliefs for an individual's background, including culture, education, and known experiences of religion. These preoccupations are incongruous with the mood of the subject. Falling within the definition also are delusions arising in psychotic depression; however, these must present within a major depressive episode and be congruous with mood.

Researchers in a 2000 study found religious delusions to be unrelated to any specific set of diagnostic criteria, but correlated with demographic criteria, primarily age. In a comparative study sampling 313 patients, those with religious delusion were found to be aged older, and had been placed on a drug regime or started a treatment programme at an earlier stage. In the context of presentation, their global functioning was found to be worse than another group of patients without religious delusions. The first group also scored higher on the Scale for the Assessment of Positive Symptoms (SAPS), had a greater total on the Brief Psychiatric Rating Scale (BPRS), and were treated with a higher mean number of neuroleptic medications of differing types during their hospitalization.

Religious delusion was found in 2007 to strongly correlate with "temporolimbic overactivity". This is a condition where irregularities in the brain's limbic system may present as symptoms of paranoid schizophrenia.

In a 2010 study, Swiss psychiatrists found religious delusions with themes of spiritual persecution by malevolent spirit-entities, control exerted over the person by spirit-entities, delusional experience of sin and guilt, or delusions of grandeur.

Religious delusions have generally been found to be less stressful than other types of delusion. A study found adherents to new religious movements to have similar delusionary cognition, as rated by the Delusions Inventory, to a psychotic group, although the former reported feeling less distressed by their experiences than the latter.

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: False belief that another person, group of people, or external force controls one's general thoughts, feelings, impulses, or behavior.

- Cotard delusion: False belief that one does not exist or has died.

- Delusional jealousy: False belief 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): Ungrounded feeling of remorse or guilt of delusional intensity.

- Delusion of mind being read: False belief that other people can know one's thoughts.

- Delusion of thought insertion: Belief that another thinks through the mind of the person.

- Delusion of reference: False belief that insignificant remarks, events, or objects in one's environment have personal meaning or significance.

- Erotomania: False belief that another person is in love with them.

- Grandiose religious delusion: Belief that the affected person is a god or chosen to act as a god.

- Somatic delusion: 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: 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: Person strongly believes they are financially incapacitated. Although this type of delusion is less common now, it was particularly widespread in the days preceding state support.

Grandiose delusions or delusions of grandeur are principally a subtype of delusional disorder but could possibly feature as a symptom of schizophrenia and manic episodes of bipolar disorder. Grandiose delusions are characterized by fantastical beliefs that one is famous, omnipotent or otherwise very powerful. The delusions are generally fantastic, often with a supernatural, science-fictional, or religious bent. In colloquial usage, one who overestimates one's own abilities, talents, stature or situation is sometimes said to have "delusions of grandeur". This is generally due to excessive pride, rather than any actual delusions. Grandiose delusions or delusions of grandeur can also be associated with megalomania.

The delusion of negation is the central symptom in Cotard's syndrome. The patient afflicted with this mental illness usually denies his or her existence, the existence of a certain body part, or the existence of a portion of their body. Cotard's syndrome exists in three stages: (i) Germination stage—the symptoms of psychotic depression and of hypochondria appear; (ii) Blooming stage—the full development of the syndrome and the delusions of negation; and (iii) Chronic stage—continued, severe delusions along with chronic psychiatric depression.

The Cotard syndrome withdraws the afflicted person from other people due to the neglect of their personal hygiene and physical health. The delusion of negation of self prevents the patient from making sense of external reality, which then produces a distorted view of the external world. Such a delusion of negation is usually found in the psychotic patient who also presents with schizophrenia. Although a diagnosis of Cotard's syndrome does not require the patient's having had hallucinations, the strong delusions of negation are comparable to those found in schizophrenic patients.

The article "Betwixt Life and Death: Case Studies of the Cotard Delusion" (1996) describes a contemporary case of Cotard delusion, which occurred in a Scotsman whose brain was damaged in a motorcycle accident:

The article "Recurrent Postictal Depression with Cotard Delusion" (2005) describes the case of a fourteen-year-old epileptic boy whose distorted perception of reality resulted from Cotard syndrome. His mental health history was of a boy expressing themes of death, chronic sadness, decreased physical activity in playtime, social withdrawal, and disturbed biological functions. About twice a year, the boy suffered episodes that lasted between three weeks and three months. In the course of each episode, he said that everyone and everything was dead (including trees), described himself as a dead body, and warned that the world would be destroyed within hours. Throughout the episode, the boy showed no response to pleasurable stimuli and had no interest in social activities.

Persecutory delusions are the most common type of delusions and involve the theme of being followed, harassed, cheated, poisoned or drugged, conspired against, spied on, attacked, or otherwise obstructed in the pursuit of goals.

Persecutory delusions are a condition in which the affected person wrongly believes that they are being persecuted. Specifically, they have been defined as containing three central elements: The individual thinks that:

1. harm is occurring, or is going to occur.

2. the persecutor(s) has(have) the intention to cause harm.

3. they are constantly being prejudged or profiled.

According to the DSM-IV-TR, persecutory delusions are the most common form of delusions in schizophrenia, where the person believes they are "being tormented, followed, sabotaged, tricked, spied on, or ridiculed." In the DSM-IV-TR, persecutory delusions are the main feature of the persecutory type of delusional disorder. When the focus is to remedy some injustice by legal action, they are sometimes called "querulous paranoia".

Diagnosis of a specific type of delusional disorder can sometimes be made based on the content of the delusions. The "Diagnostic and Statistical Manual of Mental Disorders" (DSM) enumerates seven types:

- Erotomanic type (erotomania): delusion that another person, often a prominent figure, is in love with the individual. The individual may breach the law as he/she tries to obsessively make contact with the desired person.

- Grandiose type: delusion of inflated worth, power, knowledge, identity or believes themself to be a famous person, claiming the actual person is an impostor or an impersonator.

- Jealous type: delusion that the individual's sexual partner is unfaithful when it is untrue. The patient may follow the partner, check text messages, emails, phone calls etc. in an attempt to find "evidence" of the infidelity.

- Persecutory type: This delusion is a common subtype. It includes the belief that the person (or someone to whom the person is close) is being malevolently treated in some way. The patient may believe that he/she has been drugged, spied upon, harassed and so on and may seek "justice" by making police reports, taking court action or even acting violently.

- Somatic type: delusions that the person has some physical defect or general medical condition

- Mixed type: delusions with characteristics of more than one of the above types but with no one theme predominating.

- Unspecified type: delusions that cannot be clearly determined or characterized in any of the categories in the specific types.

According to the DSM-IV-TR diagnostic criteria for delusional disorders, grandiose-type symptoms include grossly exaggerated beliefs of:

- self-worth

- power

- knowledge

- identity

- exceptional relationship to a divinity or famous person.

For example, a patient who has fictitious beliefs about his or her power or authority may believe himself or herself to be a ruling monarch who deserves to be treated like royalty.

There are substantial differences in the degree of grandiosity linked with grandiose delusions in different patients. Some patients believe they are God, the Queen of England, a president's son, a famous rock star, and so on. Others are not as expansive and think they are skilled sports-persons or great inventors.

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".

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.

The following can indicate a delusion:

1. The patient expresses an idea or belief with unusual persistence or force.

2. That idea appears to have an undue influence on the patient's life, and the way of life is often altered to an inexplicable extent.

3. Despite his/her profound conviction, there is often a quality of secretiveness or suspicion when the patient is questioned about it.

4. The individual tends to be humorless and oversensitive, especially about the belief.

5. There is a quality of "centrality": no matter how unlikely it is that these strange things are happening to him/her, the patient accepts them relatively unquestioningly.

6. An attempt to contradict the belief is likely to arouse an inappropriately strong emotional reaction, often with irritability and hostility.

7. The belief is, at the least, unlikely, and out of keeping with the patient's social, cultural, and religious background.

8. The patient is emotionally over-invested in the idea and it overwhelms other elements of their psyche.

9. The delusion, if acted out, often leads to behaviors which are abnormal and/or out of character, although perhaps understandable in the light of the delusional beliefs.

10. Individuals who know the patient observe that the belief and behavior are uncharacteristic and alien.

Additional features of delusional disorder include the following:

1. It is a primary disorder.

2. It is a stable disorder characterized by the presence of delusions to which the patient clings with extraordinary tenacity.

3. The illness is chronic and frequently lifelong.

4. The delusions are logically constructed and internally consistent.

5. The delusions do not interfere with general logical reasoning (although within the delusional system the logic is perverted) and there is usually no general disturbance of behavior. If disturbed behavior does occur, it is directly related to the delusional beliefs.

6. The individual experiences a heightened sense of self-reference. Events which, to others, are nonsignificant are of enormous significance to him or her, and the atmosphere surrounding the delusions is highly charged.

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".

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'.

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.

The 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 him- or herself 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.

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.