Delusional disorders are uncommon in psychiatric practice, though this may be an underestimation due to the fact that those afflicted lack insight and thus avoid psychiatric assessment. The prevalence of this condition stands at about 24 to 30 cases per 100,000 people while 0.7 to 3.0 new cases per 100,000 people are reported every year. Delusional disorder accounts for 1–2% of admissions to inpatient mental health facilities. The incidence of first admissions for delusional disorder is lower, from 0.001–0.003%.

Delusional disorder tends to appear in middle to late adult life, and for the most part first admissions to hospital for delusional disorder occur between age 33 and 55. It is more common in women than men, and immigrants seem to be at higher risk.

Explaining the causes of delusions continues to be challenging and several theories have been developed. One is the genetic or biological theory, which states that close relatives of people with delusional disorder are at increased risk of delusional traits. Another theory is the dysfunctional cognitive processing, which states that delusions may arise from distorted ways people have of explaining life to themselves. A third theory is called motivated or defensive delusions. This one states that some of those persons who are predisposed might suffer the onset of delusional disorder in those moments when coping with life and maintaining high self-esteem becomes a significant challenge. In this case, the person views others as the cause of their personal difficulties in order to preserve a positive self-view.

This condition is more common among people who have poor hearing or sight. Also, ongoing stressors have been associated with a higher possibility of developing delusions. Examples of such stressors are immigration, low socioeconomic status, and even possibly the accumulation of smaller daily hassles.

A paranoid reaction may be caused from a decline in brain circulation as a result of high blood pressure or hardening of the arterial walls.

Drug-induced paranoia, associated with amphetamines, methamphetamine and similar stimulants has much in common with schizophrenic paranoia; the relationship has been under investigation since 2012. Drug-induced paranoia has a better prognosis than schizophrenic paranoia once the drug has been removed. For further information, see Stimulant psychosis and Substance-induced psychosis.

Based on data obtained by the Dutch NEMISIS project in 2005, there was an association between impaired hearing and the onset of symptoms of psychosis, which was based on a five-year follow up. Some older studies have actually declared that a state of paranoia can be produced in patients that were under a hypnotic state of deafness. This idea however generated much skepticism during its time.

In researching over 1000 individuals of vast range of backgrounds, Stompe and colleagues (2006) found that grandiosity remains as the second most common delusion after persecutory delusions. A variation in the occurrence of grandiosity delusions in schizophrenic patients across cultures has also been observed. In research done by Appelbaum "et al." it has been found that GDs appeared more commonly in patients with bipolar disorder (59%) than in patients with schizophrenia (49%), followed by presence in substance misuse disorder patients (30%) and depressed patients (21%).

A relationship has been claimed between the age of onset of bipolar disorder and the occurrence of GDs. According to Carlson "et al." (2000), grandiose delusions appeared in 74% of the patients who were 21 or younger at the time of the onset, while they occurred only in 40% of individuals 30 years or older at the time of the onset.

Social circumstances appear to be highly influential on paranoid beliefs. Based on data collected by means of a mental health survey distributed to residents of Ciudad Juárez, Chihuahua (in Mexico) and El Paso, Texas (in the United States), paranoid beliefs seem to be associated with feelings of powerlessness and victimization, enhanced by social situations. Potential causes of these effects included a sense of believing in external control, and mistrust which can be strengthened by lower socioeconomic status. Those living in a lower socioeconomic status may feel less in control of their own lives. In addition, this study explains that females have the tendency to believe in external control at a higher rate than males, potentially making females more susceptible to mistrust and the effects of socioeconomic status on paranoia.

Emanuel Messinger reports that surveys have revealed that those exhibiting paranoia can evolve from parental relationships and dis-trustworthy environments. These environments could include being very disciplinary, stringent, and unstable. It was even noted that, "indulging and pampering (thereby impressing the child that he is something special and warrants special privileges)," can be contributing backgrounds. Experiences likely to enhance or manifest the symptoms of paranoia include increased rates of disappointment, stress, and a hopeless state of mind.

Discrimination has also been reported as a potential predictor of paranoid delusions. Such reports that paranoia seemed to appear more in older patients who had experienced higher levels of discrimination throughout their lives. In addition to this it has been noted that immigrants are quite susceptible to forms of psychosis. This could be due to the aforementioned effects of discriminatory events and humiliation.

The top two factors mainly concerned in the germination of delusions are: 1. Disorder of brain functioning; and 2. background influences of temperament and personality.

Higher levels of dopamine qualify as a symptom of disorders of brain function. That they are needed to sustain certain delusions was examined by a preliminary study on delusional disorder (a psychotic syndrome) instigated to clarify if schizophrenia had a dopamine psychosis. There were positive results - delusions of jealousy and persecution had different levels of dopamine metabolite HVA and homovanillyl alcohol (which may have been genetic). These can be only regarded as tentative results; the study called for future research with a larger population.

It is too simplistic to say that a certain measure of dopamine will bring about a specific delusion. Studies show age and gender to be influential and it is most likely that HVA levels change during the life course of some syndromes.

On the influence personality, it has been said: "Jaspers considered there is a subtle change in personality due to the illness itself; and this creates the condition for the development of the delusional atmosphere in which the delusional intuition arises."

Cultural factors have "a decisive influence in shaping delusions". For example, delusions of guilt and punishment are frequent in a Western, Christian country like Austria, but not in Pakistan - where it is more likely persecution. Similarly, in a series of case studies, delusions of guilt and punishment were found in Austrian patients with Parkinson's being treated with l-dopa - a dopamine agonist.

The cause of delusional disorder is unknown, but genetic, biochemical, and environmental factors may play a significant role in its development. Some people with delusional disorders may have an imbalance in neurotransmitters, the chemicals that send and receive messages to the brain. There does seem to be some familial component, and social isolation, immigration (generally for persecutory reasons), drug abuse, excessive stress, being married, being employed, low socioeconomic status, celibacy among men, and widowhood among women may also be risk factors. Delusional disorder is currently thought to be on the same spectrum or dimension as schizophrenia, but people with delusional disorder, in general, may have less symptomatology and functional disability.

Research suggests that the severity of the delusions of grandeur is directly related to a higher self-esteem in individuals and inversely related to any individual’s severity of depression and negative self-evaluations. Lucas "et al." found that there is no significant gender difference in the establishment of grandiose delusion. However, there is a claim that ‘the particular component of Grandiose delusion’ may be variable across both genders. Also, it had been noted that the presence of GDs in people with at least grammar or high school education was greater than lesser educated persons. Similarly, the presence of grandiose delusions in individuals who are the eldest is greater than in individuals who are the youngest of their siblings.

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).

According to the Mayo Clinic, it is best to start receiving treatment for paranoid schizophrenia as early as possible and to maintain the treatment throughout life. Continuing treatment will help keep the serious symptoms under control and allow the person to lead a more fulfilling life. This illness is typically unpreventable.

It has a strong hereditary component with a first degree parent or sibling. There is some possibility that there are environmental influences including "prenatal exposure to a viral infection, low oxygen levels during birth (from prolonged labor or premature birth), exposure to a virus during infancy, early parental loss or separation, and verbal, physical or sexual abuse in childhood". Eliminating any of these factors could help reduce an individual's future risk of developing paranoid schizophrenia.

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.

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.

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.

Paranoid schizophrenia is an illness that typically requires lifelong treatment with neuroleptics to allow someone to have a relatively stable and normal lifestyle. In order to be successfully treated, a person with schizophrenia should seek help from family or primary care doctors, psychiatrists, psychotherapists, pharmacists, family members, case workers, psychiatric nurses, or social workers, provided he or she is not unable to do so, due to many people with schizophrenia having the inability to accept their condition. Non-compliance with neuroleptics may also occur if the patient considers the side effects (such as extrapyramidal symptoms) to be more debilitating than the condition itself. The main options that are offered for the treatment of paranoid schizophrenia are the following: neuroleptics, psychotherapy, hospitalization, electroconvulsive therapy, and vocational skills training.

There are many different types of disorders that have similar symptoms to paranoid schizophrenia. There are tests that psychiatrists perform to achieve a correct diagnosis. They include "psychiatric evaluation, in which the doctor or psychiatrist will ask a series of questions about the patient's symptoms, psychiatric history, and family history of mental health problems; medical history and exam, in which the doctor will ask about one's personal and family health history and will also perform a complete physical examination to check for medical issues that could be causing or contributing to the problem; laboratory tests in which the doctor will order simple blood and urine tests can rule out other medical causes of symptoms".

There are side effects associated with antipsychotic medication. Neuroleptics can cause high blood pressure and high cholesterol. Many people who take them exhibit weight gain and have a higher risk of developing diabetes.

Because of reduced levels of trust, there can be challenges in treating PPD. However, psychotherapy, antidepressants, antipsychotics and anti-anxiety medications can play a role when an individual is receptive to intervention.

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.

PPD occurs in about 0.5%–2.5% of the general population. It is seen in 2%–10% of psychiatric outpatients. It is more common in males.

Men and women are diagnosed in equal numbers with depersonalization disorder. A 1991 study on a sample from Winnipeg, Manitoba estimated the prevalence of depersonalization disorder at 2.4% of the population. A 2008 review of several studies estimated the prevalence between 0.8% and 1.9%. This disorder is episodic in about one-third of individuals, with each episode lasting from hours to months at a time. Depersonalization can begin episodically, and later become continuous at constant or varying intensity.

Onset is typically during the teenage years or early 20s, although some report being depersonalized as long as they can remember, and others report a later onset. The onset can be acute or insidious. With acute onset, some individuals remember the exact time and place of their first experience of depersonalization. This may follow a prolonged period of severe stress, a traumatic event, an episode of another mental illness, or drug use. Insidious onset may reach back as far as can be remembered, or it may begin with smaller episodes of lesser severity that become gradually stronger. Patients with drug-induced depersonalization do not appear to be a clinically separate group from those with a non-drug precipitant.

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.

The underlying neurophysiology and psychopathology of Cotard syndrome might be related to problems of delusional misidentification. Neurologically, the Cotard delusion (negation of the Self) is thought to be related to the Capgras delusion (people replaced by impostors); each type of delusion is thought to result from neural misfiring in the fusiform face area of the brain (which recognizes faces) and in the amygdalae (which associate emotions to a recognized face).

The neural disconnection creates in the patient a sense that the face they are observing is not the face of the person to whom it belongs; therefore, that face lacks the familiarity (recognition) normally associated with it. This results in derealization, or a disconnection from the environment. If the observed face is that of a person known to the patient, they experience that face as the face of an impostor (the Capgras delusion). If the patient sees their own face, they might perceive no association between the face and their own sense of Self—which results in the patient believing that they do not exist (the Cotard delusion).

Cotard's syndrome is usually encountered in people afflicted with a psychosis (e.g., schizophrenia ), neurological illness, mental illness, clinical depression, derealization, brain tumor, and with migraine headache. The medical literature indicate that the occurrence of Cotard's delusion is associated with lesions in the parietal lobe. As such, the Cotard-delusion patient presents a greater incidence of brain atrophy—especially of the median frontal lobe—than do the people in the control groups.

The Cotard delusion also has resulted from a patient's adverse physiological response to a drug (e.g., aciclovir) and to its prodrug precursor (e.g., valaciclovir). The occurrence of Cotard delusion symptoms was associated with a high serum-concentration of 9-Carboxymethoxymethylguanine (CMMG), the principal metabolite of the drug aciclovir. As such, the patient with weak kidneys (impaired renal function) continued risking the occurrence of delusional symptoms, despite the reduction of the dose of aciclovir. Hemodialysis resolved the patient's delusions (of negating the Self) within hours of treatment, which suggests that the occurrence of Cotard-delusion symptoms might not always be cause for psychiatric hospitalization of the patient.

Chronic hallucinatory psychosis is a psychosis subtype, classified under "Other nonorganic psychosis" by the . Other abnormal mental symptoms in the early stages are, as a rule, absent. The patient is most usually quiet and orderly, with a good memory.

It has often been a matter of the greatest difficulty to decide under which heading of the recognized classifications individual members of this group should be placed. As the hallucinations give rise to slight depression, some might possibly be included under melancholia. In others, paranoia may develop. Others, again, might be swept into the widespread net of dementia praecox. This state of affairs cannot be regarded as satisfactory, for they are not truly cases of melancholia, paranoia, dementia praecox or any other described affection.

This disease, as its name suggests, is a hallucinatory case, for it is its main feature. These may be of all senses, but auditory hallucinations are the most prominent. At the beginning, the patient may realize that the hallucination is a morbid phenomenon and unaccountable. They may claim to hear a "voice" speaking, though there is no one in the flesh actually doing so. Such a state of affairs may last for years and possibly, though rarely, for life, and the subject would not be deemed insane in the ordinary sense of the word.

It's probable, however, that this condition forms the first stage of the illness, which eventually develops on definite lines. What usually happens is the patient seeks an explanation for the hallucinations. As none is forthcoming he/she tries to account for their presence and the result is a delusion, and, most frequently, a delusion of persecution. Also, it needs to be noted that the delusion is a comparatively late arrival and is the logical result of the hallucinations.

Clinical lycanthropy is defined as a rare psychiatric syndrome that involves a delusion that the affected person can transform into, has transformed into, or is a non-human animal. Its name is associated with the mythical condition of lycanthropy, a supernatural affliction in which humans are said to physically shapeshift into wolves. It is purported to be a rare disorder.

Delusional parasitosis is divided into primary, secondary functional, and secondary organic groups.

Delusional misidentification syndrome is an umbrella term, introduced by Christodoulou (in his book "The Delusional Misidentification Syndromes", Karger, Basel, 1986) for a group of delusional disorders that occur in the context of mental and neurological illness. They all involve a belief that the identity of a person, object, or place has somehow changed or has been altered. As these delusions typically only concern one particular topic, they also fall under the category called monothematic delusions.

This psychopathological syndrome is usually considered to include four main variants:

- The Capgras delusion is the belief that (usually) a close relative or spouse has been replaced by an identical-looking impostor.

- The Fregoli delusion is the belief that various people the believer meets are actually the same person in disguise.

- Intermetamorphosis is the belief that people in the environment swap identities with each other whilst maintaining the same appearance.

- Subjective doubles, described by Christodoulou in 1978 ("American Journal of Psychiatry" 135, 249, 1978), is the belief that there is a doppelgänger or double of him- or herself carrying out independent actions.

However, similar delusional beliefs, often singularly or more rarely reported, are sometimes also considered to be part of the delusional misidentification syndrome. For example:

- Mirrored-self misidentification is the belief that one's reflection in a mirror is some other person.

- Reduplicative paramnesia is 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 an identical-looking hospital in a different part of the country, despite this being obviously false.

- The Cotard delusion is a rare disorder in which people hold a delusional belief that they are dead (either figuratively or literally), do not exist, are putrefying, or have lost their blood or internal organs. In rare instances, it can include delusions of immortality.

- Syndrome of delusional companions is the belief that objects (such as soft toys) are sentient beings.

- Clonal pluralization of the self, where a person believes there are multiple copies of him- or herself, identical both physically and psychologically but physically separate and distinct.

There is considerable evidence that disorders such as the Capgras or Fregoli syndromes are associated with disorders of face perception and recognition. However, it has been suggested that all misidentification problems exist on a continuum of anomalies of familiarity, from déjà vu at one end to the formation of delusional beliefs at the other.

Neither antidepressants nor antipsychotics have been found to be useful, Additionally antipsychotics can worsen symptoms of depersonalisation. To date, no clinical trials have studied the effectiveness of benzodiazepines. Tentative evidence supports naloxone and naltrexone.

A combination of an SSRI and a benzodiazepine has been proposed to be useful for DPD patients with anxiety.

Modafinil used alone has been reported to be effective in a subgroup of individuals with depersonalization disorder (those who have attentional impairments, under-arousal and hypersomnia). However, clinical trials have not been conducted.