One hypothesis is that brain dysfunction (either due to physical damage or damage from an organic disorder) in the right hemisphere, temporal lobe, and/or bi-frontal lobes causes the delusion of subjective doubles. Physical damage resulting in the subjective doubles delusion often includes, but is not limited to brain lesions or traumatic brain injury. Suspected organic causes of brain damage that may lead to subjective doubles include disorders such as epilepsy.

Because other mental illnesses are commonly co-morbid with subjective doubles syndrome, it is unknown whether these types of brain injuries are linked to the delusion or the additional mental illness. For example, right hemisphere brain damage is associated with schizophrenia, which is commonly reported with the delusion of subjective doubles.

Subjective doubles is commonly comorbid with other psychiatric illnesses, such as bipolar disorder or schizophrenia. The cause of the disorder is difficult to ascertain not only because of its rarity, but also due to the simultaneous presence of other disorders. While the physiological cause of the syndrome of subjective doubles has not been found, many hypotheses exist.

Some researchers believe that the syndrome of subjective doubles appears as a symptom of another disorder instead of a disorder of its own. (see #Controversy, below)

The syndrome of subjective doubles may also be related to substance dependence.

Another hypothesis states that subjective doubles is result of hyper-identification, linked to over-activity in certain areas of the brain, thereby causing the patient see familiar aspects of the self in strangers.

Some hypothesize that this delusion is a result of facial processing deficiencies, as it has seemingly similar symptoms of prosopagnosia; however, it must be noted that recognition of most faces is impaired in this delusion. Facial processing deficiencies also do not account for the occasion in which multiple doubles are reported.

Another hypothesis is that a "disconnection" between the right and left hemispheres may cause the delusional symptoms. This hypothesis relies heavily on the theory of lateralization of brain function, or left brain vs. right brain theory. In this hypothesis, the inability of the right hemisphere to "check" the left hemisphere causes the individual to succumb to delusions of self-awareness created by the left hemisphere.

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.

Depersonalization is also a direct symptom of Lyme disease as well as other tick-borne diseases. If depersonalization is suspected a blood-test is required in search of anti-bodies.

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.

The following two case reports are examples of the Capgras delusion in a psychiatric setting:

The following case is an instance of the Capgras delusion resulting from a neurodegenerative disease:

Brief reactive psychosis generally follows a recognisably traumatic life event like divorce or homelessness, but may be triggered by any subjective experience which appears catastrophic to the person affected.

Among such stressors are the death of a loved one, professional loss such as unexpectedly losing one's job or otherwise becoming unemployed, or serious adverse changes in the patient's personal life, such as the breakdown of their family through divorce, etc.

It must be emphasised that this is by no means an exhaustive list of stressful life events, because the events which trigger brief reactive psychosis tend, due to the individualistic nature of human psychology, to be extremely personalized. BRP may be the first breakdown for someone with a chronic psychiatric disorder but only time will tell whether the disorder will be brief or lifelong, whether BRP or a chronic condition that is controlled well enough by medication that symptoms do not return.

Depersonalization has been described by some as a desirable state, particularly by those that have experienced it under the influence of mood-altering recreational drugs. It is an effect of dissociatives and psychedelics, as well as a possible side effect of caffeine, alcohol, amphetamine, and cannabis. It is a classic withdrawal symptom from many drugs.

Benzodiazepine dependence, which can occur with long-term use of benzodiazepines, can induce chronic depersonalization symptomatology and perceptual disturbances in some people, even in those who are taking a stable daily dosage, and it can also become a protracted feature of the benzodiazepine withdrawal syndrome.

Lieutenant Colonel Dave Grossman, in his book "", suggests that military training artificially creates depersonalization in soldiers, suppressing empathy and making it easier for them to kill other human beings.

Graham Reed (1974) noted that depersonalization occurs in relation to the experience of falling in love.

Capgras delusion is a psychiatric disorder in which a person holds a delusion that a friend, spouse, parent, or other close family member (or pet) has been replaced by an identical impostor. The Capgras delusion is classified as a delusional misidentification syndrome, a class of delusional beliefs that involves the misidentification of people, places, or objects. It can occur in acute, transient, or chronic forms. Cases in which patients hold the belief that time has been "warped" or "substituted" have also been reported.

The delusion most commonly occurs in patients diagnosed with paranoid schizophrenia, but has also been seen in patients suffering from brain injury and dementia. It presents often in individuals with a neurodegenerative disease, particularly at an older age. It has also been reported as occurring in association with diabetes, hypothyroidism, and migraine attacks. In one isolated case, the Capgras delusion was temporarily induced in a healthy subject by the drug ketamine. It occurs more frequently in females, with a female:male ratio of approximately 3:2. It is worth noting that there is historical and quite probably modern use of the Political decoy as well as Celebrity lookalike, and impersonation is used by criminals, as well. Although 'delusion' is defined as when a patient holds a false belief "in spite of incontrovertible evidence", such evidence is difficult to produce (in the case of Capgras symptoms), whether lookalikes exist or not. This does not deter psychiatrists from prescribing pharmaceutical chemicals for persons describing these situations with little, if any, investigation into the claims, though it seems even one actual encounter with a genuine impersonator (whether sinister or not) has a notably unnerving effect on future interactions with that actual person, and possibly leading to paranoia of others being impersonated. Carefully targeted identity thefts in this sense can cause quite a few real problems, ranging from confusion to petty theft, business and domestic situations going awry, sexual relationship sabotage or confusion (possibility of unplanned pregnancy and risk of STD exposure), and financial fraud.

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).

The Fregoli delusion, or the delusion of doubles, is a rare disorder in which a person holds a delusional belief that different people are in fact a single person who changes appearance or is in disguise. The syndrome may be related to a brain lesion and is often of a paranoid nature, with the delusional person believing themselves persecuted by the person they believe is in disguise.

A person with the Fregoli delusion can also inaccurately recall places, objects, and events. This disorder can be explained by "associative nodes". The associative nodes serve as a biological link of information about other people with a particular familiar face (to the patient). This means that for any face that is similar to a recognizable face to the patient, the patient will recall that face as the person they know.

The Fregoli delusion is classed both as a monothematic delusion, since it only encompasses one delusional topic, and as a delusional misidentification syndrome (DMS), a class of delusional beliefs that involves misidentifying people, places, or objects. Like Capgras delusion, psychiatrists believe it is related to a breakdown in normal face perception.

Levodopa, also known as L-DOPA, is the precursor to several catecholamines, specifically of dopamine, epinephrine and norepinephrine. It is clinically used to treat Parkinson's disease and dopamine-responsive dystonia. Clinical studies have shown that the use of levodopa can lead to visual hallucinations and delusions. In most patients, delusions were more salient than hallucinations. With prolonged use of levodopa, the delusions occupy almost all of a patient's attention. In experimental studies, when the concentration of levodopa decreases, the number of reported delusions decreases as well. It has been concluded that delusions related to antiparkinsonian medications are one of the leading causes of Fregoli syndrome.

Several studies have shown that the risk of suicide is higher in patients who suffer from Bipolar II than those who suffer from Bipolar I, and especially higher than patients who suffer from major depressive disorder.

In results of a summary of several lifetime study experiments, it was found that 24% of Bipolar II patients experienced suicidal ideation or suicide attempts compared to 17% in Bipolar I patients and 12% in major depressive patients. Bipolar disorders, in general, are the third leading cause of death in 15- to 24-year-olds. Bipolar II patients were also found to employ more lethal means and have more complete suicides overall.

Bipolar II patients have several risk factors that increase their risk of suicide. The illness is very recurrent and results in severe disabilities, interpersonal relationship problems, barriers to academic, financial, and vocational goals, and a loss of social standing in their community, all of which increase the likelihood of suicide. Mixed symptoms and rapid-cycling, both very common in Bipolar II, are also associated with an increased risk of suicide. The tendency for Bipolar II to be misdiagnosed and treated ineffectively, or not at all in some cases, leads to an increased risk.

As a result of the high suicide risk for this group, reducing the risk and preventing attempts remains a main part of the treatment; a combination of self-monitoring, close supervision by a therapist, and faithful adherence to their medication regimen will help to reduce the risk and prevent the likelihood of a completed suicide.

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.

Brief reactive psychosis, referred to in the DSM IV-TR as "brief psychotic disorder with marked stressor(s)", is the psychiatric term for psychosis which can be triggered by an extremely stressful event in the life of an individual.

Typically such brief psychoses last from a day to a month only, with the delusions, speech disorders and hallucinations eventually yielding to a return to normal functioning.

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.

Comorbid conditions are extremely common in individuals with BP-II. In fact, individuals are twice as likely to present a comorbid disorder than not. These include anxiety, eating, personality (cluster B), and substance use disorders. For bipolar II disorder, the most conservative estimate of lifetime prevalence of alcohol or other drug abuse disorders is 20%. In patients with comorbid substance abuse disorder and BP-II, episodes have a longer duration and treatment compliance decreases. Preliminary studies suggest that comorbid substance abuse is also linked to increased risk of suicidality.

According to research by RM Carney et al., any history of child depression influences the occurrence of adolescent cardiac risk factors, even if individuals no longer suffer from depression. They are much more likely to develop heart disease as adults.

The onset of alcohol dementia can occur as early as age thirty, although it is far more common that the dementia will reveal itself anywhere from age fifty to age seventy. The onset and the severity of this type of dementia is directly correlated to the amount of alcohol that a person consumes over his or her lifetime.

Epidemiological studies show an association between long-term alcohol intoxication and dementia. Alcohol can damage the brain directly as a neurotoxin, or it can damage it indirectly by causing malnutrition, primarily a loss of thiamine (vitamin B1). Alcohol abuse is common in older persons, and alcohol-related dementia is under-diagnosed. A discredited French study claimed that moderate alcohol consumption (up to four glasses of wine per week) protected against dementia, whereas higher rates of consumption have conclusively been shown to increase the chances of getting it.

According to research conducted by Laura P. Richardson et al., major depression occurred in 7% of the cohort during early adolescence (11, 13, and 15 years of age) and 27% during late adolescence (18 and 21 years of age). At 26 years of age, 12% of study members were obese. After adjusting for each individual's baseline body mass index (calculated as the weight in kilograms divided by the square of height in meters), depressed late adolescent girls were at a greater than 2-fold increased risk for obesity in adulthood compared with their non-depressed female peers (relative risk, 2.32; 95% confidence interval, 1.29-3.83). A dose-response relationship between the number of episodes of depression during adolescence, and risk for adult obesity was also observed in female subjects. The association was not observed for late adolescent boys or for early adolescent boys or girls.

Experiences - are characterized by the presence of the following three factors:

- disembodiment, an apparent location of the self outside one's body;

- impression of seeing the world from an elevated and distanced visuo-spatial perspective or extracorporeal, but egocentric visuo-spatial perspective;

- impression of seeing one's own body from this perspective (autoscopy).

Laboratory of Cognitive Neuroscience, École Polytechnique Fédérale de Lausanne, Lausanne, and Department of Neurology, University Hospital, Geneva, Switzerland, have reviewed some of the classical precipitating factors of autoscopy. These are sleep, drug abuse, and general anesthesia as well as neurobiology. They have compared them with recent findings on neurological and neurocognitive mechanisms of the autoscopy. The reviewed data suggest that autoscopic experiences are due to functional disintegration of lower-level multisensory processing and abnormal higher-level self-processing at the temporoparietal junction.

Somatization disorder is estimated to occur in 0.2% to 2% of females, and 0.2% of males.

There are cultural differences in the prevalence of somatization disorder. For example, somatization disorder and symptoms were found to be significantly more common in Puerto Rico. In addition the diagnosis is also more prevalent among African Americans and those with less than a high school education or lower socioeconomic status.

There is usually co-morbidity with other psychological disorders, particularly mood disorders or anxiety disorders. Research also showed comorbidity between somatization disorder and personality disorders, especially antisocial, borderline, narcissistic, histrionic, avoidant, and dependent personality disorder.

About 10-20 percent of female first degree relatives also have somatization disorder and male relatives have increased rates of alcoholism and sociopathy.

Memory distrust syndrome is a condition coined by Gísli Guðjónsson and James MacKeith in 1982, in which an individual doubts the accuracy of their memory concerning the content and context of events of which they have experienced. Since the individual does not trust their own memory, they will commonly depend on outside sources of information rather than using their ability for recollection. Some believe that this may be a defense or coping mechanism to a preexisting faulty memory state such as Alzheimer's disease, amnesia, or possibly dementia.

The condition is generally considered to be related to source amnesia, which involves the inability to recall the basis for factual knowledge. The main difference between the two is that source amnesia is a lack of knowing the basis of knowledge, whereas memory distrust syndrome is a lack of believing the knowledge that exists. The fact that an individual lacks the trust in their own memory implies that the individual would have a reason or belief that would prevent them from the trust that most of us have in our recollections. Cases concerning memory distrust syndrome have led to documented false confessions in court cases.

It is normal to have some level of memory distrust, or the lack of trusting in one's own memory. This may occur when speaking with your parents about your childhood, for example. However it seems that everyone has their own level of memory distrust, and memory distrust syndrome seems to be a severe case.

The direct cause is unknown; however, it is possibly a defense or coping mechanism to a preexisting condition that would alter one's memory. This could involve frontal lobe lesions, Alzheimer's disease, amnesia, dementia, or other conditions. Any condition that would alter either existing memories or the formation of new memories could cause a coping scheme such as memory distrust syndrome. Alternatively, an individual may have learned over time to not trust their own memory from conditioning, and as such the individual would develop a defense mechanism to remove themselves from potential embarrassment.

Autoscopy is the experience in which an individual perceives the surrounding environment from a different perspective, from a position outside of his or her own body. Autoscopy comes from the ancient Greek ("self") and ("watcher").

Autoscopy has been of interest to humankind from time immemorial and is abundant in the folklore, mythology, and spiritual narratives of most ancient and modern societies. Cases of autoscopy are commonly encountered in modern psychiatric practice. According to neurological research, autoscopic experiences are hallucinations.