Some critical psychiatrists criticize the practice of defining one and the same belief as normal in one culture and pathological in another culture for cultural essentialism. They argue that since cultural influences are mixed, including not only parents and teachers but also peers, friends, books and the internet, and the same cultural influence can have different effects depending on earlier cultural influences, the assumption that culture can be boiled down to a few traceable, distinguishable and statistically quantifiable factors and that everything that does not fall in those factors must be biological, is not a justified assumption. Other critical psychiatrists argue that just because a person's belief is unshaken by one influence does not prove that it would remain unshaken by another. For example, a person whose beliefs are not changed by verbal correction from a psychiatrist, which is how delusion is usually diagnosed, may still change his or her mind when observing empirical evidence, only that psychiatry rarely if ever present patients with such situations.

The modern definition and Jaspers' original criteria have been criticised, as counter-examples can be shown for every defining feature.

Studies on psychiatric patients show that delusions vary in intensity and conviction over time, which suggests that certainty and incorrigibility are not necessary components of a delusional belief.

Delusions do not necessarily have to be false or 'incorrect inferences about external reality'. Some religious or spiritual beliefs by their nature may not be falsifiable, and hence cannot be described as false or incorrect, no matter whether the person holding these beliefs was diagnosed as delusional or not.

In other situations the delusion may turn out to be true belief. For example, in delusional jealousy, where a person believes that their partner is being unfaithful (and may even follow them into the bathroom believing them to be seeing their lover even during the briefest of partings), it may actually be true that the partner is having sexual relations with another person. In this case, the delusion does not cease to be a delusion because the content later turns out to be verified as true or the partner actually chose to engage in the behavior of which they were being accused.

In other cases, the delusion may be assumed to be false by a doctor or psychiatrist assessing the belief, because it "seems" to be unlikely, bizarre or held with excessive conviction. Psychiatrists rarely have the time or resources to check the validity of a person’s claims leading to some true beliefs to be erroneously classified as delusional. This is known as the Martha Mitchell effect, after the wife of the attorney general who alleged that illegal activity was taking place in the White House. At the time, her claims were thought to be signs of mental illness, and only after the Watergate scandal broke was she proved right (and hence sane).

Similar factors have led to criticisms of Jaspers' definition of true delusions as being ultimately 'un-understandable'. Critics (such as R. D. Laing) have argued that this leads to the diagnosis of delusions being based on the subjective understanding of a particular psychiatrist, who may not have access to all the information that might make a belief otherwise interpretable. R. D. Laing's hypothesis has been applied to some forms of projective therapy to "fix" a delusional system so that it cannot be altered by the patient. Psychiatric researchers at Yale University, Ohio State University and the Community Mental Health Center of Middle Georgia have used novels and motion picture films as the focus. Texts, plots and cinematography are discussed and the delusions approached tangentially. This use of fiction to decrease the malleability of a delusion was employed in a joint project by science-fiction author Philip Jose Farmer and Yale psychiatrist A. James Giannini. They wrote the novel "Red Orc's Rage", which, recursively, deals with delusional adolescents who are treated with a form of projective therapy. In this novel's fictional setting other novels written by Farmer are discussed and the characters are symbolically integrated into the delusions of fictional patients. This particular novel was then applied to real-life clinical settings.

Another difficulty with the diagnosis of delusions is that almost all of these features can be found in "normal" beliefs. Many religious beliefs hold exactly the same features, yet are not universally considered delusional. These factors have led the psychiatrist Anthony David to note that "there is no acceptable (rather than accepted) definition of a delusion." In practice, psychiatrists tend to diagnose a belief as delusional if it is either patently bizarre, causing significant distress, or excessively pre-occupying the patient, especially if the person is subsequently unswayed in belief by counter-evidence or reasonable arguments.

It is important to distinguish true delusions from other symptoms such as anxiety, fear, or paranoia. To diagnose delusions a mental state examination may be used. This test includes appearance, mood, affect, behavior, rate and continuity of speech, evidence of hallucinations or abnormal beliefs, thought content, orientation to time, place and person, attention and concentration, insight and judgment, as well as short-term memory.

Johnson-Laird suggests that delusions may be viewed as the natural consequence of failure to distinguish conceptual relevance. That is, the person takes irrelevant information and puts it in the form of disconnected experiences, then it is taken to be relevant in a manner that suggests false causal connections. Furthermore, the person takes the relevant information, in the form of counterexamples, and ignores it.

Differential diagnosis includes ruling out other causes such as drug-induced conditions, dementia, infections, metabolic disorders, and endocrine disorders. Other psychiatric disorders must then be ruled out. In delusional disorder, mood symptoms tend to be brief or absent, and unlike schizophrenia, delusions are non-bizarre and hallucinations are minimal or absent.

Interviews are important tools to obtain information about the patient's life situation and past history to help make a diagnosis. Clinicians generally review earlier medical records to gather a full history. Clinicians also try to interview the patient's immediate family, as this can be helpful in determining the presence of delusions. The mental status examination is used to assess the patient's current mental condition.

A psychological questionnaire used in the diagnosis of the delusional disorder is the Peters Delusion Inventory (PDI) which focuses on identifying and understanding delusional thinking. However, this questionnaire is more likely used in research than in clinical practice.

In terms of diagnosing a non-bizarre delusion as a delusion, ample support should be provided through fact checking. In case of non-bizarre delusions, Psych Central notes, "All of these situations could be true or possible, but the person suffering from this disorder knows them not to be (e.g., through fact-checking, third-person confirmation, etc.)."

In patients suffering from schizophrenia, grandiose and religious delusions are found to be the least susceptible to cognitive behavioral interventions. Cognitive behavioral intervention is a form of psychological therapy, initially used for depression, but currently used for a variety of different mental disorders, in hope of providing relief from distress and disability. During therapy, grandiose delusions were linked to patients' underlying beliefs by using inference chaining. Some examples of interventions performed to improve the patient's state were focus on specific themes, clarification of neologisms, and thought linkage. During thought linkage, the patient is asked repeatedly by the therapist to explain his/her jumps in thought from one subject to a completely different one.

Patients suffering from mental disorders that experience grandiose delusions have been found to have a lower risk of having suicidal thoughts and attempts.

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.

In the DSM-IV-TR, paranoia is diagnosed in the form of:

- paranoid personality disorder ()

- paranoid schizophrenia (a subtype of schizophrenia) ()

- the persecutory type of delusional disorder, which is also called "querulous paranoia" when the focus is to remedy some injustice by legal action. ()

According to clinical psychologist P. J. McKenna, "As a noun, paranoia denotes a disorder which has been argued in and out of existence, and whose clinical features, course, boundaries, and virtually every other aspect of which is controversial. Employed as an adjective, paranoid has become attached to a diverse set of presentations, from paranoid schizophrenia, through paranoid depression, to paranoid personality—not to mention a motley collection of paranoid 'psychoses', 'reactions', and 'states'—and this is to restrict discussion to functional disorders. Even when abbreviated down to the prefix para-, the term crops up causing trouble as the contentious but stubbornly persistent concept of paraphrenia".

At least 50% of the diagnosed cases of schizophrenia experience delusions of reference and delusions of persecution. Paranoia perceptions and behavior may be part of many mental illnesses, such as depression and dementia, but they are more prevalent in three mental disorders: paranoid schizophrenia, delusional disorder (persecutory type), and paranoid personality disorder.

A 2012 paper suggested that, when compared with experiences today, psychiatric conditions associated with psychotic spectrum symptoms may be possible explanations for some revelatory driven experiences and activities such as those of Abraham, Moses, Jesus, and Saint Paul. However, the paper admits that the study was not aimed to deny supernatural elements, nor was it conclusive on whether their experiences were delusional in part or not at all.

According to the DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, 5th Edition), Cotard delusion falls under the category of somatic delusions, those that involve bodily functions or sensations. (Citation needed. DSM-5 does not specifically reference Cotard syndrome.)

There are no further diagnostic criteria for Cotard syndrome within the DSM-5, and identification of the syndrome relies heavily on clinical interpretation.

Cotard delusion should not be confused with Delusional Disorders as defined by the DSM-5, which involve a different spectrum of symptoms that are less severe and have lesser detrimental effect on functioning.

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.

The attribution model has been well talked about regarding paranoid or delusional individuals. The idea is that they like to assign issues to external events. A motivation behind this characteristic may involve the need for that person to develop a better self-image and maintain self-confidence. There have been debates about whether or not paranoid individuals are more likely to have a low or high self-perception, and results have been generated for both of these hypotheses. Researchers have made a distinction between positive self-esteem and negative self-esteem revealing that paranoid delusional individuals have more of a negative self-evaluation.

Because it is a rare and poorly understood condition, there is no definitive way to diagnose the Capgras delusion. Diagnosis is primarily made on psychological evaluation of the patient, who is most likely brought to a psychologist's attention by a family member or friend believed to be an imposter by the person under the delusion.

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.

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.

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.

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).

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.

In ICD-10, this disorder is called depersonalization-derealization syndrome F48.1. The diagnostic criteria are as follows:

The diagnosis should not be given in certain specified conditions, for instance when intoxicated by alcohol or drugs, or together with schizophrenia, mood disorders and anxiety disorders.

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.

Diagnosis is based on the self-reported experiences of the person followed by a clinical assessment. Psychiatric assessment includes a psychiatric history and some form of mental status examination. Since some medical and psychiatric conditions mimic the symptoms of DPD, clinicians must differentiate between and rule out the following to establish a precise diagnosis: temporal lobe epilepsy, panic disorder, acute stress disorder, schizophrenia, migraine, drug use, brain tumour or lesion. No laboratory test for depersonalization-derealization disorder currently exists.

The diagnosis of depersonalization disorder can be made with the use of the following interviews and scales:

The Structured Clinical Interview for DSM-IV Dissociative Disorders (SCID-D) is widely used, especially in research settings. This interview takes about 30 minutes to 1.5 hours, depending on individual's experiences.

The Dissociative Experiences Scale (DES) is a simple, quick, self-administered questionnaire that has been widely used to measure dissociative symptoms. It has been used in hundreds of dissociative studies, and can detect depersonalization and derealization experiences.

The Dissociative Disorders Interview Schedule (DDIS) is a highly structured interview which makes DSM-IV diagnoses of somatization disorder, borderline personality disorder and major depressive disorder, as well as all the dissociative disorders. It inquires about positive symptoms of schizophrenia, secondary features of dissociative identity disorder, extrasensory experiences, substance abuse and other items relevant to the dissociative disorders. The DDIS can usually be administered in 30–45 minutes.

The Cambridge Depersonalization Scale (CDS) is a method for determining the severity of depersonalization disorder. It has been proven and accepted as a valid tool for the diagnosis of depersonalization disorder in a clinical setting. It is also used in a clinical setting to differentiate minor episodes of depersonalization from actual symptoms of the disorder. Due to the success of the CDS, a group of Japanese researchers underwent the effort to translate the CDS into the J-CDS or the Japanese Cambridge Depersonalization Scale. Through clinical trials the Japanese research team successfully tested their scale and determined its accuracy. One limitation is that the scale does not allow for the differentiation between past and present episodes of depersonalization. It should also be noted that it may be difficult for the individual to describe the duration of a depersonalization episode, and thus the scale may lack accuracy. The project was conducted in the hope that it would stimulate further scientific investigations into depersonalization disorder.

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.

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.

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.

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

In primary delusional parasitosis, the delusions comprise the entire disease entity: there is no additional deterioration of basic mental functioning or idiosyncratic thought processes. The parasitic delusions consist of a single delusional belief regarding some aspect of health. This is also referred to as "monosymptomatic hypochondriacal psychosis", and sometimes as "true" delusional parasitosis. In the DSM-IV, this corresponds with "delusional disorder, somatic type".