Delusion

A delusion is a fixed belief that is not amenable to change in light of conflicting evidence. As a pathology, it is distinct from a belief based on false or incomplete information, confabulation, dogma, illusion, hallucination, or some other misleading effects of perception, as individuals with those beliefs are able to change or readjust their beliefs upon reviewing the evidence. However:
"The distinction between a delusion and a strongly held idea is sometimes difficult to make and depends in part on the degree of conviction with which the belief is held despite clear or reasonable contradictory evidence regarding its veracity."
Delusions occur in the context of many pathological states and are of particular diagnostic importance in psychotic disorders including schizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.

Types

Delusions are categorized into four different groups:

Bizarre delusion: Delusions are deemed bizarre if they are clearly implausible and not understandable to same-culture peers and do not derive from ordinary life experiences. An example named by the DSM-5 is a belief that someone replaced all of one's internal organs with someone else's without leaving a scar, depending on the organ in question.
Non-bizarre delusion: A delusion that, though false, reflects real–life situations and is at least technically possible; it may include feelings of being followed, poisoned, infected etc. e.g., the affected person mistakenly believes that they are under constant police surveillance.
Mood-congruent delusion: Any delusion with content consistent with either a depressive or manic state, e.g., a depressed person believes that news anchors on television highly disapprove of them, or a person in a manic state might believe they are a powerful deity.
Mood-neutral delusion: A delusion that does not relate to the patient's emotional state; for example, a belief that an extra limb is growing out of the back of one's head is neutral to either depression or mania.

French psychiatry, however, also establishes a difference between "paranoid" and "paranoiac" delusion. The paranoid delusion, observed in schizophrenia, is non-systematized and is characterized by a disorganized structure and confused speech and thoughts. The paranoiac delusion, observed in paraphrenia, is highly systematized and focuses on a single theme.

Themes

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 behaviors.
Delusional jealousy: False belief that a spouse or lover is having an affair, with no proof to back up the claim.
Delusion of guilt or sin : Ungrounded feeling of remorse or guilt of delusional intensity.
Thought broadcasting: False belief that other people can know one's thoughts.
Delusion of thought insertion: Belief that another thinks through the mind of the person.
Persecutory delusions: False belief that one is being persecuted.
Delusion of reference: False belief that insignificant remarks, events, or objects in one's environment have personal meaning or significance. "Usually the meaning assigned to these events is negative, but the 'messages' can also have a grandiose quality."
Erotomania: False belief that another person is in love with them.
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.
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 fantastical, 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.

Persecutory delusions

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. The person affected by a persecutory delusion wrongly believes that they are being persecuted. Specifically, for a person's delusion to be defined as persecutory, they must believe the following two central elements:

harm is occurring, or is going to occur
the persecutors have the intention to cause harm

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". The DSM-IV-TR also identifies persecutory delusions as the main feature of the persecutory type of delusional disorder. When the focus is remedying some injustice by legal action, they are sometimes called "querulous paranoia".

Causes

While explaining the causes of delusions remains a challenge, researchers have developed several theories. The genetic or biological theory holds that close relatives of people with delusional disorder are at increased risk of developing delusional traits. Another theory is dysfunctional cognitive processing, according to which delusions arise from distorted ways in which individuals view themselves. A third theory is motivated or defensive delusions, according to which individuals who are predisposed to delusional disorder may develop it at times when they are struggling to cope with life and maintaining high self-esteem. In this case, the person views others as the cause of their personal difficulties in order to preserve a positive self-concept.
Delusional thinking is more common among people who have poor hearing or sight. The probability of developing delusions is also higher where there are ongoing stressors, such as immigration, low socioeconomic status, and possibly the accumulation of smaller daily struggles.

Specific delusions

The two largest factors in the formation of delusions are disorders of brain functioning and background influences of temperament and personality.
Higher levels of dopamine are a sign of disorders of brain functioning. A preliminary 2002 study on delusional disorder examined the role of elevated dopamine levels in sustaining certain delusions, in order to establish whether schizophrenia was linked to dopamine abnormalities. The results confirmed the theory, showing that individuals with delusions of jealousy and persecution had different levels of dopamine metabolite HVA and homovanillyl alcohol; the cause of this may in turn be genetic. The authors cautioned that the results were preliminary and called for future research with a larger population. Also, factors beyond dopamine impact the development of a specific delusion; studies show age and gender are influential and HVA levels likely change during the life course of some syndromes.
On the influence of personality, Andrew Sims wrote: "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 Austria, but not in Pakistan, where delusions are more often about persecution. Similarly, in a series of case studies, delusions of guilt and punishment were found in Austrian patients with Parkinson's disease being treated with l-dopa, a dopamine agonist.

Pathophysiology

The two-factor model of delusions posits that dysfunction in both belief formation systems and belief evaluation systems are necessary for delusions. Neuroimaging studies support the view that dysfunction in evaluations systems is localized to the right lateral prefrontal cortex, regardless of delusion content, and this is congruent with its role in conflict monitoring in healthy persons. Abnormal activation and reduced volume in this region are seen in people with delusions, as well as in disorders associated with delusions such as frontotemporal dementia, psychosis and Lewy body dementia. Furthermore, lesions to the right lateral prefrontal cortex are associated with "jumping to conclusions", damage to this region is associated with post-stroke delusions, and hypometabolism in this region is associated with caudate strokes presenting with delusions.
The aberrant salience model suggests that delusions are a result of people assigning excessive importance to irrelevant stimuli. In support of this hypothesis, regions normally associated with the salience network demonstrate reduced grey matter in people with delusions, and the neurotransmitter dopamine, which is widely implicated in salience processing, is also widely implicated in psychotic disorders.
Abnormalities in specific regions have been associated with specific types of delusions. The volume of the hippocampus and parahippocampus is related to paranoid delusions in Alzheimer's disease, and has been reported to be abnormal post mortem in one person with delusions. Capgras delusions have been associated with occipito-temporal damage and may be related to failure to elicit normal emotions or memories in response to faces.