Delusions are categorized as either bizarre or non-bizarre and as either mood-congruent or mood-incongruent. A bizarre delusion is a delusion that is very strange and completely implausible for the person's culture; an example of a bizarre delusion would be that aliens have removed the affected person's brain. A non-bizarre delusion is one whose content is definitely mistaken, but is at least possible; an example may be that the affected person mistakenly believes that he or she is under constant police surveillance. A mood-congruent delusion is any delusion whose content is consistent with either a depressive or manic state; for example, a depressed person may believe that the world is ending, or a person in a manic state (a state in which the person feels compelled to take on new projects, has a lot of energy, and needs little sleep) believes that he or she has special talents or abilities, or is a famous person. A mood-incongruent delusion is any delusion whose content is not consistent with either a depressed or manic state or is mood-neutral. An example is a depressed person who believes that thoughts are being inserted into his or her mind from some outside force, person, or group of people, and these thoughts are not recognized as the person's own thoughts (called "thought insertion").
In addition to these categories, delusions are often categorized according to theme. Although delusions can have any theme, certain themes are more common. Some of the more common delusion themes are:
Delusion of control: This is a false belief that another person, group of people, or external force controls one's thoughts, feelings, impulses, or behavior. A person may describe, for instance, the experience that aliens actually make him or her move in certain ways and that the person affected has no control over the bodily movements. Thought broadcasting (the false belief that the affected person's thoughts are heard aloud), thought insertion, and thought withdrawal (the belief that an outside force, person, or group of people is removing or extracting a person's thoughts) are also examples of delusions of control.
Nihilistic delusion: A delusion whose theme centers on the nonexistence of self or parts of self, others, or the world. A person with this type of delusion may have the false belief that the world is ending.
Delusional jealousy (or delusion of infidelity): A person with this delusion falsely believes that his or her spouse or lover is having an affair. This delusion stems from pathological jealousy and the person often gathers "evidence" and confronts the spouse about the nonexistent affair.
Delusion of guilt or sin (or delusion of self-accusation): This is a false feeling of remorse or guilt of delusional intensity. A person may, for example, believe that he or she has committed some horrible crime and should be punished severely. Another example is a person who is convinced that he or she is responsible for some disaster (such as fire, flood, or earthquake) with which there can be no possible connection.
Delusion of mind being read: The false belief that other people can know one's thoughts. This is different from thought broadcasting in that the person does not believe that his or her thoughts are heard aloud.
Delusion of reference: The person falsely believes that insignificant remarks, events, or objects in one's environment have personal meaning or significance. For instance, a person may believe that he or she is receiving special messages from the news anchorperson on television. Usually the meaning assigned to these events is negative, but the "messages" can also have a grandiose quality.
Erotomania: A delusion in which one believes that another person, usually someone of higher status, is in love with him or her. It is common for individuals with this type of delusion to attempt to contact the other person (through phone calls, letters, gifts, and sometimes stalking).
Grandiose delusion: An individual exaggerates his or her sense of self-importance and is convinced that he or she has special powers, talents, or abilities. Sometimes, the individual may actually believe that he or she is a famous person (for example, a rock star or Christ). More commonly, a person with this delusion believes he or she has accomplished some great achievement for which they have not received sufficient recognition.
Persecutory delusions: These 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 obstructed in the pursuit of goals. Sometimes the delusion is isolated and fragmented (such as the false belief that co-workers are harassing), but sometimes are well-organized belief systems involving a complex set of delusions ("systematized delusions"). A person with a set of persecutory delusions may be believe, for example, that he or she is being followed by government organizations because the "persecuted" person has been falsely identified as a spy. These systems of beliefs can be so broad and complex that they can explain everything that happens to the person.
Religious delusion: Any delusion with a religious or spiritual content. These may be combined with other delusions, such as grandiose delusions (the belief that the affected person was chosen by God, for example), delusions of control, or delusions of guilt. Beliefs that would be considered normal for an individual's religious or cultural background are not delusions.
Somatic delusion: A 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. An example of a somatic delusion would be a person who believes that his or her body is infested with parasites.
Delusions of control, nihilistic delusions, and thought broadcasting, thought insertion, and thought withdrawal are usually considered bizarre delusions. Most persecutory, somatic, grandiose, and religious delusions, as well as most delusions of jealousy, delusions of mind being read, and delusions of guilt would be considered non-bizarre.