A delusion is a fixed, false personal belief held with absolute conviction despite all evidence to the contrary. The belief is outside the person’s normal culture or subculture and dominates their viewpoint and behavior. Delusions may be described in terms of their content (e.g. delusions of persecution or grandeur). They can be mood congruent (the content of delusion is appropriate to the mood of the patient), or mood incongruent. Delusions are described as systematized if they are united by a single theme.
A primary delusion arises fully formed without any discernible connection with previous events (also called autochthonous delusions), e.g. “I woke up and knew that my daughter was the spawn of Satan and should die so that my son could be the new Messiah”. Secondary delusions can be understood in terms of other psychopathology, for example hallucinations: “The neighbors must have connected all the telephones in the building; that’s why I can hear them all the time”.
The term delusional mood is slightly confusing in that it does not describe an abnormal belief, but refers to an ill-defined feeling that something strange and threatening is happening which may manifest as perplexity, uncertainty or anxiety. This may precede a primary delusion or a delusional perception, which involves a real perception occurring almost simultaneously with a delusional misinterpretation of that perception, e.g. “I saw the traffic lights change from red to green and knew that I was the rightful heir to the throne of England”. Overvalued ideas are unreasonable and sustained intense preoccupations maintained with a strong emotional investment but less than delusional intensity. The idea or belief held is demonstrably false and not usually held by persons from the same subculture.
Delusions may be classified in terms of their content, for example delusions of...
Persecution An outside person or force is in some way interfering with the sufferer’s life or wishes them harm, e.g. “The people upstairs are watching me by using satellites and have poisoned my food”.
Reference The behavior of others, objects, or broadcasts on the television and radio have a special meaning or refer directly to the person, e.g. “A parcel came from Sun Alliance and the radio said that ‘the son of man is here’, on a Sunday, so I am the son of God”.
Control The sensation of being the passive recipient of some controlling or interfering agent that is alien and external. This agent can control thoughts, feeling and actions (passivity experiences), e.g. “I feel as if my face is being pulled upwards and something is making me laugh when I’m sad”.
Grandeur Exaggerated belief of one’s own power or importance, e.g. “I can lift mountains by moving my hands, I could destroy you!”
Nihilism Others, oneself, or the world does not exist or is about to cease to exist (often called Cotard’s syndrome), e.g. “The inside of my tummy has rotted away. I have no bowels”.
Infidelity One’s partner is being unfaithful (also known as delusional jealousy or the Othello syndrome).
Doubles A person known to the patient, most frequently their spouse, has been replaced by another (also known as Capgras’ syndrome or, confusingly, ‘illusion’ of doubles).
Infatuation A particular person is in love with the patient (also known as erotomania or de Clerambault’s syndrome).
Somatic Delusional belief pertaining to part of the person’s body, e.g. “My arms look like they’ve been melted and squashed into a mess”.