A delusion is a fixed false belief, not amenable to reason (i.e., held despite evidence to the contrary), and not culturally sanctioned.
There are a number of common forms of delusion, including:
- Persecutory (paranoia)
- Reference: important events or people being influenced by patients thoughts, ideas
- Grandiose/expansive: occur particularly in mania
- Guilt/worthlessness: occur particularly in depression
- Thought broadcast and thought insertion
- Control by an external agency.
Specific, named, delusional syndromes are those of:
- Capgras: the "delusion of doubles", a familiar person or place is thought to be an impostor, or double; this resembles the reduplicative paramnesia described in neurological disorders, such as Alzheimer’s disease.
- Fregoli: a familiar person is identified in other people, even though they bear no resemblance; this may occur in schizophrenia.
- De Clérambault (erotomania): the belief (usually of a single woman) that a famous person is secretly in love with her ("hope"), followed by the belief that that person is persecuting her ("resentment"); may occur in schizophrenia.
Delusions are a feature of primary psychiatric disease (psychoses, such as schizophrenia; neuroses, such as depression), but may also be encountered in neurological disease with secondary psychiatric features ("organic psychiatry"), e.g., delirium, and dementing syndromes, such as Alzheimer’s disease, dementia with Lewy bodies.
Tekin S, Cummings JL. Hallucinations and related conditions. In: Heilman KM, Valenstein E (eds.). Clinical neuropsychology (4th edition). Oxford: OUP, 2003: 479-494