A delusion is a belief that evidence cannot pry loose
Psychiatry reserves delusion for a fixed idea that refuses correction when contradictory evidence appears. That is different from ordinary error, dogma, illusion or hallucination—those can yield when better information arrives. The border with a merely strong opinion is graded by how tightly conviction grips.
Karl Jaspers, psychiatrist and philosopher, set out the classic test in his 1913 book General Psychopathology. A delusional belief, he said, is held with total certainty, cannot be shifted by argument or proof, has content that is impossible or plainly false, and cannot be explained by the person's psychology. Beliefs built on value judgments count only if they cannot be shown true: a man insisting he flew to the Sun and back qualifies unless he is speaking figuratively.
Clinicians sort delusions along two lines. Bizarre ones are implausible even to people from the same culture, such as the DSM-5 example of believing a stranger has swapped out all your internal organs; non-bizarre ones, like being trailed by police, are false but possible. Separately, some fit the person's mood, as when a manic patient believes they are a deity, and others are neutral, such as feeling an extra limb growing from the back of the head.
Themes repeat across patients. Persecution is the most common, and the DSM-IV-TR names it the most frequent form in schizophrenia: being followed, cheated, poisoned or spied on. Others include jealousy about a faithful partner, the conviction that thoughts are broadcast or inserted, erotomania, grandiosity, and delusions of poverty, which were widespread before state welfare existed. Culture shapes the content: guilt and punishment dominate among patients in Austria, persecution among those in Pakistan.
Causes are unsettled. Relatives of affected people carry higher risk, poor hearing or sight makes delusional thinking more likely, and so do stressors such as immigration and low income. One theory stresses raised dopamine; the two-factor model argues that both forming and checking beliefs must fail, with imaging linking the checking fault to the right lateral prefrontal cortex. A rival account says the brain attaches too much importance to irrelevant signals.
Source: Delusion
