A patient has expressed a belief that strikes you as false and out of keeping with his background. Before revealing the answer, decide how you would explore whether this is a delusion.

Assess how fixed and unshakeable the belief is, by gently offering counter-evidence or an alternative explanation and observing whether this has any effect, since imperviousness to counter-evidence is central to the concept. Ask the patient to explain how he arrived at the belief, since a delusion is not something reached through an ordinary, understandable process of reasoning from his own cultural or community background, whereas a secondary or culturally shared belief can usually be explained in this way. Explore his degree of concern about whether others agree with him, since this is often, though not always, notably reduced. Remember that falseness alone is not the defining feature, since a delusional belief can occasionally coincide with the truth — the fixed, culturally out-of-keeping manner of holding it is what matters most.