Describing the recent death of her father, a 26-year-old woman giggles intermittently throughout the account. When her clinician gently notes the mismatch, she appears genuinely puzzled rather than embarrassed or self-conscious about it. What is this?

This is incongruity of affect: an emotion is clearly present, it is simply the wrong one for the content being discussed, and she has no insight into the mismatch. It is not blunting of affect, which is a general reduction in the range and intensity of emotional display rather than a mismatched emotion. It is not smiling depression, in which the smile is a consciously maintained front over a mood the person knows is low — here she is genuinely puzzled, not covering anything up. And it is not ordinary nervous laughter, which people recognise in themselves and which does not occur with this persistence or lack of self-awareness.