instinctive origins are quite separate. The smile derives from a submissive, appeasement gesture in non-human primates. The mouth is stretched back so as to reveal the teeth closed in a non-threatening position, essentially saying, ‘See, I’m totally harmless and won’t bite you’. The laugh is nearer to an aggressive signal, like that of a monkey that shows its teeth open and ready to bite, while making a loud glottal ‘ah-ah-ah’ sound to threaten a rival. This is especially true of the kind of laugh that children use when ridiculing one another in the playground and when adults make hostile jokes against ethnic out-groups. Naturally enough, the appeasing smile has been adopted by humans to signal friendly intent.