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This might sound funny. But I have been wondering why do humans/primates laugh or smile when they are happy or find something funny?

Why don't they cry when they are happy and laugh/*smile* when they are sad?

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I agree that we dont usually cry when we are happy :), but often I see people wich smiling is the easier way of expressing/hidding sadness for them, dun't ask me why – Enoque Duarte Jan 17 '14 at 11:57
I don't think there's any significant reason, it just happens to be the way our wiring worked out, that particular facial muscle contractions are associated with particular experiences. If we were to start mammals over again, the random mutations could have led a different direction. What's important, and possibly selected for as social creatures, is that we can distinguish cries from smiles to infer feelings of others. But it doesn't particularly matter what the symbol is as long as everyone knows it. Just like an A is an arbitrary set of squiggles and only has meaning by convention. – Keegan Keplinger Jan 17 '14 at 16:18

Subcortical unconscious programming originating in distance evolutionary relatives cause humans to smile when they feel happy.

-Consciousness, Volition, and the Neuropsychology of Facial Expressions of Emotion

There are disorders like the Pseudobulbar affect which cause people to express a different emotion than what is typical like crying instead of smiling. With PBA the emotions displayed are sudden, frequent, extreme and uncontrollable.

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