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THE DUCHENNE SMILE: INSTINCTIVE SMILING TO EXPRESS THE EMOTION OF HAPPINESS
Babies who are born both deaf and blind begin smiling (Duchenne smile) at the same period of their development as babies born with normal hearing and sight. A blind infant would not smile (make a facial signal that communicates happiness or contentment to the mother) if smiling were not inborn.
Later in life humans continue to communicate happiness to others by smiling and laughing. Humans laugh 30 times more often in the company of other people than when alone.
Laughter is involuntary, indicating its adaptive nature. And, like other expressions of emotion, laughter is contagious.