"We are part of a human herd whose behavior is often the involuntary playing out of an ancient neurological script that is so familiar that it goes unnoticed. Consider what is really happening when your body is hijacked by an observed yawn or you spontaneously join others in a communal chorus of ha-ha-ha. You don't decide to yawn or laugh contagiously — it just happens. We discover that a simple neurological mechanism that automatically replicates observed behavior is the basis for contagious yawning, laughing, crying, scratching, and coughing, and perhaps sociality and empathy. American president Bill Clinton's celebrated ability to 'feel your pain' may be related to his joining friends in a yawn or laugh. A malfunction of this mechanism of contagious behavior may be responsible for the social deficits of autism and schizophrenia.
"Laughter is revealed to be the sound of physical play, with the labored pant-pant of our cavorting primate ancestors evolving into the human ha-ha, the best example of the origin of a vocalization. Laughter also shows why we can talk and chimpanzees can't, and why the breath control conferred by bipedality is essential for this transformation.
"Emotional tears are a breakthrough in human social evolution, adding range and nuance to the face as an instrument of emotional expression. Will people recognize your sadness without tears? The evolution of the white of the eye (the sclera) is another transformative social event. We learn why the uniquely white human sclera is necessary for the display of red eyes, a signal of sadness and poor health, and why eye-drops that 'get the red out' are really beauty aids."