Why do only humans weep? The evolutionary puzzle of crying.

"Like laughter, tears express an emotional state by means other than the muscles of the face. They are involuntary, conspicuous to a perceiver, and unique to Homo sapiens (a conclusion flaunted in the title of the most comprehensive book on the subject, the psychologist Ad Vingerhoets's Why Only Humans Weep). And they seem engineered to generate common knowledge. A weeping person feels the welling in his sockets and the trickle on his cheeks."

"The sadness comes from a loss, defeat, or humiliation, and is accompanied by a feeling of helplessness and self-pity. The obvious coordination game resolved by common knowledge of a loss is Hawk-Dove. Like the white flag of surrender or throwing a towel into a boxing ring, crying signals that a person acknowledges defeat and can no longer put up a fight, sparing both sides in a conflict from the costs of further fighting when the outcome is a foregone conclusion."

The first word associated with laugh is cry, reflecting their semantic contrast. Tears express emotion by means other than facial muscles; they are involuntary, conspicuous, and unique to humans. Tears appear engineered to generate common knowledge because a weeper experiences internal sensations and also sees others observing the same tears. Crying signals sadness from loss, defeat, or humiliation, accompanied by helplessness and self-pity. In coordination games like Hawk-Dove, crying functions like a white flag or thrown towel, signaling acknowledged defeat and inability to fight, thereby sparing both sides the costs of continued conflict.

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