Why should our most explicit expression of mutual vulnerability be primarily a vocal rather than, say, a visual signal? Why did we not evolve to, instead, simply cross our eyes, raise an eyebrow, wiggle our ears, or wrinkle our nose whenever we felt amusement?
Consider the situation of two young gorillas engaging in a bout of playful wrestling. Because laughter is vocal, the tickler need not have visual contact with his or her victim’s face to know that the attack is being correctly interpreted as playful. Nor would the tickler need to adopt a particular body posture or facial expression during what would likely be an extremely chaotic, though friendly, altercation.
Moreover, a vocal signal such as laughter can go beyond those immediately involved in the physical exchange. Supervising adults, with the ability to quash what they misinterpret as an aggressive encounter, would be much more effectively informed about the nature of the physical contact with a vocal signal than they would with a visual one.
