Today, covering the mouth when yawning is considered an essential of good manners. But the original custom stemmed not from politeness but from fear—a fear that in one giant exhalation the soul, and life itself, might depart the body. A hand to the lips held back the life force. Ancient man had accurately observed (though incorrectly interpreted) that a newborn, struggling to survive, yawns shortly after birth (a reflexive response to draw additional oxygen into the lungs). With infant mortality extraordinarily high, early physicians, at a loss to account for frequent deaths, blamed the yawn. The helpless baby simply could not cover its mouth with a protective hand. Roman physicians actually recommended that a mother be particularly vigilant during the early months of life and cover any of her newborn’s yawns.
An ancient belief that the breath of life might escape the body during a yawn established the custom of covering the mouth. Today it is also considered good manners when yawning to turn one’s head. But courtesy had nothing to do with the origin of the custom, nor with the apology that follows a yawn. Ancient man had also accurately observed that a yawn is contagious to witnesses. Thus, if a yawn was dangerous to the yawner, this danger could be “caught” by others, like the plague. The apology was for exposing friends to mortal danger.
Modern science has explained the yawn as the body’s sudden need for a large infusion of oxygen, especially on awakening, when one is physically exhausted, and in the early stages of strenuous exercise. But there still is no physiological accounting for the contagiousness of yawning. We know only that the sight of a person yawning goes to the visual center of the brain and from there is transmitted to the yawn centre. Why such a particular pathway should exist is as mysterious to us today as was the yawn itself to ancient man.