One common misconception in American culture is that it takes twice as many facial muscles to frown as it does to smile. In fact, frowning uses 11 muscles while smiling uses 12.
Facial expressions are important for interactions between humans. In his book The Body: A Guide for Occupants, Bill Bryson writes:
Babies fresh from the womb are said to prefer a face, or even the general pattern of the face, to any other shape. Whole regions of the brain are devoted solely to recognizing faces. We are exquisitely sensitive to the subtlest alterations of mood or expression, even if we are not always conscious of them.
Facial expressions are also an important part of teaching language to infants. Modern motherese (a simple form of language parents often use when speaking to infants) involves not only a slower, more melodic, and repetitious form of spoken language but also incorporates facial expressions—often exaggerated—touching, and body language.
Humans can read facial expressions and there is a tendency to mirror other people’s emotions. This ability to read emotions from facial expressions generally allows social relationships to run more smoothly.