Medicine: Why Babies Beam

After boasting about their newborn's first smile, many proud parents are deflated by the pediatrician's cool remark: "It's only gas." Not so, says Psychiatrist Robert N. Emde of the University of Colorado School of Medicine. Having studied more than 300 beaming babies in the past nine years, Dr. Emde reports that infants have two quite different kinds of smiles, neither of which has anything to do with gas.

Even before a baby recognizes faces or responds to voices, says Emde, he smiles when he is drowsy or wriggling in his sleep. During his first two weeks of life, such grins occur as...