Human infants are acutely attuned to the human voice and prefer it above all other sounds. In fact, they prefer the higher pitch ranges characteristic of female voices. They are also attentive to the human face, particularly the eyes, which they stare at even more if the face is talking. These preferences are present at birth, and some research indicates that babies even listen to their mother's voice during the last few months of pregnancy.

Since the early 1970s, it has been known that babies can detect very subtle differences between English phonemes (the functional units of speech sound). For example, they can detect the difference between "pa" and "ba," or between "da" and "ga." Of course, they do not attach meaning to the differences for 12 months or more.

At the beginning of infancy, vegetative noises and crying predominate. Observers note that by the age of...