In the first few hours of life, a human infant can tell the difference between its own mother's voice and a strange female voice. An infant as young as 1 month old can hear the difference between categories of speech sounds, such as the difference between /p/ and /b/. And from the moment of birth, infants display affect signals that their caregivers interpret as meaningful. We know, then, that certain basic capacities serving communication and language are already in place at the beginning of life. By the time language begins, in the second year, and the rudiments of speech sounds have only just begun to appear, the development of affect expression is well under way. Smiles, giggles, laughs, frowns, whines, and cries appear effortless and automatic at a time when emerging words are fragile, tentative, and inconsistent. The purpose of this chapter is to show how developments in expression and the social life of infants in the first year of life bring an infant to this threshold of language.

Two aspects of expression in infancy are particularly relevant for understanding the transition to language. The first is the nature of the expression itself: what it looks like, what it sounds like, and what it means. The second is the development of the infant as a profoundly social being, virtually from the beginning of life.