2. From citizens of the world to language specialists

Adults are often said to perceive non-native speech input through the “filter” of their native language (Sapir, 1921). Indeed, a multitude of studies examine the challenges adults face when listening to a language that is not their L1. The pervasive difficulties adults encounter in perceiving non-native speech contrasts are made only more striking by the relative ease with which infants perceive such contrasts. Regardless of the language community in which an infant is born, for the first several months of her life she will discriminate contrasts that her parents cannot. For example, an infant born in Japan can discriminate English /r/ and /l/, a contrast with which Japanese-speaking adults notoriously struggle (Goto, 1971; Miyawaki et al., 1981). However, an infant’s amazing perceptual acuity diminishes during the first year of life, as she becomes specialized in perceiving the contrasts of her native language (Aslin, Pisoni, & Jusczyk, 1983; Jusczyk, 1993, 1994; Kuhl, 1991, 1993; Werker & Tees, 1984a, 1884b).