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The development of L1 speech perception begins in utero, evidence for which is manifested immediately after birth (Zhao & Kuhl, 2018). Researchers have used laboratory techniques to establish that within hours of birth, babies not only show a preference for their mother’s voice (Lee & Kisilevsky, 2014; May et al., 2018) but are also capable of distinguishing between L1 versus foreign language sounds (Moon et al., 2013). These abilities are hypothesized to result from fetal experience of the mother’s voice, and the voices of other speakers of the L1, through the abdominal wall.

Despite their proclivity toward speakers heard in utero, newborns maintain the ability to perceive fine-grained phonetic details associated with any speech sounds found in any of the world’s languages for some time after birth. By six months, however, experience with their ambient language (or languages) results in refinement of their perceptual systems, such that they begin paying less attention to any phonetic information not needed to categorize L1 speech sounds (Kuhl, 2009). By the time they are a year old, infants begin losing the ability to discriminate foreign speech sounds, while their ability to recognize ambient language sounds strengthens (Zhao & Kuhl, 2018). This loss of perceptual plasticity corresponds with increasing L1 processing efficiency, which is a necessary precursor to the learning of L1 vocabulary and higher-order language skills (Kuhl, 2009). By the time children reach the age of four, they cannot discriminate sounds in a foreign language any better than an adult can (Werker, 2018).

Extensive research describing L1 speech perception and later language development makes it clear that the foundation of L1 pronunciation is accurate speech perception, which results in automatic categorization of sounds (Werker & Curtin, 2005). Though lagging behind perception, the development of L1 speech production follows the same trajectory. As with perception, infant vocalizations first emerge in language-independent ways. By ten months, babbling begins to reflect the properties of the ambient language (Grenon et al., 2007). The observed asymmetry between L1 perception and production is in part due to physiology, since speaking is a physical activity, while perception is cognitive. Over time, the acoustic properties of L1 speech production begin to closely match that of older speakers in the community (Flege, 2003). Kuhl and Meltzoff (1996) argue that children’s vocal imitation of interlocutors explains how this happens, as was illustrated in the personal anecdote with which I began this chapter.