<p>Infants must learn to discriminate between individuals in order to determine who is familiar (and likely to provide their basic needs) from those who are unfamiliar (and possibly a threat to their survival). One cue that humans use to discriminate others is the unique sound of each individual’s voice. Until the present thesis, little research existed on the topic of voice discrimination development during infancy. I conducted the first set of studies to investigate whether voice discrimination develops through a process of perceptual narrowing. Perceptual narrowing is defined as an experience-dependent increase in sensitivity to distinctions important in the native environment and a decrease in sensitivity to distinctions not important (often foreign to) the native environment across the first year. It has been described in previous research for the processing of a number of socially-relevant stimuli in the auditory (e.g., musical rhythms and pitches, linguistic phonemes) and visual (e.g., faces) domains. In Chapter 2, I provide the first evidence that narrowing occurs for voice discrimination, with infants specializing for the discrimination of native (human, English-speaking) over foreign (rhesus monkey) vocalizations between 6 and 12 months. In Chapter 3, I establish that the specialization demonstrated in Chapter 2 resulted primarily from familiarity with the vocalization from the human species, rather than the particular language spoken. In Chapter 4, I show that sensitivity to distinctions between monkey voices can be reinstated at 12 months of age, after narrowing has taken place, with two weeks of exposure to monkey voices. Together, these findings indicate that infants become attuned to individual distinctions between human voices by the end of their first year, but that plasticity remains such that sensitivity to distinctions between voices from rarely-heard species can be reinstated with exposure, at least until the end of the first year.</p> / Doctor of Philosophy (PhD)