Infants are born with functional hearing and progressively develop sound discrimination, phoneme recognition, and sensitivity to native-language speech patterns that form the foundation for language acquisition. By 12 months, infants recognize familiar words and understand prosodic and phonotactic patterns of their native language, preparing for the language explosion of toddlerhood.
Newborns arrive with more auditory sophistication than their limited behavioral repertoire suggests. You know from neonatal sensory capabilities that hearing begins before birth — the fetus responds to sound by the third trimester and is already forming memories of auditory patterns in the womb. At birth, infants can discriminate their mother's voice from a stranger's, and they prefer it. This head start reflects months of prenatal exposure to the prosodic patterns — the rhythm, stress, and melody — of their native language. The auditory system is not starting from scratch; it is continuing a process already underway.
The first year of life is a period of perceptual narrowing in which infants progressively specialize their sound discrimination toward the contrasts that are meaningful in their native language. Newborns can distinguish virtually all phoneme contrasts used in any human language — a universal sensitivity that researchers call phonetic universalism. By 6 months, infants are beginning to show better discrimination of native-language vowel contrasts, and by 10–12 months, the same narrowing has occurred for consonants. Japanese infants, who will grow up not needing to distinguish /r/ from /l/, lose sensitivity to that contrast by the end of the first year. This is not a loss of hearing acuity — the ear still receives the sounds — but a reorganization of perceptual categories based on the statistical structure of the input.
The mechanism underlying this narrowing is statistical learning: infants track the frequency and distribution of sounds in their environment without any explicit instruction. High-amplitude sucking experiments and preferential looking procedures reveal that infants notice when the statistical patterns in sound streams change — they are unconsciously building a probabilistic model of their language's phonology. By 6–8 months, infants also respond differently to speech in their native language versus a foreign language, reflecting sensitivity to its characteristic rhythmic and stress patterns (prosody). They are not learning words yet, but they are learning the sound skeleton onto which words will be hung.
By 10–12 months, several milestones signal that the auditory system is language-ready. Infants recognize familiar words in connected speech, understand that words refer to things, and show sensitivity to the phonotactic patterns of their language — the permissible sound sequences. An English-learning infant has internalized that "str" can begin an English word but "ngb" cannot. They respond to their own name, look up at common words, and distinguish different emotional tones. The auditory groundwork laid in the first year enables the rapid word-learning and grammatical bootstrapping of the second year — what researchers call the vocabulary explosion — because the perceptual and statistical machinery for parsing speech into meaningful units is already in place.