Newborns Can Hear Every Sound in Every Human Language. By Their First Birthday, They Can't.
At birth, an infant can distinguish between every speech sound used in every human language. Mandarin tones, Czech consonant clusters, Zulu clicks, the subtle vowel contrasts of Norwegian and Hindi. By around six months old, that ability begins narrowing. By their first birthday, the contrasts they have not heard regularly have started to fade — and many of those distinctions become almost impossible for adult ears to hear at all.
At birth, a human infant can distinguish between every speech sound used in every human language. Mandarin's pitched tones, Czech's dense consonant clusters, Zulu's distinct click consonants, the subtle vowel contrasts of Norwegian, the dental and retroflex T sounds of Hindi, the difference between Japanese 'r' and English 'l' — all of these contrasts are immediately audible to the newborn brain.
The total inventory of phonetic contrasts across the world's roughly 7,000 living languages runs into the hundreds. Newborns can hear them all. The same baby, in principle, could grow up to be a native speaker of any human language.
This sensitivity does not last. Beginning around six months of age, infants enter what developmental psychologists call perceptual narrowing. The brain begins to prune its perception toward the sounds it actually encounters. Contrasts that show up frequently — the consonants and vowels of the local language — sharpen. Contrasts that do not show up start fading. By around twelve months, the process is largely complete. The infant has begun the slide from universal phonetic listener to native speaker of one particular language.
The work is not voluntary, and it is not driven by formal teaching. It is a form of unsupervised statistical learning. Patricia Kuhl and other researchers, beginning in the 1980s and 1990s, established that infants track the frequency and distribution of speech sounds in their environment and adjust their perception accordingly. By their first birthday, infants raised in monolingual environments respond strongly to native contrasts and weakly to foreign ones, while those raised bilingual maintain sensitivity to a wider set.
The cost shows up in adulthood. Native English speakers cannot reliably hear the difference between Hindi dental and retroflex T sounds. Native Japanese speakers struggle with the English 'r' and 'l' contrast. These are not failures of effort. The underlying neural circuit was pruned in infancy, before either listener could speak a word.
It is one of the few times in human development when the brain pays for specialization by giving up capacity.