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Babies Can Hear the Sounds of Every Language — Until About 10 Months: Werker and Tees' Experiment

In short
Janet Werker and Richard Tees showed that at 6–8 months, babies can tell apart speech sounds from any language in the world, even ones their parents don't speak. By 10–12 months, this ability narrows to the sounds of their native language — a process called perceptual reorganization (or perceptual narrowing).
How the experiment worked
Infants from English-speaking Canadian families heard pairs of sounds that speakers of other languages distinguish but English speakers don't:
  • two "t" sounds from Hindi (dental and retroflex, with the tongue tip curled back);
  • two "k"-type sounds from Nthlakampx, a Salish First Nations language of British Columbia.
The method was the conditioned head-turn procedure: babies learned to turn toward a toy when the sound changed. Groups aged 6–8, 8–10 and 10–12 months were tested, and some infants were followed longitudinally. Adults and infants from Hindi- and Nthlakampx-speaking families were tested for comparison.
Results
  • At 6–8 months, nearly all English-learning infants discriminated the unfamiliar sounds.
  • At 8–10 months, noticeably fewer did; at 10–12 months, only a few.
  • Infants from Hindi- and Nthlakampx-speaking homes still discriminated their native contrasts at 11–12 months.
  • English-speaking adults discriminated them poorly.
Conclusions and significance for developmental psychology
Babies are born "citizens of the world", and by the end of the first year their hearing tunes in to the sounds they hear constantly. The ability is not entirely lost but reorganized around the native language. It is a striking example of experience shaping perception during a sensitive period. Later work showed that vowel perception is already shaped by 6 months (Kuhl et al., 1992), and that live interaction in a foreign language at 9 months preserves sound discrimination, while video or audio does not (Kuhl et al., 2003).
What it means for parents
  • Live interaction does the most for your baby's language: talk, sing and read aloud from the first months.
  • If your family speaks two languages, use both — bilingualism from birth is natural for the brain.
  • Foreign-language videos don't replace a real person.
What it means for professionals
The study is key to understanding early foundations of language, bilingual development and risks for speech and language disorders. Perceptual reorganization is a reference point when assessing hearing and speech development in the first year, and an argument for early detection of hearing loss.
References
  • Kuhl, P. K., Tsao, F.-M., & Liu, H.-M. (2003). Foreign-language experience in infancy: Effects of short-term exposure and social interaction on phonetic learning. Proceedings of the National Academy of Sciences, 100(15), 9096–9101. https://doi.org/10.1073/pnas.1532872100
  • Kuhl, P. K., Williams, K. A., Lacerda, F., Stevens, K. N., & Lindblom, B. (1992). Linguistic experience alters phonetic perception in infants by 6 months of age. Science, 255(5044), 606–608. https://doi.org/10.1126/science.1736364
  • Werker, J. F. (2024). Phonetic perceptual reorganization across the first year of life: Looking back. Infant Behavior and Development, 75, Article 101935. https://doi.org/10.1016/j.infbeh.2024.101935
  • Werker, J. F., & Tees, R. C. (1984). Cross-language speech perception: Evidence for perceptual reorganization during the first year of life. Infant Behavior and Development, 7(1), 49–63. https://doi.org/10.1016/S0163-6383(84)80022-3
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