In the early months of life, infants acquire information about the phonetic properties of their native language simply by listening to adults speak. The acoustic properties of phonetic units in language input to young infants in the United States, Russia, and Sweden were examined. In all three countries, mothers addressing their infants produced acoustically more extreme vowels than they did when addressing adults, resulting in a "stretching" of vowel space. The findings show that language input to infants provides exceptionally well-specified information about the linguistic units that form the building blocks for words.

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Speech input to infants may exert an important influence on the development of language-specific perception by providing infants with information regarding the phonetic and phonological properties of their language. This study compares the acoustic structure of vowels produced by Russian and American mothers conversing with their infants, with the structure of vowels produced by the same women conversing with an adult. Mothers in both countries were asked to play with their infants (aged 2–5 months) using sets of toys whose names contained the target vowels in at least two different consonant contexts (e.g., sheep, bead). They were asked to use the same words in conversation with an adult. All mothers increased the degree of acoustic separation between vowel categories in infant-directed speech by selectively increasing or decreasing F1 and F2 values. Relative to American mothers [P. K. Kuhl et al., Science 277, 684–686 (1997)], Russian mothers showed smaller increases in fundamental frequency and less variation in vocal pitch while speaking to their infants. Nevertheless, mothers in both countries produced highly similar changes in acoustic structure, and comparable increases in the size of the acoustic space delimited by the vowels /i/, /a/, and /u/, when speaking to their infants.