Bart de Boer
This article provides an overview on infant-directed speech and language evolution. Infant-directed speech is defined as a set of speech registers that caretakers use to address infants. There are at least three different kinds of infant-directed speech, first, which is used to get the infant's attention, second, which is used to soothe the infant, and last, which is used to address the infant with linguistically meaningful utterances. All kinds of infant-directed speech are characterized by slower speech rate and larger intonation contours. Attention-getting infant-directed speech is characterized by higher volume and extreme intonation excursions, but it does not necessarily consist of meaningful utterances. Soothing infant-directed speech is characterized by lower volume, sometimes even whispered speech, and very flowing intonation contours. Caretakers use infant-directed speech automatically when addressing infants, even without being aware of doing it. They also automatically adapt the complexity of their speech to the level of linguistic competence of the infant. The infant-directed speech also appears to be nearly universal cross-culturally and a similar register is attested in signed languages. Computational experiments have revealed that it is easier to acquire vowel categories on the basis of infant-directed speech than on the basis of adult-directed speech. Other computational experiments have shown that infant-directed speech can help to preserve stability of vowel systems over time.