Acoustic regularities in infant-directed speech and song across cultures

Abstract

The forms of many species’ vocal signals are shaped by their functions^1–15. In humans, a salient context of vocal signaling is infant care, as human infants are altricial^{16, 17}. Humans often alter their vocalizations to produce “parentese”, speech and song produced for infants that differ acoustically from ordinary speech and song^18–35 in fashions that have been proposed to support parent-infant communication and infant language learning^36–39; modulate infant affect^{33, 40–45}; and/or coordinate communicative interactions with infants^46–48. These theories predict a form-function link in infant-directed vocalizations, with consistent acoustic differences between infant-directed and adult-directed vocalizations across cultures. Some evidence supports this prediction^{23, 27, 28, 32, 49–52}, but the limited generalizability of individual ethnographic reports and laboratory experiments⁵³ and small stimulus sets⁵⁴, along with intriguing reports of counterexamples^55–62, leave the question open. Here, we show that people alter the acoustic forms of their vocalizations in a consistent fashion across cultures when speaking or singing to infants. We collected 1,615 recordings of infant- and adult-directed singing and speech produced by 410 people living in 21 urban, rural, and small-scale societies, and analyzed their acoustic forms. We found cross-culturally robust regularities in the acoustics of infant-directed vocalizations, such that infant-directed speech and song were reliably classified from acoustic features found across the 21 societies studied. The acoustic profiles of infant-directedness differed across language and music, but in a consistent fashion worldwide. In a secondary analysis, we studied whether listeners are sensitive to these acoustic features, playing the recordings to 51,065 people recruited online, from many countries, who guessed whether each vocalization was infant-directed. Their intuitions were largely accurate, predictable in part by acoustic features of the recordings, and robust to the effects of linguistic relatedness between vocalizer and listener. By uniting rich cross-cultural data with computational methods, we show links between the production of vocalizations and cross-species principles of bioacoustics, informing hypotheses of the psychological functions and evolution of human communication.