First clip is adult-directed; second, infant-directed speech.
Listen to more recordings at The Music Lab.
Global consistencies in infant-directed speech included higher pitch and increased pitch range. Other features that appeared many times in the recordings were contrasting vowels (like emphasizing a long “e” and a short “e” in the same word) and pulse clarity (using a musical voice). When the researchers analyzed singing, they noticed consistent changes in tempo and timbre.
The group trained a machine-learning model to use features like pitch and loudness to guess whether a recording was infant-directed or adult-directed. The model was largely accurate in its determination across the 21 societies and strengthened the case for global consistencies, according to Courtney Hilton, a postdoctoral fellow in the Department of Psychology and a co-first author on the paper.
“Our study provides the strongest test yet of whether there are acoustic regularities in infant-directed vocalizations across cultures,” Hilton said. “It is also really the first to convincingly address this question in both speech and song simultaneously. The consistencies in vocal features offer a really tantalizing clue for a link between infant-care practices and distinctive aspects of our human psychology relating to music and sociality.”
This research builds on earlier studies showing that lullabies and altered speech patterns can have a soothing effect on infants, as well as animal findings demonstrating the clear function of vocalizations such as sounding the alarm for an approaching predator or signaling friendliness and approachability.
“When we were selecting the features with which to look at infant-directed song and speech, we specifically appealed to some baseline bioacoustic principles that can be held constant not only in humans, but across different animals,” said Cody J. Moser, a co-first author on the paper, doctoral candidate at the University of California Merced, and a visiting student in Harvard’s Graduate School of Arts and Sciences. “Things about the way that we alter our pitch toward our infants might have their roots in ways that other animals communicate with each other.”
A secondary part of the study involved members of the public trying to identify whether a vocalization was infant-directed or adult-directed. More than 50,000 English speakers with fluency in 199 languages hailing from 187 countries participated in that portion of the study. Most were accurate in their assessments regardless of their native language or culture. This aspect of the research confirmed that people can pick up on the acoustic markers of infant-directed speech and song, even if they don’t understand the words or cultural references.
