Acoustic regularities in infant-directed speech and song across cultures

Abstract

Humans often produce vocalizations for infants that differ from vocalizations for adults. Is this property common across societies? The forms of infant-directed vocalizations may be shaped by their function in parent-infant communication. If so, infant-directed song and speech should be differentiable from adult-directed song and speech on the basis of their acoustic features, and this property should be relatively invariant across cultures. To test this hypothesis, we built a corpus of 1,614 recordings of infant-and adult-directed singing and speech produced by 411 people living in 21 urban, rural, and small-scale societies. We studied the corpus in a massive online experiment and in a series of acoustic analyses. Naïve listeners (N = 13,218) reliably identified infant-directed vocalizations as infant-directed, and adult-directed speech (but not songs) as adult-directed, at rates far higher than chance. Ratings of infant-directed song were the most accurate and the most consistent across all societies; infant-directed speech was accurately identified on average, but inconsistently across societies. To determine the mechanisms underlying these results, we extracted many acoustic features from each recording and identified those that most reliably characterize infant-directed song and speech across cultures, via preregistered exploratory-confirmatory analyses and machine classification. The features distinguishing infant-and adult-directed song and speech concerned pitch, rhythmic, phonetic, and timbral attributes; a hypothesis-free classifier with cross-validation across societies reliably identified all vocalization types, with highest accuracy for infant-directed song. Last, we isolated 12 acoustic features that were predictive of perceived infant-directedness; of these, two pitch attributes (median F0 and its variability) were by far the most explanatory. These findings demonstrate cross-cultural regularities in infant-directed vocalizations that are suggestive of universality; moreover, infant-directed song appears to be more cross-culturally stereotyped than infant-directed speech, informing hypotheses of the functions and evolution of both.The forms of many animal signals are shaped by their functions, a link arising from production-and reception-2 related rules that help to maintain reliable signal detection within and across species 1-6 . This is especially 3 true of vocal signals, where form-function links have been demonstrated across many species, including 4 nonhuman primates 3 , meerkats 7 , grackles 8 , frogs 9 , and fish 10 . 5The link between form and function in vocalizations is also evident from listeners' behavior. For example, 6 humans 11 , red deer 12 , and canines 13 reliably detect the intentions of heterospecific signalers on the basis of 7 the sounds of their signals. A classic demonstration of this fact is the ability of some species to eavesdrop 8 on the alarm signals of other species, whether or not their own species has an extended vocal repertoire 14,15 . 9 In humans, an area of...

Infants relax in response to unfamiliar foreign lullabies

“…These findings support a hypothesized role for infant-directed song in the ecosystem of parental investment 21,22 -including the proclivity of parents to sing to their infants [5][6][7] ; the acoustic features that characterize infant-directed songs worldwide 1,34,[40][41][42][43][44][45] ; infants' ability to perceive them and motivation to engage with them 63,64 ; and their calming effects [48][49][50][51] -that is both universal and innately specified.…”

“…melodic and rhythmic complexity, slower tempo, less steady beat, lower arousal, lower valence, and lower pleasantness 41 . Together, these features predict the degree to which listeners perceive a song as infantdirected 34,41 .…”

“…Future studies could test these questions by comparing infants' physiological responses across different song types and across different acoustic signals; or by systematically manipulating the acoustic features present in songs to measure their relaxation effects on listeners. Prior research has outlined some musical features that parents in Western cultures exaggerate while singing lullabies 42,68 , which correspond with acoustic differences between infant-and adult-directed song across many societies 34 ; these are good candidates for possible features that may drive relaxation effects.…”

Origins of music in credible signaling

“…While few data exist with which to test these relationships, preliminary findings suggest that similarities do exist. For example, baboon contact calls are harmonically rich, whereas alarm calls are harsh and noisy (Fischer et al 2000); in a vocalization corpus from 21 human societies, infantdirected song was acoustically distinct from infant-directed speech across many pitch, rhythmic, phonetic, and timbral attributes (Moser et al 2020), with a similar pattern of results to the acoustic differences between baboon contact calls and alarm calls. Moreover, several acoustic features driving these effects were related to vocal exertion (e.g., temporal modulation, pitch rate, vowel rate), perhaps honestly signaling additional costs incurred by the signaler.…”

Citizen science can help to alleviate the generalizability crisis

“…For example, in a study of acoustic regularities in infant-directed vocalizations across cultures, we model random effects of listener characteristics, speaker/singer (i.e., the producers of the stimuli) characteristics, and stimulus categories of interest (e.g., infant-directed vs. adult-directed speech). This is only possible with large datasets (in our case, nearly 1 million listener judgements; Moser et al, 2020). Other under-used analyses also become more practical with big citizen-science data, including radical randomization (Baribault et al, 2018), prediction with cross-validation (Yarkoni & Westfall, 2017), and matching methods for causal inference (Stuart, 2010).…”