When interacting with infants, humans often alter their speech and song in ways thought to support communication. Theories of human child-rearing, informed by data on vocal signalling across species, predict that such alterations should appear globally. Here, we show acoustic differences between infant-directed and adult-directed vocalizations across cultures. We collected 1,615 recordings of infant- and adult-directed speech and song produced by 410 people in 21 urban, rural and small-scale societies. Infant-directedness was reliably classified from acoustic features only, with acoustic profiles of infant-directedness differing across language and music but in consistent fashions. We then studied listener sensitivity to these acoustic features. We played the recordings to 51,065 people from 187 countries, recruited via an English-language website, who guessed whether each vocalization was infant-directed. Their intuitions were more accurate than chance, predictable in part by common sets of acoustic features and robust to the effects of linguistic relatedness between vocalizer and listener. These findings inform hypotheses of the psychological functions and evolution of human communication.
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The audio corpus is available at https://doi.org/10.5281/zenodo.5525161. All data, including supplementary fieldsite-level data and the recording collection protocol, are available at https://github.com/themusiclab/infant-speech-song and are permanently archived at https://doi.org/10.5281/zenodo.6562398. The preregistration for the auditory analyses is at https://osf.io/5r72u.
Analysis and visualization code, a reproducible R Markdown manuscript and code for the naive listener experiment are available at https://github.com/themusiclab/infant-speech-song and are permanently archived at https://doi.org/10.5281/zenodo.6562398.
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This research was supported by the Harvard University Department of Psychology (M.M.K. and S.A.M.); the Harvard College Research Program (H.L.-R.); the Harvard Data Science Initiative (S.A.M.); the National Institutes of Health Director’s Early Independence Award DP5OD024566 (S.A.M. and C.B.H.); the Academy of Finland grant no. 298513 (J. Antfolk); the Royal Society of New Zealand Te Aparangi Rutherford Discovery Fellowship RDF-UOA1101 (Q.D.A. and T.A.V.); the Social Sciences and Humanities Research Council of Canada (L.K.C.); the Polish Ministry of Science and Higher Education grant no. N43/DBS/000068 (G.J.); the Fogarty International Center (P.M., A. Siddaiah and C.D.P.); the National Heart, Lung, and Blood Institute and the National Institute of Neurological Disorders and Stroke award no. D43 TW010540 (P.M. and A. Siddaiah); the National Institute of Allergy and Infectious Diseases award no. R15-AI128714-01 (P.M.); the Max Planck Institute for Evolutionary Anthropology (C.T.R. and C.M.); a British Academy Research Fellowship and grant no. SRG-171409 (G.D.S.); the Institute for Advanced Study in Toulouse, under an Agence nationale de la recherche grant, Investissements d’Avenir ANR-17-EURE-0010 (L.G. and J. Stieglitz); the Fondation Pierre Mercier pour la Science (C.S.); and the Natural Sciences and Engineering Research Council of Canada (S.E.T.). We thank the participants and their families for providing recordings; L. Sugiyama for supporting pilot data collection; J. Du, E. Pillsworth, P. Wiessner and J. Ziker for collecting or attempting to collect additional recordings; N. Nicolas for research assistance in the Republic of the Congo; Z. Jurewicz for research assistance in Toronto; M. Delfi and R. Sakaliou for research assistance in Indonesia; W. Naiou and A. Altrin for research assistance in Vanuatu; S. Atwood, A. Bergson, D. Li, L. Lopez and E. Radytė for project-wide research assistance; and J. Kominsky, L. Powell and L. Yurdum for feedback on the manuscript. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended Data Fig. 1 Variation across societies of infant-directed alterations.
Estimated differences between infant-directed and adult-directed vocalizations, for acoustic feature, in each fieldsite (corresponding with the doughnut plots in Fig. 2). The estimates are derived from the random-effect components of the mixed-effects model reported in the main text. Cells of the table are shaded to facilitate the visibility of corpus-wide consistency (or inconsistency): redder cells represent features where infant-directed vocalizations have higher estimates than adult-directed vocalizations and bluer cells represent features with the reverse pattern. Within speech and song, acoustic features are ordered by their degree of cross-cultural regularity; some features showed the same direction of effect in all 21 societies (for example, for speech, median pitch and pitch variability), whereas others were more variable.
Extended Data Fig. 2 Principal-components analysis of acoustic features.
As an alternative approach to the acoustics data, we ran a principal-components analysis on the full 94 acoustic variables, to test whether an unsupervised method also yielded opposing trends in acoustic features across the different vocalization types. It did. The first three components explained 39% of total variability in the acoustic features. Moreover, the clearest differences between vocalization types accorded with the LASSO and mixed-effects modelling (Figs. 1b and 2). The first principal component most strongly differentiated speech and song, overall; the second most strongly differentiated infant-directed song from adult-directed song; and the third most strongly differentiated infant-directed speech from adult-directed speech. The violins indicate kernel density estimations and the boxplots represent the medians (centres), interquartile ranges (bounds of boxes) and 1.5 × IQR (whiskers). Significance values are computed via two-sided Wilcoxon signed-rank tests (n = 1,570 recordings); *p < 0.05, **p < 0.01, ***p < 0.001. Feature loadings are in Supplementary Table 7.
Extended Data Fig. 3 Screenshots from the naive listener experiment.
On each trial, participants heard a randomly selected vocalization from the corpus and were asked to quickly guess to whom the vocalization was directed: an adult or a baby. The experiment used large emoji and was designed to display comparably on desktop computers (a) or tablets/smartphones (b).
Extended Data Fig. 4 Response biases in the naive listener experiment.
a, Listeners showed reliable biases: regardless of whether a vocalization was infant- or adult-directed, the listeners gave speech recordings substantially fewer “baby” responses than expected by chance, and gave song recordings substantially more “baby” responses. The grey points represent average ratings for each of the recordings in the corpus that were used in the experiment (after exclusions, n = 1,138 recordings from the corpus of 1,615), split by speech and song; the orange and blue points indicate the means of each vocalization type; and the horizontal dashed line represents hypothetical chance level of 50%. b, Despite the response biases, within speech and song, the raw data nevertheless showed clear differences between infant-directed and adult-directed vocalizations, that is, by comparing infant-directedness scores within the same voice, across infant-directed and adult-directed vocalizations (visible here in the steep negative slopes of the grey lines). The main text results report only d’ statistics for these data, for simplicity, but the main effects are nonetheless visible here in the raw data. The points indicate average ratings for each recording; the grey lines connecting the points indicate the pairs of vocalizations produced by the same voice; the half-violins are kernel density estimations; the boxplots represent the medians, interquartile ranges and 95% confidence intervals (indicated by the notches); and the horizontal dashed lines indicate the response bias levels (from a).
Extended Data Fig. 5 Response-time analysis of naive listener experiment.
We recorded the response times of participants in their mobile or desktop browsers, using jsPsych (see Methods), and asked whether, when responding correctly, participants more rapidly detected infant-directedness in speech or song. They did not: a mixed-effects regression predicting the difference in response time between infant-directed and adult-directed vocalizations (within speech or song), adjusting hierarchically for fieldsite and world region, yielded no significant differences (ps > .05 from two-sided linear combination tests; no adjustments made for multiple comparisons). The grey points represent average ratings for each of the recordings in the corpus that were used in the experiment (after exclusions, n = 1,138 recordings from the corpus of 1,615), split by speech and song; the grey lines connecting the points indicate the pairs of vocalizations produced by the same participant; the half-violins are kernel density estimations; and the boxplots represent the medians, interquartile ranges and 95% confidence intervals (indicated by the notches).
Supplementary Methods, Results, Figs. 1–4 and Tables 1–7.
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Hilton, C.B., Moser, C.J., Bertolo, M. et al. Acoustic regularities in infant-directed speech and song across cultures. Nat Hum Behav 6, 1545–1556 (2022). https://doi.org/10.1038/s41562-022-01410-x
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