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. 2022 Dec 30;17(12):e0279395.
doi: 10.1371/journal.pone.0279395. eCollection 2022.

Perspectives on the origin of language: Infants vocalize most during independent vocal play but produce their most speech-like vocalizations during turn taking

Helen L Long  1 , Gordon Ramsay  2   3   4 , Ulrike Griebel  5   6   7 , Edina R Bene  6 , Dale D Bowman  5   8 , Megan M Burkhardt-Reed  6 , D Kimbrough Oller  5   6   7
Affiliations

Perspectives on the origin of language: Infants vocalize most during independent vocal play but produce their most speech-like vocalizations during turn taking

Helen L Long et al. PLoS One. .

Abstract

A growing body of research emphasizes both endogenous and social motivations in human vocal development. Our own efforts seek to establish an evolutionary and developmental perspective on the existence and usage of speech-like vocalizations ("protophones") in the first year of life. We evaluated the relative occurrence of protophones in 40 typically developing infants across the second-half year based on longitudinal all-day recordings. Infants showed strong endogenous motivation to vocalize, producing vastly more protophones during independent vocal exploration and play than during vocal turn taking. Both periods of vocal play and periods of turn-taking corresponded to elevated levels of the most advanced protophones (canonical babbling) relative to periods without vocal play or without turn-taking. Notably, periods of turn taking showed even more canonical babbling than periods of vocal play. We conclude that endogenous motivation drives infants' tendencies to explore and display a great number of speech-like vocalizations, but that social interaction drives the production of the most speech-like forms. The results inform our previously published proposal that the human infant has been naturally selected to explore protophone production and that the exploratory inclination in our hominin ancestors formed a foundation for language.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Amounts of VP and TT.
A. The coders indicated that at all three Ages, the amount of VP for the 40 infants was much higher than the amount of TT. In the display, the scale has been converted so that 1 indicates No VP or TT in a coded segment, while 2 indicates Some VP or TT (any coded value from 2–5) occurred within a segment. On average, VP was deemed to have occurred considerably more than half the time during the segments, while TT was deemed to have occurred far less than half the time. The error bars are 95% confidence intervals. B. Zooming in so that mean differences and error bar information are more visible shows that VP occurred more often at 9.5 and 12 mo than at 7.5 mo. C. Zooming in also shows that TT occurred more often at 12 mo than at the younger Ages.
Fig 2
Fig 2. Canonical babbling ratios as a function of VP and TT.
A. CBR was higher in segments judged to have Some VP than No VP. The difference was clear at all three Ages. B. CBR was higher in segments with Some TT than in segments with No TT. The pattern was most notable at 9.5 mo. C. Comparing the data for Some VP and Some TT, the data showed significantly higher CBRs for segments with Some TT than for segments with Some VP. The error bars for all the panels represent 95% confidence intervals.
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