Social and non-social functions of infant vocalizations

Research on infant vocal development is focused primarily on vocal interaction with caregivers, where it appears to be largely assumed that infants vocalize mostly for the purpose of interaction. A survey of both parents and non-parents indicated that public opinion conformed to the expectation that infant vocalization is mostly socially interactive. However, we report that in laboratory recordings of infants and their parents, the bulk of infant speech-like vocalizations (“protophones”) were directed toward no one, and instead appeared to be generated endogenously in exploration of vocal abilities. The tendency to produce protophones without directing them to others occurred both during periods when parents were instructed to interact with their infants and during periods when parents were occupied with an interviewer, with the infants in the room. The results emphasize the infant as an agent in vocal learning, not as a passive recipient of vocal input.


Introduction
84 where parents were in the same room and silent or not present in the room at all, suggesting that 85 infants had an "independent inclination to vocalize spontaneously" (p. 481) (36) in the absence 86 of social interaction. Importantly, the rate of protophone production has been reported to be very 87 high, >4 protophones per minute during all-day audio recordings, across the entire first year, and 88 even when infants were judged to be alone in a room, the rate was >3 per minute (37).
89 These findings suggest vocalizations are commonly produced with non-social functions. In other 90 words, infants in these prior studies appear to have been intrinsically motivated to explore or 91 practice sounds, in essence to play with sensorimotor aspects of sound production, although the 92 evidence has been indirect. We propose that this vocal exploration may have a deeply significant 93 role in vocal development, alongside the support of caregiver interaction and ambient language 94 exposure. 95 In spite of the possible importance of exploratory vocalizations in language development, to our 96 knowledge there is no published evidence specifically targeting the social-directivity of infant 97 protophones or the lack of it. As noted above, existing evidence about social-directivity of infant 98 protophones is indirect. The necessary work requires considering gaze direction during infant 99 vocalization and the extent to which infants may bid for attention vocally even when they are not 100 in the same room with caregivers. It also requires taking into account the relative timing of infant 101 and caregiver utterances as well as the content of utterances of adults who are present at the time 102 of the recording, especially caregivers who presumably know a good deal about the capabilities 103 of a particular infant. Only with such work will it be possible to reliably quantify proportions of 128 directivity and without social directivity based solely on a reflection of their own experiences 129 around infants. In this study, we hypothesize that:  187 but one infant were exposed to an English-only speaking environment (Infant 6 was exposed to 188 English and Ukrainian at home). Parents were asked to speak English and no other language 189 during the laboratory recordings. Criteria for inclusion of infant participants included a lack of 190 impairments of hearing, vision, language, or other developmental disorders. Demographics and 191 recording ages for each infant at each recording session are provided in Table 2.
192 Interactive or Non-Interactive segments of time). These segments were then collated into a single 218 circumstance at each age for each infant to ensure all segments of the recordings were accurately 219 portrayed for analysis of the vocalization data. The amount of time pertaining to each varied 220 substantially, including two segments that included so few utterances (< 5) we did not include 221 them in the analyses (see Table 3).
308 All the data of the present study were coded for illocutionary force (from which socially-and 309 non-socially-directed categories could be derived) by the first author, and approximately 30% of 310 the total data set was coded independently for illocutionary force by the agreement coder. An 311 original coding of gaze direction had been done on three of the six infants by a previous team of 312 coders for the paper previously cited (34). This completely independent prior coding on half of 313 the data for the present study was available to offer an agreement check on the coding done for 314 the present paper. 410 Even though social directedness as determined by gaze-direction did not correspond for as many 411 individual protophones as the illocutionary judgments of social directedness, the overall 412 percentages of non-socially-directed protophones was notably similar for both methods. That is, 413 the great majority of infant protophones were judged to be produced with gaze directed 414 somewhere other than towards any person in the room, just as the illocutionary judgments found 415 the great majority of infant protophones to be non-socially directed. 72% of the infant 416 protophones were deemed not to include person-directed gaze, and 75% were deemed non-417 socially directed by illocutionary coding.

4 Discussion
419 Overall, infants used about three times as many non-socially-directed protophones as socially-420 directed ones. This predominance remained stable across the three ages. Furthermore, even in the 421 Interactive circumstance, where parents had been instructed to engage with their infants, non-511 The production of protophones in infancy at the beginning of the communicative split between 512 ancient hominins and their ape relatives, perhaps millions of years ago, seems likely to have laid 513 a foundation for a more extensive use of vocalization as a fitness signal later in life, for example, 514 in mating or in alliance formation (57). And as the amount of protophone-like vocalization 515 became more well-established in the hominin line, it surely provided a foundation for more 516 elaborate uses of vocalization, ratcheting from simple fitness signaling toward more and more 517 language-like uses (48).
518 Play is widely recognized as a theater for practice of the behaviors young mammals will need as 519 they proceed through life (67,68). But it is important to note that playful behavior can serve not 520 only as practice, but also as a fitness signal for the altricial young of many species. Our 521 suggestion is that protophones can be seen (in the substantial majority of cases) as playful 522 indicators of well-being, but they would seem to contribute at the same time to a sort of 523 preparation for the future in mating, in alliance formation, and ultimately in the development of 524 language.