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When infants talk, infants listen: pre-babbling infants prefer listening to speech with infant vocal properties.

To learn to produce speech, infants must effectively monitor and assess their own speech output. Yet very little is known about how infants perceive speech produced by an infant, which has higher voice pitch and formant frequencies compared to adult or child speech. Here, we tested whether pre-babbling infants (at 4-6 months) prefer listening to vowel sounds with infant vocal properties over vowel sounds with adult vocal properties. A listening preference favoring infant vowels may derive from their higher voice pitch, which has been shown to attract infant attention in infant-directed speech (IDS). In addition, infants' nascent articulatory abilities may induce a bias favoring infant speech given that 4- to 6-month-olds are beginning to produce vowel sounds. We created infant and adult /i/ ('ee') vowels using a production-based synthesizer that simulates the act of speaking in talkers at different ages and then tested infants across four experiments using a sequential preferential listening task. The findings provide the first evidence that infants preferentially attend to vowel sounds with infant voice pitch and/or formants over vowel sounds with no infant-like vocal properties, supporting the view that infants' production abilities influence how they process infant speech. The findings with respect to voice pitch also reveal parallels between IDS and infant speech, raising new questions about the role of this speech register in infant development. Research exploring the underpinnings and impact of this perceptual bias can expand our understanding of infant language development.