Acoustic Features of Emotional Expression in Preverbal Infant Vocalizations

The musicality of human communication before the onset of words enables mutual interaction from early infancy and lays the foundation for language development. The present study investigated the development of infants' emotional vocalizations across the first year of life, with a specific focus on their acoustic properties. Analyses revealed systematic age-related changes in acoustic features, with pronounced shifts in pitch height, spectral clarity, and formant stability occurring between 3-6 and 6-9 months. These findings mark the second half of the first year as a key milestone in the development of vocal communication. Linear discriminant analysis confirmed improved age classification beyond this stage. Emotional valence was encoded in distinct acoustic profiles, particularly involving spectral energy, pitch, vocal intensity, and resonance. Negative vocalizations, especially in younger infants, exhibited consistently higher pitch and intensity, along with stable acoustic signatures over time, suggesting their function as early, distinct affective signals. Valence-specific developmental trajectories were also identified. While formant frequency changes typically clustered around the 6-month age, an earlier shift was evident in positive vocalizations, adding nuance to general developmental trends. Taken together, these findings challenge the notion of full emotional neutrality in preverbal vocalizations and suggest that emotional valence is partially encoded in stable acoustic parameters. Such cues may complement contextual information in supporting caregivers' interpretation of infants' emotional states. This work contributes to understanding how functional flexibility emerges through the developmental tuning of prosody and further calls for research into whether adults can reliably decode emotion from vocal signals alone, shedding light on the perceptual foundations of early social communication.