Nonlinear acoustic phenomena affect the perception of pain in human baby cries.

What makes the painful cries of human babies so difficult to ignore? Vocal traits known as 'nonlinear phenomena' are prime candidates. These acoustic irregularities are common in babies' cries and are typically associated with high levels of distress or pain. Despite the vital importance of cries for a baby's survival, how these nonlinear phenomena drive pain perception in adult listeners has not previously been systematically investigated. Here, by combining acoustic analyses of cries recorded in different contexts with playback experiments using natural and synthetic cries, we show that baby cries expressing acute pain are characterized by a pronounced presence of different nonlinear phenomena, and that these nonlinear phenomena drive pain evaluation by adult listeners. While adult listeners rated all cries presenting any of these nonlinear phenomena as expressing more pain, they were particularly sensitive to the presence of chaos. Our results thus show that nonlinear phenomena, especially chaos, encode pain information in baby cries and may be critically helpful for the development of vocal-based tools for monitoring babies' needs in the context of paediatric care.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.