Speech-like sounds dominate the human infant landscape

Abstract

Early human infant vocalization is viewed as forming not only a critical foundation for vocal learning of language, but also a crucial realm of communication affecting emotional and social development. Although speech-like sounds are rare or absent in other ape infants, they share distress sounds (shrieks and cries) and laughter with humans, forming a potential basis for especially informative cross-species comparisons as well as potential insights regarding usage and learning of vocal sounds. A fundamental need to make such comparisons possible is empirical research to document frequency of occurrence of vocalizations of various types in natural environments. The present work focuses on laughter in the human infant, a topic that has been viewed by many as a key factor in social development for humans and other apes. Yet we know of no research quantifying frequency of occurrence of human infant laughter in natural environments across the first year. In the past two decades it has been shown that the predominant vocalizations of the human infant are "protophones", the precursor sounds to speech. Longitudinal research has indicated unambiguously that protophones outnumber cries by a factor of at least five based on data from random-sampling of all-day recordings across the whole first year. The present work expands on the prior reports by reporting data showing that human infant laughter occurs even more rarely than cry in all-day recordings. Yet laughter is clearly a salient and important aspect of social development. We reason about the dominance of protophones in the infant vocal landscape in light of their role in illuminating human vocal learning and the origin of language.