Endogenous and social factors influencing infant vocalizations as fitness signals

This dissertation evaluated the role of social and endogenous prelinguistic vocalizations as vocal fitness signals in human development. It consists of three studies. The first investigated the saliency of infant vocal imitation using listener judgments regarding the degree of imitativeness in parent-infant vocal turn pairs. Participating listeners demonstrated moderate to high intra- and inter-rater agreement, suggesting vocal imitation has the potential to be used as a signal of fitness to caregivers in early development. The work also showed that vocal imitation in infancy is rare. The second study sought to quantify the extent to which infants produce vocalizations socially (directed to a caregiver) vs endogenously (not directed to a caregiver) in laboratory settings where parents either attempted to engage them or talked with another adult. The infants produced three times as many vocalizations endogenously as socially in both settings. High rates of endogenously produced sounds may result from evolutionary pressures to signal wellness to caregivers through vocalization. Extensive independent vocal play may offer infants the opportunity to explore sensorimotor characteristics of the vocal system and provide the raw material that parents can use in face-to-face interactions. The third study examined social and endogenous motivations in the emergence of advanced vocal forms. Specifically, it compared canonical babbling ratios of infants at low- and high-risk for autism across high and low levels of both vocal turn-taking and vocal play. Both groups showed a tendency to produce more canonical babbling during high turn-taking and high vocal play. The findings highlight a potentially robust internal social motivation for vocalization, even in the presence of likely social-cognitive differences such as risk for autism. High rates of endogenously produced canonical syllables in high-risk infants support the idea of robust evolutionary pressures for infants to signal fitness through vocalization. Furthermore, differences in vocal production across settings can inform our understanding of the importance of both vocal interaction and independent infant exploration of vocalization. This dissertation offers perspective on the ways in which social and endogenous factors reveal natural selection pressures on fitness signaling in the human infant.