UCLA

Laughter is a universal human behavior that likely evolved from a play vocalization in the common ancestor of great apes. Comparative research on play signals, their associated emotions, and their cognitive effects can shed light on laughter’s origins and on human sociality more broadly. This dissertation investigates the evolutionary roots of play and laughter through theoretical and empirical studies in humans and our primate relatives. Chapter 1 provides a general introduction to the evolution of play and laughter and their significance for our understanding of human emotion, cognition, and social bonding. I highlight the importance of social bonds in research I conducted on the development of sex differences in play in white-faced capuchins and on playful teasing behaviors in apes. Chapter 2 is a literature review synthesizing research on laughter and play vocalizations across the animal kingdom, adding support to the theory that human laughter likely evolved from a pant-like play signal. I present a comprehensive review of species that vocalize during play, along with acoustic descriptions of the calls. I also discuss potentially unique aspects of human laughter and implications for the evolution of language. Chapters 3 and 4 describe a set of cognitive bias experiments I conducted with humans and one of our closest ape relatives, the bonobos. I tested whether hearing laughter leads to optimism bias in judgements of ambiguous stimuli, which would indicate a positive affective response. The study in chapter 3 found that bonobos tended to approach ambiguous stimuli more often after hearing unfamiliar conspecific laughter. However, the study in chapter 4 did not find evidence that humans show greater optimism bias or more positive self-reported feelings after hearing infant laughter. In chapter 5, I conclude with a general discussion of these findings and future directions. While only apes produce human-like laughter, many animals have play vocalizations that appear to involve positive emotions. Laughter perception and contagion likely include ancient emotional mechanisms, and perhaps some derived ones. Further comparative research can illuminate the extent to which emotional and behavioral contagion underpin play vocalizations across species and the unique psychosocial mechanisms associated with human laughter. Overall, this dissertation grounds the study of laughter in evolutionary theory and research on animal play, using an interdisciplinary approach to tackle both proximate and ultimate explanations for this ubiquitous behavior.