The development of play face mimicry: comparative insights from human infants, adults and orangutans.

Laughter and its associated facial expression, the play face, are critically important in the social lives of animals. Perhaps owing to this, play faces are mimicked in social interactions across many species. Despite growing interest, few works have directly compared play face mimicry across species and age groups within the same experimental framework, limiting our understanding of its developmental and evolutionary origins. In this study, we examined play face mimicry in human infants (n = 63), human adults (n = 36) and orangutans (Pongo pygmaeus; n = 6), using a tightly controlled design. We found that both human infants and adults reliably mimicked play faces, however, infants produced these robustly more than adults. Additionally, while adult humans did not produce as many play faces as infants, they produced reliably more smiles in response to the play face stimuli. When considering the orangutans, we found no evidence of play face mimicry, with only one instance observed across the entire experiment. This suggests that while our paradigm reliably induced mimicry across both human groups, our set-up may have been suboptimal to evoke mimicry responses in orangutans. This article is part of the theme issue 'Mechanisms, development, phylogeny and functions of emotional expressions'.