Human adult laughter is characterized by vocal bursts produced predominantly during exhalation, yet apes laugh while exhaling and inhaling. The current study investigated our hypothesis that laughter of human infants changes from laughter similar to that of apes to increasingly resemble that of human adults over early development. We further hypothesized that the more laughter is produced on the exhale, the more positively it is perceived. To test these predictions, novice ( n = 102) and expert (phonetician, n = 15) listeners judged the extent to which human infant laughter ( n = 44) was produced during inhalation or exhalation, and the extent to which they found the laughs pleasant and contagious. Support was found for both hypotheses, which were further confirmed in two pre-registered replication studies. Likely through social learning and the anatomical development of the vocal production system, infants' initial ape-like laughter transforms into laughter similar to that of adult humans over the course of ontogeny.

People make rapid inferences about others' thoughts and intentions. For example, they observe facial movements and pupil size of others and unwittingly make use of this information when deciding whether to trust someone or not. However, whether spontaneous mimicry depends on visual awareness of the stimulus and whether these processes underlie trust decisions is still unknown. To investigate whether visual awareness modulates the relationship between emotional expressions, mimicry and trust, participants played a series of trust games and saw either their partners’ faces with a neutral, happy or fearful expression, or their partners' eyes in which the pupil size was large, medium or small. Subjects' trust investments, facial movements and pupil responses were measured. In half of the trials, the stimuli were rendered invisible by continuous flash suppression. Results showed that facial expressions were mimicked and influenced trust decisions during the conscious condition, but not during the unconscious (suppressed) condition. The opposite was found for pupil size, which influenced trust decisions during states of unawareness. These results suggest that the neurobiological pathway linking the observation of facial expressions to mimicry and trust is predominantly conscious, whereas partner pupil size influences trust primarily when presented unconsciously. This article is part of the theme issue ‘Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience’.

Laughter occurs across all great ape species, yet human laughter differs from that of other primates: Human laughter is primarily produced on the exhale, whereas other primates laugh on both the inhale and exhale. In the current study, we asked whether human infants laugh in a similar manner to apes, given that human infants, like non-human primates, tend to laugh in the context of tickling or rough-and-tumble play. Human adults, in contrast, laugh across many different kinds of social interactions. To test this hypothesis, we examined whether human infant laughter is acoustically more similar to non-human apes' laughter. Laughter clips from infants aged 3 to 18 months were annotated by phoneticians and evaluated by two listener samples (naïve listeners and phoneticians, respectively). The results provide support for the prediction that the proportion of infants’ laughter produced on the exhale increases with age. These results suggest that at younger ages, human infants’ laughter is more similar to that of other great apes. These findings are discussed in the context of vocal control maturation and social learning.