Human neonates prefer listening to speech compared to many nonspeech sounds, suggesting that humans are born with a bias for speech. However, neonates' preference may derive from properties of speech that are not unique but instead are shared with the vocalizations of other species. To test this, thirty neonates and sixteen 3-month-olds were presented with nonsense speech and rhesus monkey vocalizations. Neonates showed no preference for speech over rhesus vocalizations but showed a preference for both these sounds over synthetic sounds. In contrast, 3-month-olds preferred speech to rhesus vocalizations. Neonates' initial biases minimally include speech and monkey vocalizations. These listening preferences are sharpened over 3 months, yielding a species-specific preference for speech, paralleling findings on infant face perception.

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Humans often produce vocalizations for infants that differ from vocalizations for adults. Is this property common across societies? The forms of infant-directed vocalizations may be shaped by their function in parent-infant communication. If so, infant-directed song and speech should be differentiable from adult-directed song and speech on the basis of their acoustic features, and this property should be relatively invariant across cultures. To test this hypothesis, we built a corpus of 1,614 recordings of infant-and adult-directed singing and speech produced by 411 people living in 21 urban, rural, and small-scale societies. We studied the corpus in a massive online experiment and in a series of acoustic analyses. Naïve listeners (N = 13,218) reliably identified infant-directed vocalizations as infant-directed, and adult-directed speech (but not songs) as adult-directed, at rates far higher than chance. Ratings of infant-directed song were the most accurate and the most consistent across all societies; infant-directed speech was accurately identified on average, but inconsistently across societies. To determine the mechanisms underlying these results, we extracted many acoustic features from each recording and identified those that most reliably characterize infant-directed song and speech across cultures, via preregistered exploratory-confirmatory analyses and machine classification. The features distinguishing infant-and adult-directed song and speech concerned pitch, rhythmic, phonetic, and timbral attributes; a hypothesis-free classifier with cross-validation across societies reliably identified all vocalization types, with highest accuracy for infant-directed song. Last, we isolated 12 acoustic features that were predictive of perceived infant-directedness; of these, two pitch attributes (median F0 and its variability) were by far the most explanatory. These findings demonstrate cross-cultural regularities in infant-directed vocalizations that are suggestive of universality; moreover, infant-directed song appears to be more cross-culturally stereotyped than infant-directed speech, informing hypotheses of the functions and evolution of both.

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Young infants’ successful performance on false belief tasks has led several researchers to argue that there may be a core knowledge system for representing the beliefs of other agents, emerging early in human development and constraining automatic belief processing into adulthood. One way to investigate this purported core belief representation system is to examine whether non-human primates share such a system. Although non-human primates have historically performed poorly on false belief tasks that require executive function capacities, little work has explored how primates perform on more automatic measures of belief processing. To get at this issue, we modified Kovács et al. (2010)’s test of automatic belief representation to examine whether one non-human primate species—the rhesus macaque (Macaca mulatta)—is automatically influenced by another agent’s beliefs when tracking an object’s location. Monkeys saw an event in which a human agent watched an apple move back and forth between two boxes and an outcome in which one box was revealed to be empty. By occluding segments of the apple’s movement from either the monkey or the agent, we manipulated both the monkeys’ belief (true or false) and agent’s belief (true or false) about the final location of the apple. We found that monkeys looked longer at events that violated their own beliefs than at events that were consistent with their beliefs. In contrast to human infants, however, monkeys’ expectations were not influenced by another agent’s beliefs, suggesting that belief representation may be an aspect of core knowledge unique to humans.