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.

People often believe that orderly structures were created by agents. We examine the cognitive basis of this tendency, asking if learned associations or causal reasoning drives us to link order with agents. Causal reasoning predicts that knowledge of an alternative physical-mechanical cause should ‘explain away’ orderliness, weakening the link with agents. In a preregistered experiment, we manipulated the context to provide (or not provide) a physical-mechanical explanation for orderly outcomes, and participants judged if an object or agent had been present. We compared outcomes differing in (a)levels of orderliness and (b)whether context provided an alternative explanation. We found that environmental context ‘explained away’ orderliness, such that participants observing order inferred agency only when there was no alternative explanation. The link between order and agents is moderated by causal reasoning, and is malleable: It can be weakened by understanding alternative causal mechanisms by which order could arise.

People often believe that orderly structures were created by agents. We examine the cognitive basis of this tendency, and its malleability, focusing on orderly musical sounds as a case of interest. If simple associations mediate the link between order and agents, then detection of orderly stimuli should lead people to infer the presence of agents, independent of other factors. If causal reasoning mediates the link, then an alternative physical explanation for order should explain away its link with agents. We presented participants with orderly or disorderly outcomes in the form of musical sound sequences, and manipulated whether there was an alternative physical explanation for each outcome. Participants saw a scene involving a staircase-like xylophone, where a ball rolling downhill would produce an orderly descending scale. This provided an alternative physical explanation for how this orderly outcome could be generated. We found that when this scene was shown, then occluded, people did not infer the presence of an agent after hearing the orderly descending scale. In contrast, people expected an agent after hearing a scrambled sequence of tones, which could not be produced by gravity in this context (Exp. 1). When we subtly changed the scene by shuffling the xylophone’s bars, so gravity would not create an orderly outcome, participants readily inferred the presence of an agent from the same orderly stimulus. When we controlled for the presence of an alternative explanation, and manipulated orderliness, orderliness had no effect (Exp. 2). Across both experiments, alternative causal explanations flipped participants’ judgments such that disorder, and not order, led people to infer the presence of an agent. These findings show that the link between order and agents is mediated by causal reasoning, and can be weakened by understanding an alternative mechanism by which order could arise.

Dance is a universal human behavior and a crucial component of human musicality. When and how does the motivation and tendency to move to music develop? How does this behavior change as a process of maturation and learning? We characterize infants’ earliest dance behavior, leveraging parents’ extensive at-home observations of their children. Parents of infants aged 0–24 months (N = 278, 82.7% White, 84.5% in the United States, 46.0% of household incomes ≥$100,000) were surveyed regarding their child’s current and earliest dance behavior (movement by the child, during music, that the parent considered dance), motor development, and their own infant-directed dance. We found that dance begins early: 90% of infants produced recognizable dance by 12.8 months, and the age of onset was not solely a function of motor development. Infants who produced dance did so often, on average almost every day. We also found that dance shows qualitative developmental change over the first 2 years, rather than remaining stable. With motor development, age, and more time dancing, infants used a greater variety of movements in dance, and began to incorporate learned, imitated gestures (80% of infants by 17.9 months). 99.8% of parents reported dancing for or with their infants, raising questions about the role of infant-directed dance. These findings provide evidence that the motivation and tendency to move to music appears extremely early and that both learning and maturation lead to qualitative change in dance behavior during the first 2 years, informing broad questions about the origins of human musicality.

In 2020, millions of children shifted to using video chat for core aspects of education and social interaction. While video chat allows for genuine social interaction-in which the partner can see and hear you-affordances in other modalities are limited (e.g., touch). Do children understand the nuanced affordances and limitations of video chat? Here we provide evidence of these abilities at preschool age. Prior to COVID-19, we conducted an experiment with 4-year-old children (N = 44). Each child was introduced to people over video chat, in person, and in a photograph. Children judged whether each person could see, hear, feel, and physically interact with them. We found that children made nuanced judgments about the affordances of video chat, judging that a person over video chat can see, but cannot feel a touch nor physically interact through the screen. Children's answers about hearing were divided, with children answering that the person over video chat could hear more often than for a photograph, but less often than for in-person interaction. Overall, by age four, children understand that video chat has a mixture of life-like affordances and picture-like limitations, showing the presence of a necessary cognitive prerequisite to effective use of video chat in early education.