Infant-directed speech

When speaking to infants, adults in all cultures modify their speech to have a higher pitch, greater pitch variability, slower speed, and longer pauses. Our research suggests that young infants' social preferences can be modified by the presence or absence of infant-directed speech. We suggest that infants encode the appropriateness of vocal behavior and use that information to guide subsequent preference for individual social partners, an ability that may serve as an important foundation for social reasoning.

Entrainment in vocal learning species

See sample videos of birds entrained to music.

Whether in a military march, an urban club or a tribal dance ritual, humans spontaneously synchronize their movements to auditory beats, a phenomenon termed entrainment. Other primates do not appear to share this ability. However, our work suggests that entrainment is not a uniquely human capacity, as previously believed. We find strong evidence of entrainment in multiple species of parrot.

To our knowledge, there is no evidence that parrots entrain to sound in the wild. Since entrainment does not exist in their natural behavioral repertoire, the capacity for entrainment could not have been directly selected for by natural selection, but must have emerged as the byproduct of selection for another capacity. Both parrots and humans are proficient vocal learners (able to imitate sound), while non-human primates and most animals in contact with humans are not. This raises the question: Did entrainment emerge as a byproduct of selection for vocal learning? If this were the case, we should only find entrainment in vocal learning animals.

We tested the hypothesis that vocal learning is necessary for entrainment by systematically querying an internet video database for evidence of entrainment in both vocal learning and vocal non-learning animals, sampling a wide variety of birds and mammals. Across thousands of videos and hundreds of species, only vocal learning species showed evidence of entrainment, in spite of the high representation of vocal non-learners in the database and comparable exposure to both humans and music. We conclude that entrainment is not unique to humans, and may have evolved as a by-product of selection for vocal learning.

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