Babies learn their first language from the people around them. Scientists wanted to know whose voices babies like to hear. They played recordings to babies in a village in Peru and in a city in Switzerland.

Some recordings had young children talking together. Others had adults talking together. In both places, the babies listened longer to the children. Maybe older brothers and sisters are important language teachers, too.

Most research on how babies learn language has focused on adults speaking to them. But in many cultures, babies spend a lot of time around other children. Researchers from the University of Zurich tested 127 babies aged 8 to 20 months. Of these, 67 were from Shipibo-Konibo villages in the Peruvian Amazon and 60 were from Zurich, Switzerland.

The babies heard natural recordings of young children talking among themselves and of adults talking among themselves. In both cultures, the babies paid attention longer to the children's conversations. Attention is not proof of learning, but the researchers call it a first step. In a second study, Swiss babies listened just as long to children speaking to a baby as to adults using baby talk.

For decades, studies of early language development have centered on child-directed speech from adults, the slow, high-pitched style that infants are known to prefer. But in many societies, infants hear much less adult baby talk but are exposed to a lot of speech from other children. A 2025 study in Developmental Science examined whether this overlooked input matters. Using a head-turn preference procedure, researchers measured how long 127 infants aged 8 to 20 months listened to recordings of young children chatting among themselves versus adults chatting among themselves. Both Shipibo-Konibo infants in the Peruvian Amazon, who had nearly four siblings on average, and Swiss infants from Zurich, with fewer than one, attended longer to the children's speech, with no meaningful cultural difference.

A follow-up with 61 Swiss infants found that children addressing a baby were as engaging as adults using baby talk. Higher pitch may partly explain the effect, but the authors also point to infants' social interest in older peers. The study measured only how long babies listened, and the follow-up included only Swiss infants, so whether this attention helps vocabulary grow was not tested.