Infants Learn Better When Listening to Human Speech—or Lemurs

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New experiments suggest infants narrow their attention to focus on humans not because they’re human, but because humans are what they know.

By Nathan Collins


(Photo: Wikimedia Commons)

Babies are born knowing very little about the world or what to pay attention to—they’re not blank slates, but they’re not exactly full ones either. A good example is faces: When they’re just out of the womb, infants will pay about equal attention to human faces as they would to other primates’ faces. Over time, babies learn to look more at humans. Why is that? Is there something innate to us that eventually draws our attention to our fellow human beings.

Probably not, according to new experiments—it’s just that babies see and hear many more people than they do lemurs.

The new research, from Northwestern University psychologists Danielle Perszyk and Sandra Waxman, builds on a series of experiments in the last few years that’s investigated the role of speech in cognitive development. In a 2010 study, the researchers found that playing a recording of human speech to infants while they looked at new objects helped the babies learn object categories like “dinosaur” and “fish.” But a 2013 follow-up study produced their most interesting result: Until infants are around five or six months old, listening to a lemur while getting familiar with a set of objects accomplishes exactly the same thing. The dinosaur/fish task is sort of incidental here. What’s intriguing is that, when babies are very young, they don’t differentiate between babbling humans and babbling lemurs. Only later on do they make the distinction.

“Experience plays a vital role as infants specify which signals, from a broad set of possibilities, they will harness.”

The question, Perszyk and Waxman write, is why. Their hunch: We don’t see and hear lemurs nearly as much as we do humans, so we end up just ignoring their vocalizations. The end result is that only human speech, and not lemur sounds, influences our thinking.

To test that hypothesis, the researchers needed to vary how much exposure young children have to humans and lemurs. To do so, they gathered a total of 28 children (aged seven and a half months on average) and played them 10 minutes of a Mozart piano concerto interspersed with lemur vocalizations. Next, each infant went through the same procedure as in Waxman and her colleagues’ 2013 experiment: They listened to lemur sounds while getting familiar with a set of objects, such as pictures of dinosaurs. Finally, the babies were shown both a familiar and an unfamiliar object—a stegosaurus and a trout, for example.

During that last phase, researchers tracked which objects the kids looked at, helping them figure out whether the children have familiarized themselves with the idea of a dinosaur (or whatever the objects were). In particular, if babies look at the trout more than the dinosaur, it’s a sign they’ve developed an idea of what a dinosaur is, and trout are something new.

So what happened? When primed with lemur sounds before becoming familiar with dinosaurs (or another category of objects), infants spent a bit more time—57 percent—looking at the fish (or another novel object).

In a second experiment, the researchers habituated children to lemur sounds over six weeks, then had them come back into the laboratory for the learning and test phases of the experiment. The results were the same. Apparently, the six-month-olds in the researchers’ earlier experiment hadn’t grown out of paying attention to monkeys—they just hadn’t been exposed to them enough to warrant their attention.

The results suggest “that experience plays a vital role as infants specify which signals, from a broad set of possibilities, they will harness” for learning, Perszyk and Waxman write in Cognition. In other words, it’s not that the sounds are human. At least partly, it’s that the sounds are familiar.