It was long believed that babies learned to talk by listening. But new research suggests that youngsters lip-read, a development that could have implications for diagnosing autism earlier.

The findings come from a study published this week in the Proceedings of the National Academy of Sciences looking at how children ages 4 months, 6 months, 8 months, 10 months and 12 months react to speech.

For the study, researchers at Florida Atlantic University monitored eye gaze while the kids watched videos of women speaking in English — the children’s only native language — or Spanish.

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As the children started to babble, the researchers found that they focused on the mouth of the person speaking to them. But months later, as the kids became more comfortable with language, they shifted their attention to the eyes.

This pattern was interrupted, however, when children heard a foreign tongue. Then, they tended to revert back to lip-reading in an attempt to decipher the unfamiliar sounds.

The new understanding of how children acquire language skills could offer a window to identifying kids with autism at younger ages, the researchers said. Previous research has suggested that children with autism focus on the mouth of the person speaking to them at 2-years-old while their typically developing peers focus on the talker’s eyes.

“When these facts are combined with our findings, it is likely that, contrary to typically developing children, infants who are as yet undiagnosed but who are at risk for autism may continue to focus on the mouth of a native-language talker at 12 months of age and beyond,” said David Lewkowicz, a professor of psychology at Florida Atlantic University who led the study.

“If so, this would provide the earliest behavioral confirmation of impending developmental disability and would give clinicians an early start on intervention procedures aimed at lessening or preventing the most devastating effects of autism and other communicative disorders,” he said.