Twelve live Mandarin sessions taught American babies new sounds. TV taught them nothing.

Patricia Kuhl, a researcher who studies how babies learn language, brought Mandarin speakers into her laboratory to spend time with American babies. It was “like having Mandarin relatives come and visit for a month and move into your house,” she said. The adults talked with the babies across 12 sessions.

Kuhl wanted to know whether babies who were starting to focus on English could still absorb the sounds of a completely new language. The answer was yes, and the learning was strikingly fast.

To be sure that simply coming into the laboratory did not explain any gain, her team brought in a control group of babies who came just as often but heard only English. Exposure to English did not improve their Mandarin. But the babies who heard Mandarin for 12 sessions scored as well as babies in Taiwan who had been listening to Mandarin for 10 and a half months. In 12 sessions, they had matched nearly a year of exposure.

Kuhl said what it demonstrated is that “babies take statistics on a new language.” “Whatever you put in front of them, they’ll take statistics on,” she said.

The team then tested whether the person mattered. One new group heard the same amount of Mandarin in the same number of visits, but only on video. Another group heard only the audio, while looking at a teddy bear on a screen. Neither the video group nor the audio group learned anything.

“It takes a human being for babies to take their statistics,” Kuhl said. “The social brain is controlling when the babies are taking their statistics.”

In short, babies learned from a live speaker but not from a screen with the same sounds and time.