He and doctoral student Amy Hansen-Tift tested nearly 180 babies, groups of them at ages 4, 6, 8, 10 and 12 months.
How? They showed videos of a woman speaking in English or Spanish to babies of English speakers. A gadget mounted on a soft headband tracked where each baby was focusing his or her gaze and for how long.
They found a dramatic shift in attention: When the speaker used English, the 4-month-olds gazed mostly into her eyes. The 6-month-olds spent equal amounts of time looking at the eyes and the mouth. The 8- and 10-month-olds studied mostly the mouth.
At 12 months, attention started shifting back toward the speaker's eyes.
It makes sense that at 6 months, babies begin observing lip movement, Lewkowicz says, because that's about the time babies' brains gain the ability to control their attention rather than automatically look toward noise.
But what happened when these babies accustomed to English heard Spanish? The 12-month-olds studied the mouth longer, just like younger babies. They needed the extra information to decipher the unfamiliar sounds.
That fits with research into bilingualism that shows babies' brains fine-tune themselves to start distinguishing the sounds of their native language over other languages in the first year of life. That's one reason it's easier for babies to become bilingual than older children or adults.
Image: Happy baby girl, via Shutterstock