Without hearing a word, a new study asserts, a four-month-old child can tell when speakers switch to another language, simply by observing changes in facial contortions, such as shapes made by the mouth as well as mannerisms, like the head-bobbing rhythm that varies between different tongues.

It has been well documented since the late 1980s that very young children can discriminate between languages when they are spoken, but researchers wanted to determine if they could also recognize changes based on a speaker's gestures.

Toward that end, researchers from the University of British Columbia (U.B.C.) in Vancouver separated 36 infants into three separate groups of four-, six- and eight-month-olds. They had the babies sit on their mothers' laps and watch bilingual speakers—all women—on a muted television screen read from a children's book in either English or French.

Working first with babies from homes where only English was spoken, researchers began a video with a storyteller who read in one language, then switched to reading in the other tongue when the baby started to lose interest. After the transition, the two youngest sets of infants showed renewed interest, indicating they recognized that something had changed, piquing their curiosity.

"People assume French and English are very similar," says Janet Werker, a professor of psychology and senior author of the study published this week in Science. "In fact, English and French are very dissimilar along many dimensions, such as the rhythm of the language…. We assume that babies are using the visual correlates of some of those differences to distinguish" them.

Although the two younger sets of subjects apparently caught on, the group of eight-month-olds seemed oblivious to the language shift, says lead study author Whitney Weikum, a graduate student in U.B.C.'s Infant Studies Center.

In an attempt to explain this, the team repeated the trials with six- and eight-month-old tots from families in which both English and French were spoken at home. This time, all of them seemed to recognize when the speaker switched between languages.

Weikum and Werker believe that infants may be born with an innate ability to visually discriminate language, but that it fades with time and age if they are only exposed to one. As children grow older they begin to specialize and fine-tune their understanding of that one language, usurping and erasing their ability to recognize others. It is as if, "they stop discriminating what they don't need and start discriminating what they do need," says Werker, who previously found a similar decline in babies' ability to discern subtle sounds not used in their own languages (such as the varying "d" intonations in Hindi). Tots from bilingual homes retain their ability to discern between their own languages, she says, because they are routinely exposed to and, therefore, familiar with both.

Researchers are not positive which visual cues babies pick up, but they are currently tweaking their experiments to try to find out.