Babies can see the differences between languages, even without hearing them.Babies as young as four months can tell their native language from a foreign tongue—even if they can't hear it. This according to a study led by psychologists Whitney Weikum and Janet Werker of the University of British Columbia in Canada. They showed babies silent videos of bilingual adults, speaking alternatively in English and French.

WHITNEY WEIKUM (University of British Columbia):
So we found that at four and six months, babies from the home where only English is spoken can tell the difference between the languages. But their ability to tell the difference between the languages declined by eight months of age.