A couple of recent news stories describe recent work on how children learn language. One study looked at bilingual babies and monolingual babies and found somewhat different learning sequences in each case. In a laboratory setting, the monolingual babies who were learning English were able to distinguish two similar-sounding words a few months earlier than bilingual babies who were learning English and another language. Bilingual babies tend to reach the major milestones in language learning at about the same age as monolingual babies, so the relative delay evidently doesn’t slow them down. In fact, researchers think it’s helpful for the bilingual babies to focus for longer on associating words with the right objects without worrying about the finer details of how the words sound, because they’ve got twice as many words to associate with objects. This article from Science Daily has the details.
Another study examined the way that toddlers learn to tune out speech sounds that don’t occur in the language they’re learning. Babies are born with the ability to distinguish the finer phonological distinctions found in any human language, but as they grow up they retain only a sensitivity toward the sounds found in their own language. They must know the sounds of their native language so well that they can hear them pronounced in markedly different accents without thinking they are different sounds–for example, to know that a short sharp “Hey!” from a person you accidentally bump into is the same as the delighted, drawn-out “Heeeeyyyy!” from a friend who’s happy to see you.
Evidently children start learning these things at a very young age. An experiment with 18-month-old toddlers compared English learners and Dutch learners as they were taught made-up names for toys. The English learners didn’t distinguish between two names that differed only in the length of the vowel sound (as in the “Hey” example above), because they’ve figured out that in English, a vowel can differ in that respect without signifying something different. The Dutch babies, on the other hand, noted the difference and learned the two distinct words; in Dutch, the length of the vowel does affect meaning. The English learners were able to distinguish two words with vowels that differed in a way that conveys meaning in English (“hey” versus “who”, to follow up on my example above, although the babies were actually learning nonsense words). You can read more about it in this story from Science Daily.
