So, in the first year of life there may not be sentences coming out of the child.
There are often words, but even before words, there's babbling.
So before we get to the point where we recognize that children are using language more or less like adults do, there's actually a lot going on in the child's mind that's building the foundation for language production.
Within that first year of life... for hearing children, for hearing families, there's evidence that even before they are born, they're already learning things about their target language.
So they can hear in utero, before they're born, and while they may not hear specific sounds very clearly, they can certainly hear the intonational patterns of their language.
That means that by the time they're born, they've already been exposed to some language, or languages, that the mother uses...
...and they have internalized these intonational patterns. There have been clever experiments where you can test newborns, a few days old, and you can find if they can distinguish between their native language, and a language that has very different rhythmic properties.
They do this on the basis of prosody, on rhythm, on stress, how the shape of the overall utterance is.
So they can do this at a very early stage and that helps them in their subsequent language acquisition.
So even before they're born, they're already acquiring language, if they have access to it.
A deaf child may not have access to the spoken language that the mother is using, if the parents are hearing, and even if the parents are signing, we don't know what kind of linguistic information they could have access to before they're born.
But once they are born and you start exposing them to the language around them, whether it be spoken language or sign language, they are very well equiped at that very young age to already be looking for patterns in the input they have.
A deaf child is going to be looking at the sign language he's being exposed to, or the hearing child to the spoken language they're exposed to, and they're already analyzing that and finding patterns of their language.
Being able to identify those patterns helps them set up a phonological system, a system of sounds or the system of hand shapes or movements, the components that make up a sign or a word.
And having that knowledge in place is what allows them to then move on to the part where they are speaking, where they are making noises themselves, or making signs themselves, and each part builds on the previous part.
If you take away that foundation that's normally built in that first year, then the child arrives at a point where they're trying to do more advanced development, but without a strong foundation.
This has repercussions well into childhood, into learning languages.
So it's not just, we can afford to wait until the child's starting to speak, for the second year, and then if things aren't working out we'll expose them to sign language.
By then you've already missed out on a year, and a very crucial year for their later success in language learning.