There is a critical period for learning language. It’s a narrow window – if it doesn’t happen by a certain point, you won’t learn a language. There have been studies done with birds – if they don’t learn their song at a young age, they won’t learn it. There’s also some case studies, such as Genie, which show that when deprived of language, language will never develop.

Perception precedes production. That is, babies can understand a lot more than they can physically say.

There’s a few interesting studies that show how L1 is discriminated by infants.

Infants are born with physical capabilities to hear and produce bilabial (p, b, m…) syllables in any language. As infants are conditioned, only experiencing sounds from their L1 environment, the sounds not produced in L1 drop off in production.

Did you know that deaf babies babble with their hands?

There is the same order of acquisition for sign language as their is for spoken language.

Babies can also discriminate their language from a second language. By a certain stage (I’m thinking 7 months, but I can’t remember exactly), a baby will direct her attention longer to a person speaking L1, than a person speaking L2. There’s a visual component, too. Without sound, babies will hold their attention lip-reading L1, more than L2.

There’s some non-nativist theories out there, too, such as the social interactionist theories, that state that there is a more of a nurture-component to language development, and the interaction between care givers and children play an important role.

I don’t see these camps as mutually exclusive. It’s the whole nature/nurture argument. I think they all work together.

L2 is a different matter. Are we talking about pre- or post-lateralization?