check this website: http://plato.stanford.edu/entries/innateness-language/

For innate capability, newly born infants up to 8 months old are universal listeners, they can distinguish speech sounds that are in their mother tongues and those that are not. For linguistic input, just think of the cases of animal children, or the case of Genie. It seems that nature and nurture both play inseparable roles in the acquisition of language, whether it is L1 or L2. 

Thanks for your sharing Matthew and Feng Teng, it is very helping.

Regards 

Hello,

If I was you, I'd try to investigate the competence of prosodic recognition in SLA - I've read in Di Cristo that for 1st language acquisition children can recognize several types of languages with the prosody - maybe it could be interesting to investigate that for SLA...

And... I think that you should define exactly what is innate : biological ? 1st day we are inside our mother ? 1st day we see the light of the world ? etc.

hi

children brought up in orphanages, ''wolf children'', or children under any circumstance of input derivation  have shown faulty linguistic comprehension and production , hence the significance of input . As Mathew stated , the interplay and confrontation of '' nature vs nurture '' is a perpetuated and probably,  as Edi pointed out, a combination of the two approaches could be safest and most revealing .

A useful reference, and also textbook for many graduate students in many universities, is Ortega, L. (2009). Understanding Second Language Acquisition.  Routledge. I think Chapter 2 Age is especially helpful concerning the question in your post.