The basic argument for the innateness of
language is
poverty of stimulus. This
contends that our general
human intelligence and the
samples of a given language fed to us in the first five or so years of
life just
aren't good enough to produce the
linguistic competence of your average five-year-old. In order to understand the
input, the
infant must have some tendency to look for the kinds of
patterns that appear in language,
at the very least.
A five-year-old's command of language, if it's just another complex, patterned input for babies and small children to take on board, is a pretty good trick. As unlikely as a child playing Handel's Messiah on the piano after hearing a couple of CDs of unrelated music. And this is almost every child, whatever their level of general intelligence. Put like that, the mysterious language instinct is the least unlikely explanation.