>What is behind this contradiction? If language is as instinctive to humans as dam-building is to beavers, if the design of syntax is coded in our DNA, why does the average American sound like a gibbering fool every time he opens his mouth or puts pen to paper?
Because just having language faculties doesn't necessarily make you smart or informed or well-read, nor it makes you someone that pays attention to what they say and how they say it.
Sex is an instinct too -- but how many do it badly?
That's my point too. Nature has a different definition of good language too. A human ape barely being able to communicate is enough, eloquence is not necessary.
>Hmmm, I suppose it depends on whether 'eloquence' contributes to intent / being understood.
It might, but it probably goes beyond the mere "understood", so it's more cultural than necessary for evolution beyond a point.
(Same with sex though: as parent commenter noted, being good at it is not any great evolutionary advantage. Just being able to do it to reproduce is enough).
Because just having language faculties doesn't necessarily make you smart or informed or well-read, nor it makes you someone that pays attention to what they say and how they say it.
Sex is an instinct too -- but how many do it badly?