(I blog by sending myself emails when I have an idea.  These emails are stored in a separate folder to think about later.  Some ideas have gathered dust and I am cleaning them out.)

Bilingual kids have better pronunciation than those who acquire a second language as adults.  I have read that this is because of specialization both in the brain and physically in terms of the kinds of sounds we are able to make.  But I bet there is another reason:  adults know how to read.

Take the hard ‘r’ sound in Spanish.  It can be farily well approximated by an English ‘d’ sound which any English speaker has the wiring and hardware to make.  But native Enlish speakers do not mispronounce ‘corazon’ by saying ‘codazon.’  Instead, they say ‘core a zone.’  And the reason is presumably that they have seen that word written out and the association between the written ‘r’ and the familiar sound has been highly reinforced.  This is also a problem with most vowel sounds.

I would bet that adults would more easily learn fluency in another language if they were taught exclusively orally.  Now it would seem that the obvious test case would be Chinese to English and vice versa.  I don’t know but I would guess that it would not be possible to write English words phonetically using written Chinese.  Despite this “advantage,” native Chinese speakers have a hard time with English fluency.  Bad for the theory. But I think that Chinese to English is already too difficult for other reasons to consider this a good test.  First of all the sound palettes are very different.  Second, the rhythms of the languages are very different, even with good pronunciation.

Instead, to hold other factors constant, I would look to the blind.    My guess is that the blind have a weaker association between pronunciation and the written language.  (How much of a role does Braille play when a blind person learns a new language? ) The key prediction then would be that among native English speakers who learn Spanish as adults, the blind are more fluent.