I completely agree zmjezhd.
Yes, I think we've reached some kind of consensus.
Any adult who studies for long enough will learn the language. And if what you say is true that "these people [who succeed] are few and far between", it is due not to a physical impediment in the brain, but insufficient determination.
Well, here we disagree slightly. There does seem to be something physical going on. That humans before a certain age learn language with what we call native fluency in a different manner than humans past a certain age suggest to me that something has changed. And, yes, I was ambiguous in one of my statements. The people who are "few and far between" are those who've slogged through second language acquisition and succeeded. I did not mean that people physically capable of the arduous work were few. Ostensibly ever adult human can learn a second language, short of some brain-damaged individuals, I suppose.
In other words, few succeed because few are prepared to make the effort required to succeed; not because few can succeed.
Yes.
The marked difference in difficulty, together with my observation that rote learning a second language as an adult is worse than useless but that mnemonics are very effective, was what prompted me to wonder about possible differences in brain dynamics.
I distinguish between normal first language acquisition and second language study. If by rote learning, you mean simply memorizing a written grammar of the language, some vocabulary, and sample sentence patterns, then I agree with you, it's not enough. But it seems to me that using mnemonics is just rote learning done better. I wonder if any studies have been made about second language acquisition in people with so-called eidetic memories.
Wikipedia has some interesting information on that question.
Yes, I was looking at them when you'd posted. Lenneberg was the fellow I was thinking of. His 1967 book is a classic. I merely skimmed it 30 or so years ago. Pinker is probably the most accessible.
At this late juncture, I'd like to say that most of my linguistic studies have been focused on historical-comparative linguistics, and I've only taken some survey courses and done light reading in the biology of language and applied linguistics.