Perhaps their native languages were phoneme-rich and could map those to their L2?

> people who've learned their second language as an adult and can speak nearly accent free

Perhaps it's just a capacity that some people have, similar to perfect pitch hearing. Possible, but rare.

Funny that you mention perfect-pitch because in a similar vein it seems like one of those things that can be taught but the window closes very early in childhood.

https://journals.sagepub.com/doi/abs/10.1177/030573561246394...

I was gonna bring this relation of also being a skill needing to be developed in early age, but you beat me to it.

good thing there's already a drug for that https://www.npr.org/2014/01/04/259552442/want-perfect-pitch-...

science is never fixed is it

> To take your point farther, I've met plenty of people who've learned their second language as an adult and can speak nearly accent free.

I've seen some instances of this, for instance Russians learning French.

My guess is that it's possible if the native language phonemes are a superset of the second language.

Russian doesn't have the guttural 'r' of French.

It’s not accent free, you learn the accent when you learn the language.

>As far as I can tell, the only difference between those with and without an accent is conscious effort.

In my completely unscientific sampling, I'm going to agree with you. This holds true too for native speakers who make no effort to improve their grammar and vocabulary. If one wants to be a better speaker, writer, and communicator, they will be by effort.

Yes, this is the real answer. The claims being made are not as articulate as they should be. You could perhaps say “before 12 a person can pick up an accent for ‘free’” but after twelve its certainly possible to pick up correct accent but it requires conscious effort, which many people are not interested in after they get their spoken L2 to the “good enough” point.

Yeah I don’t think this true at all. I spent six years in a foreign country with very different phonemes and, while I didn’t develop a very big vocabulary, I was complimented on my clear pronunciation for what I could say.