There’s motivation and then there’s motivation. Children are too young to respond to the requirement incorrectly enough to completely fail at it. If your System-2 must learn a new language, but you can still System-1 speak your native tongue within family or with friends, it is a different situation from where a child has no way of communicating at all, and communication is not just an instrumental requirement, it is a powerful instinct. Of course, some older adults will have damaged learning ability and won’t be able to learn a new language, but it’s an entirely different issue.
You’re saying all adults could learn a new language and become fluent if the motivation was on the level of “not being able to communicate with anyone at all”. Is there hard evidence of this? In particular, once such people achieve basic communication, do they reliably move on to fluency?
There’s motivation and then there’s motivation. Children are too young to respond to the requirement incorrectly enough to completely fail at it. If your System-2 must learn a new language, but you can still System-1 speak your native tongue within family or with friends, it is a different situation from where a child has no way of communicating at all, and communication is not just an instrumental requirement, it is a powerful instinct. Of course, some older adults will have damaged learning ability and won’t be able to learn a new language, but it’s an entirely different issue.
You’re saying all adults could learn a new language and become fluent if the motivation was on the level of “not being able to communicate with anyone at all”. Is there hard evidence of this? In particular, once such people achieve basic communication, do they reliably move on to fluency?