Ming from Agents of Speech breaks down why speech-delayed children don't talk: either they can't or they won't. Usually it's both, tangled up in confusion about what's expected and what they get out of it. Adults have to make the purpose and reward explicit.
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You're probably confusing speech delay children. And the reason I'm saying this is because people, or children in general, they don't do something because of two major reasons. Number one, they don't know how to, or number two, they don't want to. Right. It's either ability problem or a motivation problem. And usually speaking, it's a mixture of both for children. Okay. And usually what happens is that when I say usually is like when I'm working with parents and when I'm working at the clinic and I work with therapists and so on, I see that children are confused. They don't know what they're going to get out of this interaction, and therefore they don't want to, or that they don't know what you're doing, and therefore they don't know how to respond. So it's kind of both things at the same time. And both of these reasons are up to us. That's our fault. Not being able to communicate. Exactly. And clearly what we want, what structure that is. Like, what's in it for the kid, how fun it is, why is it rewarding?