The key to maximal language development is parent to child communication.
The key is adult to child dialogue! Parents who talk to their children a lot and encourage them to respond verbally will have children with advanced speech skills; and hence advanced language skills; an hence advanced thought processes. The reason is quite obvious. When parent talk to their children, they provide rich models for all the languagerules we have discusses: phonology, morphology, syntax and pragmatics. They even provide special help in the way of using motherese talk (using short sentences with special prosody) as described in the text.
They repeat and slightly rephrase or extend what the child has said, thus providing the best models for the child’s language acquisition device, to coin a phrase of Chomsky’s.