Jerome Bruner (who turns 100 today!) writes in his 1983 autobiography (emphasis in original):

“How puzzling that there should be so much emphasis … on the underlying genetic program that makes language acquisition possible and so little on the ways in which the culture, the parents and more “expert” speakers (including other, older children) help the genetic program to find expression in actual language use. The educational level of parents deeply affects how well, richly and abstractly their children will talk (and listen). It is not just the grammar of sentences that is at issue, but discourse, dialogue, the capacity to interpret spoken and written language.

…

In the end, I came to the conclusion that the need to use language fully as an instrument for participating in a complex culture (just as the infant uses it to enter the simple culture of his surround) is what provides the engine for language acquisition. The genetic ‘program’ for language is only half the story. The support system is the other half.”

Three decades later, proposals for the other half, what Bruner calls “the engine of language acquisition”, have become increasingly well-articulated and supported by rich empirical data (cf., for instance, all the research reviewed in Tomasello’s (2008) Constructing a language). But the two halves (genetic underpinnings and cultural scaffolding) are still not regularly talking to each other. Indeed they’re frequently pretending that the other half has no story at all… Why?

Bruner, Jerome S. 1983. In Search of Mind: Essays in Autobiography. Alfred P. Sloan Foundation Series. New York u.a: Harper [and] Row.