If children had to enumerate and test all possible grammatical hypotheses against the data, they could not acquire this knowledge given the impoverished input.

Suppose that the primary linguistic data were impoverished in all the ways that nativists claim and suppose, too, that children know a bunch of things for which there is no evidence available — suppose, as Hornstein and Lightfoot (1981:9) put it, that “[p]eople attain knowledge of the structure of their language for which no evidence is available in the data to which they are exposed as children.” What follows from this is that there must be constraints on the learning mechanism: children d