The linguistic data to which children are exposed appear to be insufficient to determine, by themselves, the linguistic knowledge that children eventually attain. The gap between available experience and attained competence forms what has been called the logical problem of language acquisition. The approach to a solution, which has been followed in linguistic theory over the past two decades, is to suggest that the gap is bridged by an innate Universal Grammar: a system of knowledge of what a human language can be and innate domain-specific procedures for arriving at a grammar. The classic statement is that of Chomsky:

Adults may also learn foreign languages. Abstractly, the logical problem of adult foreign language learning is the same as for childhood language learning: explaining how acquisition takes place, given the limitations of the data. But the problem is also different, in important ways. Foreign language learning differs in degree of success, in the character and uniformity of the resulting systems, in its susceptibility to factors such as motivation, and in the previous state of the organism: The learner already has a knowledge of one language and a powerful system of general abstract problem-solving skills.