  As further support for the critical period hypothesis, others have  added the observation that although children are able to learn a  second language rapidly and to native speaker fluency, adult learners of second languages typically are not: the capacity to learn a second language tapers off after puberty, no matter how much exposure to the language one has. (Newport 1990). Thus, it was speculated, the  innate knowledge base for language learning (e.g., knowledge of UG)  becomes unavailable for normal acquisition at puberty, and adult  learners must rely on less efficient learning methods. (Johnson and  Newport 1989.)