Stephen Krashen received a PhD. in Linguistics from the University of California, Los Angeles in 1972.[2] Krashen has among papers (peer-reviewed and not) and books, more than 486 publications, contributing to the fields of second-language acquisition, bilingual education, and reading.[3] He is known [citation needed] for introducing various hypotheses related to second-language acquisition, including the acquisition-learning hypothesis, the input hypothesis, the monitor hypothesis, the affective filter, and the natural order hypothesis.[4] Most recently, Krashen promotes the use of free voluntary reading during second-language acquisition, which he says "is the most powerful tool we have in language education, first and second."[5]