In the late 60’s and early 70’s of twenty-first century, the focus of heated discussion of CPH revolved around whether children have advantages in learning a foreign language than adults. Scholars who held an opposing opinion state that both children and adults have their merits at the early period of language acquisition according to positive data. From the 80’s of twenty-first century, the topic of this debate on CPH was transferred from proving children’s advantages on second language acquisition to exploring the reasons of those advantages.
Contrarily, those who supported this hypothesis states that CPH could not be refuted in a quite long period, which based on related researches. In 1999, David Birdsong, an American linguist, published a book named Second Language Acquisition and the Critical Period Hypothesis, which reflect the latest research findings of CPH due to the new and creative perspective on the age difference and some viewpoints about CPH.

2.2 Educational Linguistics
Educational linguistics (EL) was first named as a field 30 years ago and defined in two introductory books (Spolsky,1978; Stubbs,1986) the title “educational linguistics” was proposed by Bernard Spolsky in 1972 for a discipline whose primary task would be “ to offer information” relevant to the formulation of language education policy and to its implementation”(Spolsky,1974:554). It describes the commingling of an academic discipline (linguistics) with a practical academic profession (education). Spolsky regards EL is an interdiscipline of academic linguistics and practical education.

Richards et al. made a definition of EL in Longman Dictionary of Applied Linguistics: education linguistics is a term sometimes used in the USA to refer to a branch of applied linguistics which deals with relationship between language and education (Richards et al., 1985). They treated EL as a subdiscipline of applied linguistics rather than an equivalent of applied linguistics. The research of EL is the relation between language and education.

EL’s grew from the discomfort with the ambiguity of the term “applied linguistics”. Therefore, the history of Educational Linguistics is inextricably linked to applied linguistics. Since it has been proposed, in the past forty years, there is a constantly heated controversy on its disciplinary attribute. Whether it is a subdiscipline of applied linguistics or an independent academic subject remains dissenting among domestic linguists and educationists. It was later understood that “it’s a unified field within the wider discipline of applied linguistics” (Spolsky, 1978: 2). And today, it has turned out to be an independent field whose “starting point is always the practice of education and the focus is squarely on the role of language in learning and teaching (Hornberger, 2001: 19). Now, it is widely believed that it is EL which should be responsible for L1 and L2 learning, not applied linguistics.

2.3 The Integration of the Views
All child language acquisition theories talk about the roles of two factors to different degrees: the linguistic environment children are exposed to and the age they start to learn the language. These two factors bear remarkable relevance to their language development (Dai & He, 2010). On one side, the pro side of CPH proves the function of age determines the ultimate attainment of language acquisition. On the other side, EL’s broad view of the educational cultural environment highlights the non-biological factors of language acquisition, which compensate the deficiency of CPH.

The possible existence of a critical period states that children have innate propensity and language-specific ability that ease and limit language learning for language acquisition is also a nativist argument. Critical periods are the time frames during which environmental exposure is needed to stimulate an innate trait. Nevertheless, EL originated from resolving the problems of applied linguistics, it aims to explain the process of language acquisition from a problem-oriented and trans-disciplinary perspective, which pays much attention to non-biological factors of language teaching and learning. Therefore, CPH and EL stands for the innate and acquired separately, which both make up the whole influential elements of language acquisition. 儿童语言习得关键期假说的教育语言学重估(3):http://www.youerw.com/yingyu/lunwen_5978.html