The information reviewed in this paper is about Korean-English bilinguals; specifically it is about native Koreans who emigrated to the US at different ages. The findings to be reported are colored by the fact that at the time of testing for the main study the participants were long-term residents of the US living in communities in and around the Washington, DC area. The participants lived with their families and spoke Korean at home and Korean was their only language during their preschool years; however, the language environment outside their homes was predominantly English. In studying bilingualism and evaluating the achievement of bilingual individuals it is important to recognize the linguistic make-up of the country of residence. Studies carried out in other linguistic environments may very well produce different results from those reported here.
Theories of first (L1) and second (L2) language acquisition invariably get embroiled in the issue of the Critical or Sensitive Period Hypothesis (CPH). Advocates of the CPH use age of learning as the primary index of the individuals' neural maturational state and do not worry much about the effects of experiential factors that co-vary with age. The majority of studies of L2 pronunciation carried out in the US with immigrant populations show an age of learning effect that favors early L2 learners (Asher & Garcia, 1969; Fathman, 1975; Oyama, 1976; Tahta, Wood & Lowenthal, 1981b; Patkowski, 1990, 1994; Flege, Munro & MacKay, 1995; Flege, Yeni-Komshian & Liu, 1999; Yeni-Komshian, Flege & Liu, 2000; see also research reviewed by Scovel, 1988; Long, 1990; Harley & Wang, 1997, and Birdsong, 1999).