The issue of critical or sensitive periods affecting the outcome of second language (L2) acquisition has been the subject of intense investigation and debate for many years, with people arguing for or against maturational effects on ultimate attainment. In their influential paper, Johnson and Newport (1989) identify two hypotheses: the exercise hypothesis and the maturational state hypothesis. According to the former, if the capacity for acquiring language is exercised early in life (in first language acquisition), then language learning abilities will remain intact throughout life: in other words, permitting successful L2 acquisition regardless of age. In contrast, according to the maturational state hypothesis, the language learning capacity declines with age, affecting L2 acquisition as well as late L1. Johnson and Newport take their results, which show an age-related decline in performance during childhood and adolescence, to support the maturational state hypothesis. Many L2 researchers have adopted a maturational perspective and have reached similar conclusions as to the presence of critical or sensitive periods (e.g., Abrahamsson, 2012; DeKeyser & Larson-Hall, 2005; Long, 1990; Oyama, 1976; Patkowski, 1980). Some researchers have pointed out that age effects continue into adulthood, contrary to the claim for a critical period (e.g., Birdsong & Mollis, 2001). Others have suggested that what looks like an age-related maturational decline may be accounted for by confounding factors, such as task effects, effects of L1, amount of L2 use, education or input (e.g., Bialystok & Miller, 1999; Flege, Yeni-Komshian & Liu, 1999).
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