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Age effects in (second) language learning are widely acknowledged, but their exact nature remains controversial, in particular, the concept of a critical period for second language acquisition (SLA). In about the last 15 years, numerous arguments against the critical period hypothesis (CPH) have been formulated: a few studies have failed to find a clear correlation between age of acquisition and ultimate attainment; many more researchers accept the negative correlation as a fact, but they argue that it is attributable to a confound between age of acquisition and one or more other variables, such as length of residence, age at testing, the nature of the input received as a function of age, the extent to which education was provided in the second language (L2) or the first language (L1), the (lack of) motivation to integrate fully with the L2 society, or simply the amount of practice in the L2 as opposed to the L1. Some have also taken the rather different results found for learners of different L1s learning the same L2 as evidence that age of acquisition is not an important predictor by itself. (For recent critical overviews of the literature for and against the CPH, see Birdsong, 2005; DeKeyser & Larson-Hall, 2005; Herschensohn, 2007; Hyltenstam & Abrahamsson, 2003; Ioup, 2005; Long 2007, chap. 3.) The argumentation for and against the CPH that has received the most attention in the literature of the last 5 to 10 years, however, has been about the very nature of the age of acquisition-ultimate attainment function, which is centered on the question of whether the discontinuity in development implied by the CPH is found in the various data sets that were analyzed.
THE SHAPE OF THE AGE OF ACQUISITION-ULTIMATE ATTAINMENT FUNCTION
For about the last 10 years a number of researchers have analyzed data sets that appear to show a negative correlation between age of acquisition and ultimate attainment throughout the life span (Bialystok & Hakuta, 1999; Bialystok & Miller, 1999; Hakuta, Bialystok, & Wiley, 2003) or even a stronger negative correlation in adulthood than in childhood or adolescence (Birdsong & Molis, 2001). Therefore, these researchers argue, the decline as a function of age, which has been documented in dozens of studies, should...