  However, these results have failed to be replicated (Birdsong and Molis 2001) and while it still has its supporters, the ‘critical period’ hypothesis regarding second language acquisition is increasingly being criticized (Hakuta, Bialystok and Wiley 2003; Nikolov and Djugunovich 2006). Newer studies have argued, for instance, that the degree of proficiency in a second language correlates better with, such factors as the learner's level of educational attainment in that language, her length of residence in the new country,) and the grammatical similarities between the first and second languages, and/or length of residence in the new country. (Flege, Yeni-Komshian and Liu 1999; Bialystok, 1997)[22]