* - According to Fernandez and Cairns (2011), Critical Period is “the optimal period for first language acquisition before the early teen years, after which a fully complex linguistic system will not develop” (Fernandez and Cairns, 2011, p. 79). This hypothesis argues that after the critical period, it is difficult for an individual to learn a new language when compared to learning a language at a young age.

  • - Age Constraints on Second-Language Acquisition study- Flege, Yeni-Komshian, & Liu, (1999) studied the critical period hypothesis for second language acquisition with 240 native Korean speaker participants. They were evaluated on their sentence grammar structures and pronunciation of English. The results showed that the participants who arrived later at a mean age of 16.6 years of age, had a stronger foreign accent compared to those who arrived at a younger mean age of 9.7 years of age. As the participants' age of arrival increased, foreign accents grew stronger and the scores on grammar tests decreased.
  • - Critical Period Effects in Second Language Learning: The Influence of Maturational State on the Acquisition of English as a Second Language- Johnson and Newport(1989) studied the relationship between age and learning the grammar of a second language. They had a total of 46 participants who were native Chinese or Korean speakers who spoke English as their second language who arrived in the US from ages 3- 39. There were 23 participants who arrived in the US before the age 15 and 23 subjects who arrived in the US after the age 17. As a result, the researchers found that “subjects who began acquiring English in the United States at an earlier age obtained higher scores on the test than those that began later” (Johnson and Newport, 1989). They also found that those who came to the US “before the age of seven reached native performance on the test”, while those who arrived later resulted in “a linear decline in performance up through puberty”. This study clearly supports the critical period hypothesis, in which language acquisition is optimal before puberty and declines with age.
  • - A Test of the Critical-Period Hypothesis for Second-Language Acquisition- Hakuta, Bialystok and Wiley (2003) studied the relationship between the age of acquisition on second-language proficiency. They had 2,016,317 Spanish speakers and 324,444 Chinese speakers. As a result, they found that as the age of initial second-language exposure increased, there was a decline in second-language proficiency. Hakuta, Bialystok and Wiley (2003) also stated that “in addition to age of immigration, socioeconomic factors, and in particular the amount of formal education, are important in predicting how well immigrants learn English” (Hakuta, Bialystok and Wiley, 2003). So not only does the age of exposure have an effect on language acquisition, but also other social factors as well.