One of the crucial underlying concepts that account for the differences in second language acquisition (SLA) between children and adults is the critical period hypothesis. The boundaries between what is defined as a ‘child’ in SLA and what is defined as an ‘adult’ in this blog is mainly based on this theory.
Eric Lenneberg (1976) hypothesised that language acquisition in human beings was affected by biological growth (i.e. as a person ages). In his claims, the age of adolescence (after puberty, about 13 years old) is a transitional point at which the brain reaches a biologically mature state. Because of this, there is a firm localisation of language processing in the left hemisphere of the brain (i.e. language processing has already been ‘fixed’ in a particular part of the brain) after puberty. This then, causes difficulties in language acquisition after puberty.
Although the critical period hypothesis was originally applied to L1 acquisition, studies have shown that the effects are also extended to L2 acquisition. For example, a research conducted by Jacqueline S. Johnson, and Elissa L Newport (1989) compared L2 English proficiency of 46 native Korean or Mandarin speakers who had arrived in the US between the ages of 3 and 26. The subjects were tested on structures of English grammar, using a grammaticality judgement task. The test results showed that participants who arrived in the US before the age of puberty (i.e. before 13 years old) performed better at the test, as compared to participants who had arrived in the US after the age of puberty. It is also important to note that the performance among participants who arrived in the US after 13 years old was variable (at lower scores). The results generally supported the authors’ prediction that the critical period for the acquisition of language is also extended to second language acquisition.