What makes the "critical period" of language learning critical?

There is consensus in the field of second language acquisition research that a period of time exists in which the acquisition or learning of a second language can be accomplished more rapidly and easily than times falling outside of this period (Brown, 1994; Larsen-Freeman & Long, 1991; Scoval, 1998). Specifically, the hypothesis posits that children between birth and somewhere around the age when a child enters puberty can more easily acquire language than post-pubescent children. Learners of a second language that begin the learning process after this "critical period" (around age 10 or 12) are indeed still able to acquire a second language. According to research, however, individuals who begin their study of a second language after this critical period show marked performance deficits in the language, most notably the lack of a native-like accent (Brown, 1994).

The foundation of the critical period hypothesis rests on neurological research that suggests that brain functions become lateralized after puberty. As we know there are two sides of the human brain - the right and the left. Some language functions appear to be controlled or stationed in the right while others are located in the left portion. Language functions appear mainly controlled by the left side. It is believed that, before puberty, these functions are not completely assigned to either portion of the brain. The brain is viewed as elastic. The specific assignment or lateralization of brain functions is believed to be complete and set sometime during or just after puberty. According to this theory, the pre-pubescent brain is like a "sponge" - - all learning, knowledge, and experiences are merely "absorbed." This "absorption" of aspects of language to non-specific locations in the brain supposedly makes the learning of language, first or second language, easier for children than adults or older adolescents.

Fromkin and Rodman (1993) provide several examples to support this hypothesis. Genie was a young girl who received very little contact with other humans from birth to the age of 14. As a result, she did, once re-introduced to society, begin to acquire some language, but she was never able to fully acquire the syntax and morphology of children who do receive input from a very age.

So if this critical period does indeed exist, what does this mean for teachers of second language? First of all, this hypothesis states that post-pubescent learners are not going to be likely to acquire a native-like pronunciation and possibly not be able to fully acquire the syntax and morphology of the second language as implied by the Genie case study. As a high school teacher Spanish teacher should I resign to the notion that my students will never acquire a native-like pronunciation? On the contrary, I will use the critical age hypothesis in order to restructure the way in which I teach pronunciation of these post-pubescent learners. As Brown (1994) states, "some adults have been known to acquire an authentic accent in a second language after the age of puberty, but such individuals are few and far between" (p. 56). As such, rather than expecting my students to naturally acquire the pronunciation as they did easily as children learning their native language, I will give the structured pronunciation drills and compare phonemes from the second language to those in the first language. Teachers of second languages should view this theory with caution, however. Though the "few and far between" individuals who acquire critical period skills after the critical period may be few, the empirical support of this theory, in my view, appears to be just as "few and far between." The research in this area has not employed the tight experimental controls required to draw inferences on how and when children learn second languages best. Until more and better research in this area emerges, we are again left to best guesses or gut feelings in terms of student learning.

That is when our brains are best able to decode and comprehend language.