The textbook, in chapter 8, described what they called critical periods for learning language. A study was conducted in which adults who had immigrated to the United States were tested on their English grammar skills. The test subjects who were tested immigrated at various ages, so as to test whether being exposed to English at certain ages has an effect on proficiency and retention. The results showed that for the subjects who were exposed to the language between the ages of 1 and 7 had skills similar to someone who was born in the United States. After 7 years of age, the English skills steadily drop with age increase. This shows evidence for a critical period for learning a language between ages 1 and 7. Another application presented in the book was that of deaf children who received cochlear implants. Studies showed that younger children were able to develop better language skills than were older children. Yet another study was the study of the young girl Genie, who was locked in a dark room with no human interaction or language development whatsoever. This horrible abuse was not discovered until she was an adolescent, and by then it was tough for her to develop any language skills. I think that these studies are fairly good evidence that a critical period for learning language does exist, but like the book states, it is unclear that the period is strictly between ages 1 and 7, but that those ages have tested very well. The text talks about sensitive periods, which are periods where the human mind could be considered more malleable, or able to acquire language easier, which I think is also a good argument.