Why is it easier to learn a second language as a kid? Scientists argue that it is because the neural substrate need for language acquisition cease to be fully available after a certain period. The actual age of this critical period, however, is still under debate. The lost of easy language learning ability is due to the physical change in the brain where the Universal Gammar (UG) is lost. According to wisegeek.com, UG is the theory of universal grammar that believes that, "there are certain fundamental grammatical ideas which all humans possess, without having to learn them." The loss of UG disables the language learner to never reach a native like level. In Beverí's 1981 study, he found that linguistic acquisition requires the speech production and speech perception to work simultaneously. However, with the lack of use, the two systems grow to work independently of one another in adults. Smith's article provides an excellent example:
If [she] was in front of a class of grade schoolers learning Spanish and said, "Sientense y abren los libros a la pajina diez," the child's mind would lead them to think, "Awesome, I know that libro means book. So she said something about a book and page ten. She must want us to open our books to page ten. And why would we do that standing up?" The child would then proceed to sit down and open their book to page ten, which is exactly what I had asked. An adult on the other hand, would not think that way. Because of the maturation of their brains, they would be more likely to focus on the exact words. They would become so hooked on the fact that they didn't know what 'sientense' and 'abren' meant.