There may be noticeable changes in a controlled learning environment, but they tend not to last (Bley-Vorman 9). .
Compared to the one hundred percent success rate of children, there must be a reason for the discrepancy. The first hypothesis that Bley-Vorman rejects is that of the L1 interference hypothesis. This theory states that the existence of the first, or primary, language interferes with an adult's ability to acquire a second language. However, this argument does not acknowledge that second language acquisition is much easier for children than it is for adults or why a third language is easier than a second for an adult (Bley-Vroman 24). The second theory is known as the input hypothesis: adults do not get the same constant stimulation and attention that a child learning a language would get. The flaw in this theory, however, is that studies have shown that adult immigrants who have lived in other countries for decades still do not reach the level that perhaps their young child does(Bley-Vroman 26). The third popular hypothesis is known as the affect or socialization hypothesis which suggests that children have a certain attitude or state or state of mind that affects the way in which they learn language that adults lack. These factors do influence an adult's ability to learn a new language, but they seem to have no influence whatsoever over children (Bley-Vroman 26). Finally, there is the competing cognitive systems hypothesis, in which a Language Acquiring Device found in children becomes dominated by their problem solving system. With this theory it would follow that people who are good with language would be bad at problem solving, and vice versa, but studies have shown this is not the case. Also, it seems absurd that a cognitive system would not do what it was intended to do while it was still functioning (Brey-Vroman 33).
This leads into the hypothesis that Bley-Vroman favors: the fundamental difference hypothesis.