accepted as evidence supporting the CPH. Incorporating the idea, "younger equals better" by Penfield, David Singleton (1995) states that in learning a second language there are many exceptions, noting that five percent of adult bilinguals master a second language even though they begin learning it when they are well into adulthood—long after any critical period has presumably come to a close. The critical period hypothesis holds that first language acquisition must occur before cerebral lateralization completes, at about the age of puberty. One prediction of this hypothesis is that second language acquisition is relatively fast, successful, and qualitatively similar to first language only if it occurs before the age of puberty. To grasp a better understanding of SLA, it is essential to consider linguistic, cognitive, and social factors rather than age alone, as they are all essential to the learner's language acquisition. Over the years, many experimenters have tried to find evidence in support of or against the critical periods for second language acquisition. Many have found evidence that young children acquire language more easily than adults, but there are also special cases of adults acquiring a second language with native-like proficiency. Thus it has been difficult for researchers to separate correlation from causation. In 1989, Jacqueline S. Johnson and Elissa L. Newport found support for the claim that second languages are more easily acquired before puberty, or more specifically before the age of seven. They tested second language learners of English who arrived in the United States at various ages ranging from three to thirty-nine, and found that there was a decline in grammatical correctness after the age of seven. Johnson and Newport attributed this claim to a decline in language learning ability with age. Opponents of the critical period argue that the difference in language ability found by Johnson and Newport could be due to the different types of input that children and adults receive; children received reduced input while adults receive more complicated structures. Additional evidence against a strict critical period is also found in the work of Pallier et al. (2003) who found that children adopted to France from Korea were able to become native-like in their performance of French even after the critical period for phonology. Their experiment may represent a special case where subjects must lose their first language in order to more perfectly acquire their second. There is also some debate as to