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to sign language during their early years will likely never learn it (Mayberry, Lock, & Kazmi,
2002). [3]
Research Focus: When Can We Best Learn Language? Testing the Critical Period
Hypothesis
For many years psychologists assumed that there was a critical period (a time in which learning can easily occur) for
language learning, lasting between infancy and puberty, and after which language learning was more difficult or
impossible (Lenneberg, 1967; Penfield & Roberts, 1959). [4] But more recent research has provided a different
interpretation.
An important study by Jacqueline Johnson and Elissa Newport (1989) [5]using Chinese and Korean speakers who had
learned English as a second language provided the first insight. The participants were all adults who had immigrated
to the United States between 3 and 39 years of age and who were tested on their English skills by being asked to
detect grammatical errors in sentences. Johnson and Newport found that the participants who had begun learning
English before they were 7 years old learned it as well as native English speakers but that the ability to learn English
dropped off gradually for the participants who had started later. Newport and Johnson also found a correlation
between the age of acquisition and the variance in the ultimate learning of the language. While early learners were
almost all successful in acquiring their language to a high degree of proficiency, later learners showed much greater
individual variation.
Johnson and Newport’s finding that children who immigrated before they were 7 years old learned English fluently
seemed consistent with the idea of a “critical period” in language learning. But their finding of a gradual decrease in
proficiency for those who immigrated between 8 and 39 years of age was not—rather, it suggested that there might
not be a single critical period of language learning that ended at puberty, as early theorists had expected, but that
language learning at later ages is simply better when it occurs earlier. This idea was reinforced in research by Hakuta,
Bialystok, and Wiley (2003), [6] who examined U.S. census records of language learning in millions of Chinese and
Spanish speakers living in the United States. The census form asks respondents to describe their own English ability
using one of five categories: “not at all,” “not well,” “well,” “very well,” and “speak only English.” The results of this
research dealt another blow to the idea of the critical period, because it showed that regardless of what year was used
as a cutoff point for the end of the critical period, there was no evidence for any discontinuity in language-learning
potential. Rather, the results (Figure 9.12 "English Proficiency in Native Chinese Speakers") showed that the degree of
success in second-language acquisition declined steadily throughout the respondent’s life span. The difficulty of