Language Development, Critical Periods in 737 Language Development, Critical Intermediate article Periods in EL Newport, University of Rochester, Rochester, New York, USA CONTENTS Introduction Questions concerning a critical or sensitive period for Evidence for a critical or sensitive period for language language acquisition acquisition First language acquisition typically occurs in infancy beginning early in life and ending at puberty. Out- and early childhood. An important question con- side of this time period, he suggested, language cerns whether the acquisition of a first or a second could be acquired only with difficulty or by a dif- language shows a critical or sensitive period: that is, ferent learning process. He also suggested a neural whether acquisition displays a normal course and mechanism for this developmental change: he hy- leads to full proficiency in the language only when it pothesized that the critical period for language ac- begins early in life. quisition ended with the establishment of cortical lateralization of function, as the brain reached its INTRODUCTION mature organization in late puberty. In many species, including humans, important Since the time of Lenneberg's book, an extensive and species-typical behaviors develop through an research literature has asked whether there is intricate combination of innate and experiential indeed a critical or sensitive period for human lan- factors. One hallmark of such systems is the appear- guage acquisition. These studies have provided ance of a critical or sensitive period for normal strong support for the existence of such a critical development. or sensitive period (particularly for acquiring the A critical period is a maturational time period phonology and grammar of language), though not during which some crucial experience will have its for Lenneberg's specific hypothesis about the rela- peak effect on development or learning, resulting tionship between lateralization and the end of the in normal behavior attuned to the particular envir- critical period. onment to which the organism has been exposed. If The term `critical period' is sometimes used the organism is not exposed to this experience until when there is an abrupt decline in plasticity and after this time period, the same experience will no residual plasticity after this period is over, have only a reduced effect, or in extreme cases whereas the term `sensitive period' is used when may have no effect at all. Well-studied examples there is a more gradual decline and some (reduced) of species-typical behaviors showing peak plasti- plasticity remaining throughout life. However, city within a critical or sensitive period include the recent research has shown that most critical periods identification of a species member as an attachment show more gradual offsets and more complex object (called `imprinting') in ducks and birds, the interactions between maturational and experiential acquisition of the species mating song by finches factors than the original concept of a critical period and sparrows, and the spatial tuning of auditory had anticipated. The terms are therefore often used localization in barn owls. In contrast, in other interchangeably, as will be done in the present domains and systems, there may be plasticity uni- article. formly throughout life (open-ended learning), or plasticity may increase with age as experience or EVIDENCE FOR A CRITICAL OR higher-level cognitive skills increase. SENSITIVE PERIOD FOR LANGUAGE In his seminal book Biological Foundations of Lan- ACQUISITION guage, Eric Lenneberg (1967) hypothesized that human language acquisition was an example of A number of lines of research, both behavioral and biologically constrained learning, and that it neural, suggest that there is a critical or sensitive was normally acquired during a critical period, period for language acquisition. Case studies of 738 Language Development, Critical Periods in individual feral or abused children, isolated from grammaticality judgments for morphology and exposure to their first language until after puberty, syntax, and syntactic processing speed and accur- have shown extreme deficits in phonology, morph- acy. For example, Johnson and Newport (1989) ology, and syntaxresulting from this deprivation. have shown that Chinese or Korean immigrants The best studied of these cases is a girl named who move to the United States and become ex- Genie, who was followed closely for a number of posed to English as a second language show strong years after her discovery and placement in a effects of their age of exposure to the language on normal linguistic environment at age 13 (Curtiss, their ability to judge its grammatical structure 1977). While Genie did successfully acquire some many years later, even when the number of years English after puberty, her phonology was abnor- of exposure is matched. These effects are not due mal, and her control over English syntaxand merely to interference of the first language on the morphology was limited to only the simplest learner's ability to acquire the second language: aspects of the language. deaf adults, acquiring American Sign Language as However, in cases of isolated children, general their primary language, show effects of age of ex- physical and cognitive status may be a concern. In posure on their grammatical skills in ASL as much studies of populations of normal individuals, one as 50 years later, even though they may not control can systematically examine proficiency in relation any other language with great proficiency (New- to age of linguistic exposure without concern about port, 1990; Mayberry and Eichen, 1991). the physical status of the learning brain. These While there are effects of age of acquisition on studies show a strong relationship between the age both first and second languages and on both of exposure to a language and the ultimate profi- spoken and signed languages, an important ques- ciency achieved in that language (Johnson and tion is how these effects compare. Does the acquisi- Newport, 1989; Krashen et al., 1982; Long, 1990; tion of a language early in life reduce the effects of Newport, 1990), though typically with many age on later language learning? This question has fewer extreme deficits in adult learning than those been examined by comparing hearing and deaf found in the case studies of isolated children. individuals' acquisition of English or American Learning during the first months or year of expos- Sign Language as either a first or a second lan- ure may show an advantage for adult learners, guage, and (if as a second language) after early particularly in the acquisition of vocabulary and exposure to either a spoken or a signed language the speed of using certain complexsentence (Mayberry et al., 2002). The results show that age of forms; however, long-term outcome clearly favors first language onset has a significant effect, while those who start learning the language during child- language modality does not: late first language ac- hood. Peak proficiency in the language, in control quisition results in lower performance than does over the sound system as well as the grammatical late second language acquisition, regardless of structure, is displayed by those whose exposure to whether the languages in question were spoken or that language begins in infancy or very early child- signed. According to one recent finding, even over- hood. Such early learners show not only flawless hearing a language during early childhood, with- control over the accent and rhythm of the language out producing it or hearing it again for many but also full and productive control over the syntax years, can result in learning to pronounce that and morphology. With increasing ages of exposure language with a more native accent as an adult there is a decline in average proficiency, beginning (Au et al., 2002). as early as ages 4 to 6 and continuing until profi- However, age of exposure does not affect all ciency plateaus for adult learners (Johnson and aspects of language learning equally. The acquisi- Newport, 1989; Newport, 1990). Learners exposed tion of vocabulary and semantic processing occur to the language in adulthood show, on average, a relatively normally in late learners. Critical period lowered level of performance in many aspects of effects thus appear to focus on the formal proper- the language, though individual variation also in- ties of language (phonology, morphology, and creases with age (Johnson and Newport, 1989), and syntax) and not the processing of meaning. Even some individuals may approach the proficiency of within the formal properties of language, though, early learners (Birdsong, 1992). various aspects of the language may be more and These effects have been shown for both first less dependent on age of language exposure. For and second languages, and for measures of profi- example, late learners acquire the basic word order ciency including degree of accent, production of a language relatively well, but more complex and comprehension of morphology and syntax, aspects of grammar show strong effects of late Language Development, Critical Periods in 739 acquisition (Johnson and Newport, 1989; Newport, QUESTIONS CONCERNING A CRITICAL 1990). Further research is needed to characterize OR SENSITIVE PERIOD FOR the structures that do and do not show strong LANGUAGE ACQUISITION effects of age of learning. Age of exposure also affects the way language is Several questions have been raised about whether represented in the