Language acquisition refers to the process of attaining a specific variant of human language, such as English, Navajo, American Sign Language, or Korean. The fundamental puzzle in understanding this process has to do with the open-ended nature of what is learned: children appropriately use words acquired in one context to make reference in the next, and they construct novel sentences to make known their changing thoughts and desires. In light of the creative nature of this achievement, it is striking that close-to-adult proficiency is attained by the age of 4-5 years despite large differences in children's mentalities and motivations, the circumstances of their rearing, and the particular language to which they are exposed. Indeed, some linguists have argued that the theoretical goal of their discipline is to explain how children come to have knowledge of language through only limited and impoverished experience of it in the speech of adults (i.e., "Plato's problem"; Chomsky 1986). For closely related reasons, philosophers, evolutionary biologists, and psychologists have long used language acquisition as a testbed for exploring and contrasting theories of learning, development, and representation. Neither the natural communication systems of infrahumans nor the outcomes for apes specially tutored in aspects of spoken or signed systems approach in content or formal complexity the achievements of the most ordinary 3-year-old human (see also PRIMATE LANGUAGE). Because children are the only things (living or nonliving) that are capable of this learning, computer scientists concerned with simulating the process study language acquisition for much the same reason that Leonardo Da Vinci, who was interested in building a flying machine, chose to study birds.
Language acquisition begins at birth, if not earlier. Children only a few days old can discriminate their own language from another, presumably through sensitivity to language-specific properties of prosody and phonetic patterning. In the first several months of life, they discriminate among all known phonetic contrasts used in natural languages, but this ability diminishes over time such that by about 12 months, children distinguish only among the contrasts made in the language they are exposed to. Between about the seventh and tenth month, infants begin reduplicative babbling, producing sounds such as "baba" and "gaga" (see also PHONOLOGY, ACQUISITION OF). At this age, deaf children too begin to babble -- with their hands -- producing repetitive sequences of sign-language formatives that resemble the syllabic units of vocal babbling. In general, the acquisition sequence does not seem to differ for spoken and signed languages, suggesting that the language-learning capacity is geared to abstract linguistic structure, not simply to speech (see SIGN LANGUAGES).
Comprehension of a few words has been demonstrated as early as 9 months, and first spoken words typically appear between 12 and 14 months. The most common early words are names for individuals (Mama), objects (car), and substances (water); these nominal terms appear early in language development for children in all known cultures. Other common components of the earliest vocabulary are animal names, social terms such as bye-bye and -- of course -- no. Verbs and adjectives as well as members of the functional categories (such as the, -ed, is, and with) are rare in the early vocabulary compared to their frequency in the input corpus. At this initial stage, new words appear in speech at the rate of about two or three a week and are produced in isolation (that is, in "one word sentences"). The rate of vocabulary growth increases and so does the character of the vocabulary, with verbs and adjectives being added and functional morphemes beginning to appear. This change in growth rate and lexical class typology coincides with the onset of syntax (for further discussion and references, see WORD MEANING, ACQUISITION OF and SYNTAX, ACQUISITION OF).
The most obvious sign of early syntactic knowledge is canonical phrasal order: Children's speech mirrors the canonical sequence of phrases (be it Subject-Verb-Object, Verb-Subject-Object, etc.) of the exposure language as soon as they begin to combine words at all (Pinker 1984). English-speaking children's first word combinations were originally called "telegraphic" by investigators (Brown and Bellugi 1964) because they are short and because they lack most function words and morphemes, giving the speech the minimalist flavor of telegrams and classified ads. But more recent investigation shows this characterization to be inadequate, in part because this pattern of early speech is not universal. In languages where closed-class morphemes are stressed and syllabic, they appear before age 2 (Slobin 1985).
Another problem with calling children's language "telegraphic" is that this characterization underestimates the extent of child knowledge. There is significant evidence that infants' knowledge of syntax, including the forms and semantic roles played by functional elements, is radically in advance of their productive speech. One source of evidence is preferential looking behavior in tasks where a heard sentence matches only one of two visually displayed scenes. Such methods have shown, for example, that appreciation of word order and its semantic implications are in place well before the appearance of two-word speech. For example, infants will look primarily to the appropriate action video in response to hearing Big Bird tickles Cookie Monster versus Cookie Monster tickles Big Bird (Hirsh-Pasek and Golinkoff 1996) and show a rudimentary understanding of the semantic implications of functional elements (Snedeker 1996). Another compelling source of evidence for underlying syntactic knowledge is that analyses of the relative positioning of subjects, negative words, and verbs, and their interaction with verb morphology, makes clear that children have significant implicit knowledge of functional projections; for example, they properly place negative words with respect to finite versus infinitive forms of the verb. That is, the French toddler regularly says "mange pas" but "pas manger" (Deprez and Pierce 1993).
By the age of 3 years or before, the "telegraphic" stage of speech ends and children's utterances increase in length and complexity. For instance, at age 3 and 4 we hear such constructions as inverted yes/no questions (Is there some food over there?), relative clauses (the cookie that I ate), and control structures (I asked him to go). Production errors at this stage are largely confined to morphological regularizations (e.g., goed for went) and syntactic overextensions (She giggled the baby for She made the baby giggle; see Bowerman 1982), though even these errors are rare (Marcus et al. 1992).
One important commitment that all acquisition theories make is to the sort of input that is posited to be required by the learner. After all, the richer and more informative the information received, the less preprogrammed capacity and inductive machinery the child must supply "from within." Adults tend to communicate with their offspring using slow, high-pitched speech with exaggerated intonation contours, a relatively simple and restricted vocabulary, and short sentences. Although whimsically dubbed "Motherese" (Newport, Gleitman, and Gleitman 1977), this style of speech is characteristic of both males and females, and even of older children when talking to younger ones. DARWIN, who was interested in language learning and kept diaries of his children's progress, called this style of speech "the sweet music of the species." Infants resonate to it, strongly preferring Motherese to the adult-to-adult style of speech (Fernald and Kuhl 1987). Although it is possible that these apparent adult simplifications might facilitate aspects of learning, there is plenty of evidence that the abused and neglected children of the world, regardless of their other difficulties, adequately acquire the language of their communities.
Because speech to children is almost perfectly grammatical and meaningful, it seems to offer a pretty straightforward model for acquisition. Yet in principle such a "good sample" of, say, English, is a limited source of information for of necessity it doesn't explicitly expose the full range of structure and content of the language. Notice, as an example, that although an adjective can appear in two structural positions in certain otherwise identical English sentences (e.g., Paint the red barn and Paint the barn red), this is not always so: Woe to the learner who generalizes from See the red barn to See the barn red. It has been proposed, therefore, that "negative evidence" -- information about which sentences are ill-formed in some way -- might be crucial for deducing the true nature of the input language. Such information might be available in parents' characteristic reactions to the child's ill-formed early speech, thus providing statistical evidence as to right and wrong ways to speak the language. However, a series of studies beginning with Brown and Hanlon (1970) have demonstrated that there is little reliable correlation between the grammaticality of children's utterances and the sorts of responses to these that their parents give, even for children raised in middle-class American environments. Moreover, children learn language in cultures in which nobody speaks to infants until the infants themselves begin to talk (Lieven 1994).
As mentioned above, neonates can detect and store at least some linguistic elements and their patterning from minimal distributional information (Saffran, Aslin, and Newport 1997). Recent computational simulations suggest that certain lexical category, selectional, and syntactic properties of the language can be gleaned from such patterns in text (e.g., Cartwright and Brent 1997). Most theories of language development, following Chomsky (1965) and Macnamara (1972), assume that children also have access to some nonlinguistic encoding of the context. That is, they are more likely to hear hippopotamus in the presence of hippos than of aardvarks, and The cat is on the mat is, other things being equal, a more likely sentence in the presence of cats and mats than in their absence. The effects of such contextual support are obvious for learning the meanings of words, but they are likely to be critical as well for the acquisition of syntax (Bloom 1994; Pinker 1984).
Whatever the detailed cause-and-effect relations between input properties and learning functions, it seems a truism that the variant of human language attained by young children is that modeled by the adult community of language users. Yet even in this regard, there are significant exceptions. For example, isolated deaf children not exposed to signed languages spontaneously generate gestural systems that share many formal and substantive features with the received languages (e.g., Feldman, Golden-Meadow, and Gleitman 1978; see also SIGN LANGUAGES). Similarly, children whose linguistic exposure is to "pidgin" languages (simple contact systems with little grammatical structure) elaborate and regularize these in the course of learning them (Bickerton 1981; Newport 1990; see CREOLES). Much of language change can be explained as part of this process of regularization and elaboration of received systems by children. In a very real sense, then, children do not merely learn language; they create it.
The acquisition properties just described sketch the known facts about this process as it unfolds in young children, under all the myriad personal, cultural, and linguistic circumstances into which they are born. Acquisition is astonishingly "robust" to environmental circumstance. This finding is at the heart of theorizing that assigns the child's acquisition in large part to internal, biological, predispositions that can correct for adventitious properties of the environment. One more stunning indication of the merit of such a view is that young children simultaneously exposed to two -- or even three -- languages in infancy and early childhood acquire them all as rapidly and systematically as the child in monolingual circumstances acquires one language. That is, if the child in a bilingual home sleeps as much as her monolingual cousin, she necessarily hears a smaller sample of each of the languages; yet attainment of each is at age-level under conditions where both languages are in regular use in the child's immediate environment.
If it is true that biologically "prepared" factors in young children significantly support and constrain the learning process, then learning might look different when those biological factors vary. And indeed it does. The most obvious first case to inspect is that of learners who come into a language-learning situation at a later brain-maturational state. In contrast to the almost universal attainment of a high level of language knowledge by humans exposed to a model in infancy or early childhood stand the sharply different results for such late learners. In the usual case, these are second-language learners, and the level of their final attainment is inferior to infant learners as a direct negative function of maturational status at first exposure. However, the same generalizations apply to acquisition of a primary language late in life (Newport 1990). Apparently, these increasing decrements in language-acquisition skills over maturational time apply both to learning idiosyncratic details of the exposure language and to language universals, properties common across the languages of the world. Such "critical period" or "sensitive period" effects, though undeniable on the obtained evidence, cannot by themselves reveal just what is being lost or diminished in the late learner: This could be some aspects of learning specific to language itself, general capacities for structured cognitive learning, or some combination of the two (for discussion of the complex interweave of nature and nurture in accounting for critical period effects, see Marler 1991).
A large and informative literature considers tragedies of nature, populations of learners with conceptually or linguistically relevant deficits. The effect of these studies, taken as a whole, is to demonstrate, first, that linguistic and general-cognitive capacities are often dissociated in pathology. For example, in Williams syndrome (Bellugi et al. 1988), language-learning abilities are virtually unscathed but these children's IQs are severely below normal; in SLI (Specific Language Impairment; van der Lely 1997), children with normal and even superior IQs exhibit defects in the timing and content of language acquisition. The second overall finding from these unusual populations is that language deficiencies are not across-the-board but may affect only a single aspect. For example, both in SLI (Rice and Wexler 1996) and in Down's syndrome (Fowler 1990), word learning and some aspects of syntax (e.g., word order) are adequate, but there is a specific deficit for functional projections (i.e., knowledge of the interaction of verb morphology and syntactic structure).
To many theorists the bottom-line message from joint investigation of normal and unusual learning is this: Language acquisition is little affected by the ambiance in which learners find themselves, so long as they have intact young human brains; whereas any change in mentality (even: growing up!) massively, and negatively, impacts the learning process.
Beyond questions of nature-nurture in the acquisition process are more specific questions about acquisition functions for language at various levels in the linguistic hierarchy, including PHONOLOGY, SYNTAX, SEMANTICS, and word learning. At this moment in the progress of the field, interpretation of such results is limited to the extent that a number of viable theories of language design -- that is, of the targets of acquisition -- contend in the linguistic literature. Moreover, theoretical pressure to account for the emerging facts about language learning often motivates revision of the linguistic theory. One way to think about how language and its learning are jointly explored, then, is to contrast two different questions that investigators ask, depending on their orientations: "What is a child, such that it can learn language?" versus "What is language, such that every child can learn it?"
Particularly informative in adjudicating these issues are studies that contrast learning and language effects crosslinguistically (a classic collection of such articles appears in Slobin 1985). Another useful direction has been to consider acquisition across various levels of the linguistic hierarchy, to examine the extent of their interaction. For example, phonological and semantic knowledge underlie aspects of how children understand the syntax of a sentence; syntactic and morphological cues support the acquisition of word meaning; and so on. Recent technological developments are beginning to enable acquisition work in directions unforeseen and infeasible up until the last few years. One important example concerns child on-line language processing, an area that has remained largely closed to investigation (for an exception, see McKee 1996) until the advent of eyetracking equipment that can monitor the child's linguistic representations as these are constructed, in milliseconds, dependent on the reference world (Trueswell, Sekerina, and Hill 1998). Another is neuropsychological investigation employing techniques such as POSITRON EMISSION TOMOGRAPHY and functional MAGNETIC RESONANCE IMAGING (fMRI) recording of neural activity during linguistic processing. Finally, the increasing use of computer models and simulations allows explicit and detailed theories to be tested on large bodies of computerized linguistic data.
In light of the growing armamentarium of investigative technique and increasingly sophisticated linguistic analysis, understanding of language acquisition can be expected to increase rapidly in the coming decade. All current theoretical positions, no matter how different in other ways, acknowledge that language acquisition is the result of an interaction between aspects of the human mind and aspects of the environment: children learn language but rocks and dogs do not; children in France learn French and children in Mawu learn Mawukakan. Substantive debates in the present literature largely center on the nature of the learning mechanism. One issue is whether language is learned through a specialized organ or module or is the product of more general learning capacities. A further issue, logically distinct from the question of specialization, is whether the mechanisms of language acquisition and representation exploit symbolic rules, associations, or some combination of the two.
In conclusion, we should stress that language acquisition is a diverse process. It is extremely likely, for instance, that the right explanation for how children learn to form wh-questions will involve different cognitive mechanisms from those required for learning proper names or learning to take turns in conversation. We suspect, in fact, that there is no single story to be told about how children acquire language. Rather, the formatives, the learning machinery, and the combinatorial schemata at each level of the final system can be expected to vary. At the same time, knowledge of one aspect of the system can be expected to piggyback on the next in a complex incremental learning scheme, some of whose dimensions and procedures cannot now even be guessed at.
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