Developmental Psychology: Language

One of the most remarkable developmental feats of human children is the acquisition of language. Speaking and understanding are immensely complicated tasks that no other species has mastered. The question of how children acquire language is one that has occupied the careers of many developmental psychologists. Although that question remains far from completely answered, enormous progress in our understanding of the time course of acquisition and the factors that influence it has been made in the past half- century.

In the first section of this Topic, we sketch the major milestones in language acquisition, from exploratory babbling to the production of grammatically correct sentences. In the second section, we delve into the mechanisms that allow human infants to acquire language and the theories behind the psychology of language acquisition. In the third section, we discuss two interesting cases in which language acquisition does not follow the normal pathway: deafness and Williams syndrome.

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Terms

Babbling -Pre-speech vocalizations made by infants around nine months old.

Language Acquisition Device -A innate "mental module" postulated by Noam Chomsky to explain the ability of children to learn despite the "poverty of the stimulus."

Naming Burst - A dramatic increase in the rate of word learning that takes place around two to three years of age.

Overextension - When children use a word to refer to a class of objects that is broader than in adult usage. For instance, using "dog" to refer to all four- legged creatures.

Overgeneralization -When children apply a grammatical rule across all members of a grammatical class (e.g. verbs) without making the appropriate exceptions. For instance, using the -ed suffix to indicate past tense for verbs like "go" and "think."

Poverty of the Stimulus -Term used by Noam Chomsky to describe the impossibility of learning language from verbal input without some kind of innate constraints on the kind of language that can be learned.

Underextension - When children use a word to refer to a class of objects that is narrower than in adult usage. For instance, using "dog" to refer only to German shepherds.

Williams Syndrome- A genetic disorder that results in elfin-like facial characteristics, often severe mental retardation, and a surprising sociability and facility with language.

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Overview of Language Acquisition

1.1 Babbling

In the first nine months of life, infants are far from silent, as any sleep-deprived parent can attest. Around nine months of age, however, a significant change in the quality of sounds produced by the infant occurs. In addition to screaming, cooing, laughing, and other non-linguistic sounds, the child begins to utter speech-like sounds such as "ba" or "da," often in long nonsense strings of syllables. This "pre-linguistic speech" is called babbling. Babbling is thought to be the first stepping-stone to spoken language. By producing a variety of language-like sounds, the child is exercising his or her mental and vocal skills in a way that will provide the foundation for true speech.

1.2 Word Learning

Between nine months and one year of age, the child usually begins to produce whole words. At first, words are produced in isolation: the child will say "mama" in response to the presence of the mother or "cookie" to request a snack. Typically, the first words learned are monosyllabic; as the child grows, the phonological complexity of the words produced increases. In English, at least, nouns are learned earlier than verbs. Early in the word- learning process, children often make mistakes of overextension or underextension. In overextension, the child applies a word to a broader class of objects or actions than in adult usage. For instance, the word "dog" might be used to describe any four-legged animal, or the word "eat" might be used to describe both eating and drinking. Underextension is the exact opposite: the child applies the word to a smaller class of objects than he or she should. For instance, the word "cat" might be used to describe the family pet but not unfamiliar cats. Around eighteen months, the child begins to use multi-word utterances such as "give ball." Around two or three years, a "naming burst" occurs during which an enormous number of new words are acquired. One estimate is that one to two words are learned per waking hour between the ages of three and five.

1.3 Grammar

The problems of grammar first appear when the child begins to produce multi-word utterances and, in English and many other languages, to alter single words in order to indicate number (the –s ending in English), gender (the –e ending in French), tense (-ed in English), or other inflections of meaning. Just as in word learning, children frequently make errors of overgeneralization. For instance, when English-speaking children learn that the –ed ending indicates past tense, they tend to use it for all verbs, including those that are irregular and do not take the –ed ending in adult speech (such as "go" or "think"). Only after extensive practice with both the rule and its exceptions does the child learn to speak as an adult.

In English, word order is a crucial part of grammar and the learning of word order rules can be tracked as infants produce increasingly large numbers of multi-word utterances between the ages of two and five. Even two-word utterances in the second year include some kind of grammatical information: the phrase "hit Bob" means something different to a child than "Bob hit." Children appear to be able to produce this kind of difference as soon as they begin to produce multi-word utterances, and they comprehend it even earlier. This brings up an important point in language acquisition and other parts of developmental psychology: an inability to produce a certain behavior does not mean that the corresponding cognitive structures are absent. Children are able to understand grammatically complex sentences and words long before they are able to produce them.

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Theories of Language Acquisition

What are the basic mechanisms that make language possible? Is the kind of grammar that a child learns completely arbitrary, or is it constrained by biology? In this section we address these questions by describing Noam Chomsky's influential arguments about language learning.

2.1 "The Poverty of the Stimulus"

In the 1950's, a major debate raged between American psychologists who believed that language could be explained by the patterns of reinforcement that children received for their verbal behavior and psychologists who believed that behaviorist accounts could never satisfactorily explain language. B. F. Skinner and Noam Chomsky were the primary representatives of the two camps. Chomsky's scathing review of Skinner's book Verbal Behavior is commonly seen as the beginning of the "cognitive revolution" in psychology. In that review and in later works, Chomsky argued that several properties of language--primarily its "deep structure" and the fact that it can be used to generate an infinite number of different behaviors (sentences)- -made it impossible for the child to learn according to the rules of behaviorism. The fact that children did learn to speak, and with apparent ease, was a strong argument that 1) children were genetically endowed with biological structures that aided language acquisition and 2) behaviorism was wrong.

The "deep structure" of language is its basic grammatical categories and the ways in which they interact in particular sentences. Chomsky's argument against behaviorism was based on the fact that deep structure, while crucial to the correct use of language, is not directly perceivable. Chomsky called this the "poverty of the stimulus," by which he meant not that children receive very little linguistic input, which is untrue, but that no matter how much language children heard they could not learn it according to behavioristic rules. Even if children were to receive perfect feedback on their use of grammar--which is far from the actual case--it would be logically impossible for them to extract language's deep structure unless they were equipped with some innate constraints on what that structure could be.

2.2 The "Language Acquisition Device"

As a solution to the problem of the poverty of the stimulus, Chomsky suggested that each human child is equipped with a "language acquisition device" (LAD) that drives the child to acquire language and constrains the kinds of grammars that the child can learn. Evidence for something like a LAD comes from several sources: 1) children who are raised without a complete language develop a grammatical, full language of their own (such as Creole languages); 2) some family lines appear to have genetic impairments in the acquisition of grammar although the rest of their mental faculties are intact; 3) children with Williams syndrome are skilled at language despite often being severely mentally retarded; and 4) across cultures, languages seem to share a wide range of basic grammatical features.

This evidence is convincing but not conclusive. The original claim that there was a dedicated module for language acquisition has been tempered somewhat in recent years, for several reasons. First, closer studies of people with specific language impairments or specific language enhancements (such as in Williams syndrome) have shown that other aspects of cognition are invariably affected as well. Language is part of a large system of cognitive abilities, and it is likely that many of the aspects of the mind that make language possible also make non-linguistic aspects of thought possible as well. Second, children receive a wide variety of linguistic and non-linguistic input in their years of language acquisition that make the task of acquisition, although still formidable, less difficult than it may seem at first. The existence of some complex and powerful learning mechanisms is obviously necessary, but their specificity to language is not clear.

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Special Cases: Deafness and Williams Syndrome

As we noted in the introduction to this Topic, language acquisition is a remarkable achievement that most children appear to accomplish with ease. For some children, however, language acquisition is made more difficult by physical or cognitive impairments. In this section we briefly discuss two special populations within which language acquisition follows a different pathway than it does in most children. Both of these cases illustrate important points about language acquisition in general.

3.1 Deafness

Deaf children have an obvious disadvantage when it comes to learning spoken language: they can't hear it. For children who are born with complete hearing loss, the journey to spoken language production and comprehension is difficult if not impossible. However, there is another pathway toward language that illustrates the remarkable plasticity of the human brain: sign language. Sign language is a complete linguistic system--or, more precisely, a variety of linguistic systems, some of which differ from each other as much as English does from Japanese--based on gestures of the face, arms, hands, and upper body. Children who are raised in environments where they are frequently exposed to sign language learn it just as rapidly and successfully as hearing children learn spoken language. In some cases, they may even learn to express basic signed words earlier than hearing children produce spoken words. Even more amazing, there is evidence that deaf children who are raised together in the absence of an established signed language come to develop one of their own. What is the significance of this for our understanding of language acquisition more generally? First, language is not tied to audition and vocal production; it can function just as well through the medium of vision and gesture. Second, even in the absence of linguistic input children appear to have a drive towards language acquisition--what Steven Pinker has called, in his book of the same name, "the language instinct."

3.2 Williams Syndrome

Williams syndrome is an extremely rare genetic disorder that is characterized by "elfin" facial characteristics, mental retardation (IQs in the range of 40-80), sociability, and high levels of linguistic functioning. This strange mixture of abilities-- particularly the paradox of very low cognitive functioning and very high linguistic functioning--has led some researchers to view Williams syndrome children as evidence for a specific, genetically-determined language module. A more moderate assessment of the evidence suggests that Williams syndrome children show an abnormal pattern of linguistic functioning that may reflect preserved cognitive abilities in a few select areas-- just those areas that are crucial for language acquisition and production. What can we learn about language acquisition from children with Williams syndrome? First, language and cognition are not inextricably bound together; it is likely that certain cognitive functions are necessary for language but that others are completely irrelevant. Second, genes play an important role in providing children with the basic machinery for language acquisition.