Document Page 135 Â Pinker, S.), An Invitation to Cognitive Science, 2nd edition: Language (pp. Chapter 6 Language Acquisition Steven Pinker 6.1 Introduction Language acquisition is one of the central topics in cognitive science. Every theory of cognition has tried to explain it; probably no other topic has aroused such controversy. Possessing a language is the quintessentially human trait: all normal humans speak, no nonhuman animal does.
Language is the main vehicle by which we know about other people's thoughts, and the two must be intimately related. Every time we speak we are revealing something about language, so the facts of language structure are easy to come by; these data hint at a system of extraordinary complexity. Nonetheless, learning a first language is something every child does successfully in a matter of a few years and without the need for formal lessons. With language so close to the core of what it means to be human, it is not surprising that children's acquisition of language has received so much attention.
Anyone with strong views about the human mind would like to show that children's first few steps are steps in the right direction. Language acquisition is not only inherently interesting; studying it is one way to look for concrete answers to questions that permeate cognitive science: 6.1 Modularity Do children learn language using a "mental organ," some of whose principles of organization are not shared with other cognitive systems such as perception, motor control, and reasoning (Chomsky 1975, 1991; Fodor 1983)? Or is language acquisition just another problem to be solved by general intelligence, in this case the problem of how to communicate with other humans over the auditory channel (Putnam 1971; Bates 1989)? Preparation of the chapter was supported by NIH grant HD 18381 and NSF grant BNS 91-09766, and by the McDonnell-Pew Center for Cognitive Neuroscience at MIT.html [1/15/2005 5:44:28 PM] Document Page 136 Â 6.2 Human Uniqueness A related question is whether language is unique to humans. At first glance the answer seems obvious. Other animals communicate with a fixed repertoire of signals, or with analogue variation like the mercury in a thermometer.
But none appears to have the combinatorial rule system of human language, in which symbols are permuted into an unlimited set of combinations, each with a determinate meaning. On the other hand, many other claims about human uniqueness, such as that humans were the only animals to use tools or to fabricate them, have turned out to be false. Some researchers have thought that apes have the capacity for language but never profited from a humanlike cultural milieu in which language was taught, and they have thus tried to teach apes languagelike systems. Whether they have succeeded, and whether human children are really "taught" language themselves, are questions we will soon come to.3 Language and Thought Is language simply grafted on top of cognition as a way of sticking communicable labels onto thoughts (Fodor 1975; Piaget 1926)? Or does learning a language somehow mean learning to think in that language? A famous hypothesis, outlined by Benjamin Whorf (1956), asserts that the categories and relations we use to understand the world come from our particular language, so that speakers of different languages conceptualize the world in different ways.
Language acquisition, then, would be learning to think, not just learning to talk. This is an intriguing hypothesis, but virtually all modern cognitive scientists believe that it is false (see Pinker 1994a). Babies can think before they can talk (chapter 1 and chapter 8 of volume 2.) Cognitive psychology has shown that people think not just in words but in images (see chapter 7 of volume 2) and abstract logical propositions (see chapter 12). And linguistics has shown that human languages are too ambiguous and schematic to use as a medium of internal computation; when people think about "spring," surely they are not confused as to whether they are thinking about a season or something that goes "boing"—and if one word can correspond to two thoughts, thoughts cannot be words.
But language acquisition has a unique contribution to make to this issue. As we shall see, it is virtually impossible to show how children could learn a language unless one assumes that they have a considerable amount of nonlinguistic cognitive machinery in place before they start.4 Learning and Innateness All humans talk but no house pets or house plants do, no matter how pampered, so heredity must be involved in language. But a child growing http://127.html [1/15/2005 5:44:29 PM] Document Page 137 Â up in Japan speaks Japanese, whereas the same child brought up in California would speak English, so the environment is also crucial. Thus, there is no question about whether heredity or environment is involved in language or even whether one or the other is "more important." Instead, language acquisition might be our best hope of finding out how heredity and environment interact.
We know that adult language is intricately complex, and we know that children become adults; therefore, something in the child's mind must be capable of attaining that complexity. Any theory that posits too little innate structure, so that its hypothetical child ends up speaking something less than a real language, must be false. The same is true for any theory that posits too much innate structure, so that the hypothetical child can acquire English but not, say, Bantu or Vietnamese. And not only do we know about the output of language acquisition, we know a fair amount about the input to it, namely, parents' speech to their children.
So even if language acquisition, like all cognitive processes, is essentially a "black box," we know enough about its input and output to be able to make precise guesses about its contents. The scientific study of language acquisition began around the same time as the birth of cognitive science, in the late 1950s. We can see now why that is not a coincidence. The historical catalyst was Noam Chomsky's review of Skinner's Verbal Behavior (Chomsky 1959).
At that time Anglo-American natural science, social science, and philosophy had come to a virtual consensus about the answers to the questions listed above. The mind consisted of sensorimotor abilities plus a few simple laws of learning governing gradual changes in an organism's behavioral repertoire. Language, therefore, must be learned, it cannot be a module, and thinking must be a form of verbal behavior, since verbal behavior is the prime manifestation of "thought" that can be observed externally. Chomsky argued that language acquisition falsified these beliefs in a single stroke: Children learn languages that are governed by highly subtle and abstract principles, and they do so without explicit instruction or any other environmental clues to the nature of such principles.
Hence, language acquisition depends on an innate, species-specific module that is distinct from general intelligence. Much of the debate in language acquisition has attempted to test this once-revolutionary, and still controversial, collection of ideas. The implications extend to the rest of human cognition.2 The Biology of Language Acquisition Human language is made possible by special adaptations of the human mind and body that occurred in the course of human evolution and which are put to use by children in acquiring their mother tongue.html [1/15/2005 5:44:31 PM] Document Page 138 Â 6.1 Evolution of Language Most obviously, the shape of the human vocal tract seems to have been modified in evolution for the demands of speech. Our larynxes are low in our throats, and our vocal tracts have a sharp right-angle bend that creates two independently modifiable resonant cavities (the mouth and the pharynx or throat) which define a large two-dimensional range of vowel sounds (see the chapter by Liberman in this volume).
But it comes at a sacrifice of efficiency for breathing, swallowing, and chewing (Lieberman 1984). Before the invention of the Heimlich maneuver, choking on food was a common cause of accidental death in humans, causing six thousand deaths a year in the United States. The evolutionary selective advantages for language must have been very large to outweigh such a disadvantage. It is tempting to think that if language evolved by gradual Darwinian natural selection, we must be able to find some precursor of it in our closest relatives, the chimpanzees.
In several famous and controversial demonstrations, chimpanzees have been taught some hand-signs based on American Sign Language, to manipulate colored switches or tokens, or to understand some spoken commands (Gardner and Gardner 1969; Premack and Premack 1983; Savage-Rumbaugh 1991). Whether one wants to call these abilities "language" is not really a scientific question but a matter of definition: how far we are willing to stretch the meaning of the word language. The scientific question is whether the chimps' abilities are homologous to human language—that is, whether the two systems show the same basic organization owing to descent from a single system in their common ancestor. For example, biologists do not debate whether the winglike structures of gliding rodents may be called "genuine wings" or something else (a boring question of definitions).
It is clear that these structures are not homologous to the wings of bats, because they have a fundamentally different anatomical plan, reflecting a different evolutionary history. Bats' wings are modifications of the hands of the common mammalian ancestor; flying squirrels' wings are modifications of its rib cage. The two structures are merely analogous: similar in function. Though artificial chimp signaling systems have some analogies to human language (for example, use in communication, combinations of more basic signals), it seems unlikely that they are homologous.
Chimpanzees require massive regimented teaching sequences contrived by humans to acquire quite rudimentary abilities, mostly limited to a small number of signs, strung together in repetitive, quasi- random sequences, used with the intent of requesting food or tickling (Terrace, Petitto, Sanders, and Bever 1979; Seidenberg and Petitto 1979, 1987; Seidenberg 1986; Wallman 1992; Pinker 1994a). This contrasts sharply with human children, who pick up thousands of words spontaneously, combine them in structured http://127.html [1/15/2005 5:44:32 PM] Document Page 139 Â sequences where every word has a determinate role, respect the word order of the adult language, and use sentences for a variety of purposes such as commenting on interesting events. This lack of homology does not, by the way, cast doubt on a gradualistic Darwinian account of language evolution. Humans did not evolve directly from chimpanzees.
Both derived from a common ancestor, probably around six or seven million years ago. This leaves about 300,000 generations in which language could have evolved gradually in the lineage leading to humans, after it split off from the lineage leading to chimpanzees. Presumably, language evolved in the human lineage for two reasons: Our ancestors developed technology and knowledge of the local environment in their lifetimes, and they were involved in extensive reciprocal cooperation. This allowed them to benefit by sharing hard-won knowledge with their kin and exchanging it with their neighbors (Pinker and Bloom 1990).2 Dissociations between Language and General Intelligence Humans evolved brain circuitry, mostly in the left hemisphere surrounding the sylvian fissure, that appears to be designed for language, though how exactly its internal wiring gives rise to rules of language is unknown (see the chapter by Zurif in this volume).
The brain mechanisms underlying language are not just those allowing us to be smart in general. Strokes often leave adults with catastrophic losses in language (see the chapter by Zurif; also Pinker 1994a), though not necessarily impaired in other aspects of intelligence, such as those measured on the nonverbal parts of IQ tests. Similarly, there is an inherited set of syndromes called Specific Language Impairment (Gopnik and Crago 1993; Tallal, Ross, and Curtiss 1989), which is marked by delayed onset of language, difficulties in articulation in childhood, and lasting difficulties in understanding, producing, and judging grammatical sentences. By definition, specifically language impaired people show such deficits despite the absence of cognitive problems like retardation, sensory problems like hearing loss, and social problems like autism.