Most social species have systems of communication that allow them to transmit messages to each other. Honeybees communicate the location of food sources by means of a “waggle dance” that indicates both the direction and distance of the food source from the hive (Kirchner & Towne, 1994; Von Frisch, 1974). Vervet monkeys have three different warning calls that uniquely signal the presence of their main predators: a leopard, an eagle, and a snake (Cheney & Seyfarth, 1990). A leopard call provokes them to climb into a tree; an eagle call makes them look up into the sky. Each different warning call conveys a particular meaning and functions like a word in a simple language.

Language is a system for communicating with others using signals that are combined according to rules of grammar and convey meaning. Grammar is a set of rules that specify how the units of language can be combined to produce meaningful messages. The complex structure of human language distinguishes it from simpler signaling systems used by other species; it also allows us to express a wide range of ideas and concepts, including intangible concepts, such as unicorn or democracy.

Compared with other forms of communication, human language is a relatively recent evolutionary phenomenon, emerging as a spoken system no more than 1 to 3 million years ago and as a written system as recently as 6,000 years ago. There are approximately 4,000 human languages, which linguists have grouped into about 50 language families (Nadasdy, 1995). Despite their differences, all of these languages share a basic structure involving a set of sounds and rules for combining those sounds to produce meanings.

Basic Characteristics

The smallest units of sound that are recognizable as speech rather than as random noise are phonemes. These building blocks of spoken language differ in how they are produced. For example, when you say ba, your vocal cords start to vibrate as soon as you begin the b sound, but when you say pa, there is a 60-millisecond lag between the time you start the p sound and the time your vocal cords start to vibrate.

Every language has phonological rules that indicate how phonemes can be combined to produce speech sounds. For example, the initial sound ts is acceptable in German but not in English. Typically, people learn these phonological rules without instruction, and if the rules are violated, the resulting speech sounds so odd that we describe it as speaking with an accent.

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Phonemes are combined to make morphemes, the smallest meaningful units of language (see FIGURE 9.1). For example, your brain recognizes the p sound you make at the beginning of pat as a speech sound, but it carries no particular meaning. The morpheme pat, on the other hand, is recognized as an element of speech that carries meaning. Morphological rules are a set of rules that indicate how morphemes can be combined to form words. Some morphemes—content morphemes and function morphemes—can stand alone as words. Content morphemes refer to things and events (e.g., “cat,” “dog,” “take”). Function morphemes serve grammatical functions, such as tying sentences together (“and,” “or,” “but”) or indicating time (“when”). About half of the morphemes in human languages are function morphemes, and it is the function morphemes that make human language grammatically complex enough to permit us to express abstract ideas.

Words can be combined and recombined to form an infinite number of new sentences, which are governed by syntactical rules, a set of rules that indicate how words can be combined to form phrases and sentences. A simple syntactical rule in English is that every sentence must contain one or more nouns and one or more verbs (see FIGURE 9.2). So, the utterance “dogs bark” is a full sentence, but “the big gray dog over by the building” is not.

Meaning: Deep Structure versus Surface Structure

Language usually conveys meaning quite well, but everyday experience shows us that misunderstandings can occur. These errors sometimes result from differences between the deep structure of sentences and their surface structure (Chomsky, 1957). Deep structure refers to the meaning of a sentence. Surface structure refers to how a sentence is worded. “The dog chased the cat” and “The cat was chased by the dog” mean the same thing (they have the same deep structure) even though on the surface, their structures are different.

To generate a sentence, you begin with a deep structure (the meaning of the sentence) and create a surface structure (the particular words) to convey that meaning. When you comprehend a sentence, you do the reverse, processing the surface structure in order to extract the deep structure. After the deep structure is extracted, the surface structure is usually forgotten (Jarvella, 1970, 1971). In one study, researchers played tape-recorded stories to volunteers and then asked them to pick the sentences they had heard (Sachs, 1967). Participants frequently confused sentences they heard with sentences that had the same deep structure but a different surface structure. For example, if they heard the sentence “He struck John on the shoulder,” they often mistakenly claimed they had heard “John was struck on the shoulder by him.” In contrast, they rarely misidentified “John struck him on the shoulder” because this sentence has a different deep structure from the original sentence.

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Language is a complex cognitive skill, yet we can carry on complex conversations with playmates and family before we begin school. Three characteristics of language development are worth bearing in mind. First, children learn language at an astonishingly rapid rate. The average 1-year-old has a vocabulary of 10 words, which expands to more than 10,000 words in the next 4 years, requiring the child to learn, on average, about 6 or 7 new words every day. Second, children make few errors while learning to speak, and as we’ll see shortly, the errors they do make usually result from applying, but overgeneralizing, grammatical rules they’ve learned. Third, children’s passive mastery of language (ability to understand) develops faster than their active mastery (ability to speak). At every stage of language development, children understand language better than they speak.

Distinguishing Speech Sounds

At birth, infants can distinguish all the sounds that occur in all human languages. Within the first 6 months of life, they lose this ability, and, like their parents, can only distinguish among the contrasting sounds in the language they hear being spoken around them. For example, two distinct sounds in English are the l sound and the r sound, as in lead and read. These sounds are not distinguished in Japanese; instead, the l and r sounds fall within the same phoneme. Japanese adults cannot hear the difference between these two phonemes, but American adults can distinguish between them easily—and so can Japanese infants.

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In one study, researchers constructed a tape of a voice saying “la-la-la” or “ra-ra-ra” repeatedly (Eimas et al., 1971). They rigged a pacifier so that whenever an infant sucked on it, a tape player that broadcast the la-la tape was activated. When the la-la sound began playing in response to their sucking, the infants were delighted and kept sucking on the pacifier to keep the la-la sound playing. After a while, they began to lose interest, and sucking frequency declined to about half of its initial rate. At this point, the experimenters switched the tape so that ra-ra was repeatedly broadcast. The Japanese infants began sucking again with vigor, indicating that they could hear the difference between the old, boring la sound and the new, interesting ra sound.

Infants can distinguish among speech sounds, but they cannot produce them reliably, relying mostly on cries, laughs, and other vocalizations to communicate. Between the ages of about 4 and 6 months, they begin to babble speech sounds. Babbling involves combinations of vowels and consonants that sound like real syllables but are meaningless. Regardless of the language they hear spoken, all infants go through the same babbling sequence. For example, d and t appear in infant babbling before m and n. Even deaf infants babble sounds they’ve never heard, and they do so in the same order as hearing infants do (Ollers & Eilers, 1988). This is evidence that infants aren’t simply imitating the sounds they hear and suggests that babbling is a natural part of the language development process. Recent research has shown that babbling serves as a signal that the infant is in a state of focused attention and ready to learn (Goldstein et al., 2010).

In order for vocal babbling to continue, however, infants must be able to hear themselves. In fact, delayed babbling or the cessation of babbling merits testing for possible hearing difficulties. Babbling problems can lead to speech impairments, but they do not necessarily prevent language acquisition. Deaf infants whose parents communicate using American Sign Language (ASL) begin to babble with their hands at the same age that hearing children begin to babble vocally—between 4 and 6 months (Petitto & Marentette, 1991). Their babbling consists of sign language syllables that are the fundamental components of ASL.

Language Milestones

At about 10 to 12 months of age, infants begin to utter (or sign) their first words. By 18 months, they can say about 50 words and can understand several times more than that. Toddlers generally learn nouns before verbs, and the nouns they learn first are names for everyday, concrete objects (e.g., chair, table, milk) (see TABLE 9.1). At about this time, their vocabularies undergo explosive growth. By the time the average child begins school, a vocabulary of 5000 words is not unusual. By college, the average student’s vocabulary is about 10,000–15,000 words. Fast mapping, in which children map a word onto an underlying concept after only a single exposure, enables them to learn at this rapid pace (Kan & Kohnert, 2008; Mervis & Bertrand, 1994). This astonishingly easy process contrasts dramatically with the effort required later to learn other concepts and skills, such as arithmetic or writing.

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Around 24 months, children begin to form two-word sentences, such as “more milk” or “throw ball.” Such sentences are referred to as telegraphic speech because they are devoid of function morphemes and consist mostly of content words. Yet despite the absence of function words, these two-word sentences tend to be grammatical; the words are ordered in a manner consistent with the syntactical rules of the language children are learning to speak. So, for example, toddlers will say, “throw ball” rather than “ball throw” when they want you to throw the ball to them, and they will say, “more milk” rather than “milk more” when they want you to give them more milk. With these seemingly primitive expressions, 2-year-olds show an appreciation of the syntactical rules of the language they are learning.

The Emergence of Grammatical Rules

If you listen to average 2- or 3-year-old children speaking, you may notice that they use the correct past-tense versions of common verbs, as in the expressions “I ran” and “you ate.” By the age of 4 or 5, the same children will be using incorrect forms of these verbs, saying such things as “I runned” or “you eated,” forms most children are unlikely to have ever heard (Prasada & Pinker, 1993). The reason is that very young children memorize the particular sounds (i.e., words) that express what they want to communicate. But as children acquire the grammatical rules of their language, they tend to overgeneralize. For example, if a child overgeneralizes the rule that past tense is indicated by -ed, then run becomes runned or even ranned instead of ran.

These errors show that language acquisition is not simply a matter of imitating adult speech. Instead, children acquire grammatical rules by listening to the speech around them and using the rules to create verbal forms they’ve never heard. They manage this without explicit awareness of the grammatical rules they’ve learned. In fact, few children or adults can articulate the grammatical rules of their native language, yet the speech they produce obeys these rules.

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By about 3 years of age, children begin to generate complete simple sentences that include function words (e.g., “Give me the ball” and “That belongs to me”). The sentences increase in complexity over the next 2 years. By 4 to 5 years of age, many aspects of the language acquisition process are complete. As children continue to mature, their language skills become more refined, with added appreciation of subtler communicative uses of language, such as humor, sarcasm, or irony.

Language Development and Cognitive Development

Language development typically unfolds as a sequence of steps in which one milestone is achieved before moving on to the next. Nearly all infants begin with one-word utterances before moving on to telegraphic speech and then to simple sentences that include function morphemes. This orderly progression could result from general cognitive development that is unrelated to experience with a specific language (Shore, 1986; Wexler, 1999). For example, perhaps infants begin with one- and then two-word utterances because their short-term memories are so limited that initially they can only hold in mind a word or two; additional cognitive development might be necessary before they have the capacity to put together a simple sentence. By contrast, the orderly progression might depend on experience with a specific language, reflecting a child’s emerging knowledge of that language (Bates & Goodman, 1997; Gillette et al., 1999).

To tease apart these possibilities, researchers examined the acquisition of English by internationally adopted children who did not know any English prior to adoption (Snedeker, Geren, & Shafto, 2007, 2012). If the orderly sequence of milestones that characterizes the acquisition of English by infants is a by-product of general cognitive development, then different patterns should be observed in older internationally adopted children, who are more advanced cognitively than infants. However, if the milestones of language development are critically dependent on experience with a specific language—English—then language learning in older adopted children should show the same orderly progression as seen in infants. The main result was clear-cut: Language acquisition in preschool-aged adopted children showed the same orderly progression of milestones that characterizes infants. These children began with one-word utterances before moving on to simple word combinations. Furthermore, their vocabularies, just like those of infants, were initially dominated by nouns, and these children produced few function morphemes. These results indicate that some of the key milestones of language development depend on experience with English.

We know a good deal about how language develops, but what underlies the process? The language acquisition process has been the subject of considerable controversy and (at times) angry exchanges among scientists coming from three different approaches: behaviorist, nativist, and interactionist.

Behaviorist Explanations

According to B. F. Skinner’s behaviorist explanation of language learning, we learn to talk in the same way we learn any other skill: through reinforcement, shaping, extinction, and the other basic principles of operant conditioning that you learned about in the Learning chapter (Skinner, 1957). As infants mature, they begin to vocalize. Those vocalizations that are not reinforced gradually diminish, and those that are reinforced remain in the developing child’s repertoire. So, for example, when an infant gurgles “prah,” most parents are pretty indifferent. However, “da-da” is likely to be reinforced with smiles, whoops, and cackles of “Goooood baaaaaby!” by doting parents. Maturing children also imitate the speech patterns they hear. Then parents or other adults shape those speech patterns by reinforcing those that are grammatical and ignoring or punishing those that are ungrammatical. “I no want milk” is likely to be squelched by parental clucks and titters, whereas “No milk for me, thanks” will probably be reinforced.

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The behavioral explanation is attractive because it offers a simple account of language development, but it cannot account for many fundamental characteristics of language development (Chomsky, 1986; Pinker, 1994; Pinker & Bloom, 1990).

Nativist Explanations

In a blistering reply to Skinner’s behaviorist approach, linguist Noam Chomsky (1957, 1959) argued that language-learning capacities are built into the human brain and are separate from general intelligence. This nativist theory is the view that language development is best explained as an innate, biological capacity. According to the nativist view, studies of people with genetic dysphasia suggest that normal children learn the grammatical rules of human language with ease in part because they are “wired” to do so.

The story of Christopher, whom you met earlier in the chapter, is consistent with the nativist view of language development: His genius for language acquisition, despite his low overall intelligence, indicates that language capacity can be distinct from other mental capacities. Other individuals show the opposite pattern: People with normal or nearly normal intelligence can find certain aspects of human language difficult or impossible to learn. This condition is known as genetic dysphasia, a syndrome characterized by an inability to learn the grammatical structure of language despite having otherwise normal intelligence. For example, when asked to describe what she did over the weekend, one child wrote, “On Saturday I watch TV.” Her teacher corrected the sentence to “On Saturday, I watched TV,” drawing attention to the -ed rule for describing past events. The following week, the child was asked to write another account of what she did over the weekend. She wrote, “On Saturday I wash myself and I watched TV and I went to bed.” Notice that although she had memorized the past-tense forms watched and went, she could not generalize the rule to form the past tense of another word (washed).

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Also consistent with the nativist view is evidence that language can be acquired only during a restricted period of development. This was dramatically illustrated by the tragic case of Genie (Curtiss, 1977). At the age of 20 months, Genie was tied to a chair by her parents and kept in virtual isolation. Her father forbade Genie’s mother and brother to speak to her, and he himself only growled and barked at her. She remained in this brutal state until she was rescued at the age of 13. Genie’s life improved substantially, and she received years of language instruction, but it was too late. Her language skills remained extremely primitive. She developed a basic vocabulary and could communicate her ideas, but she could not grasp the grammatical rules of English.

Less dramatic evidence for a restricted period in human learning comes from studies of language acquisition in immigrants. In one study, the proficiency with which immigrants spoke English depended not on how long they’d lived in the United States, but on their age at immigration, with those who had arrived as children being more proficient than those who arrived after puberty (Johnson & Newport, 1989). More recent work using fMRI shows that acquiring a second language early in childhood (between 1 and 5 years of age) results in very different representation of that language in the brain than does acquiring that language much later (after 9 years of age; Bloch et al., 2009).

Interactionist Explanations

Nativist theories are often criticized because they do not explain how language develops; they merely explain why. The interactionist approach holds that although infants are born with an innate ability to acquire language, social interactions also play a crucial role in language. Interactionists point out that parents tailor their verbal interactions with children in ways that simplify the language acquisition process: They speak slowly, enunciate clearly, and use simpler sentences than they do when speaking with adults (Bruner, 1983; Farrar, 1990).

Further evidence of the interaction of biology and experience comes from a fascinating study of deaf children’s creation of a new language (Senghas, Kita, & Ozyurek, 2004). Prior to about 1980, deaf children in Nicaragua stayed at home and usually had little contact with other deaf individuals. In 1981, some deaf children began to attend a new school. At first, the school did not teach a formal sign language, and none of the children had learned to sign at home, but the children gradually began to communicate using hand signals that they invented. Initially, the gestures were simple, but over the past three decades, the Nicaraguan sign language has developed considerably, and it now contains many of the same features as more mature languages, including signs to describe separate components of complex concepts (Pyers et al., 2010). These acts of creation nicely illustrate the interplay of nativism (the predisposition to use language) and experience (growing up in an insulated deaf culture).

In early infancy, language processing is distributed across many areas of the brain. But as the brain matures, language processing gradually becomes more and more concentrated in two areas, Broca’s area and Wernicke’s area. Broca’s area is located in the left frontal cortex and is involved in the production of the sequential patterns in vocal and sign languages (see FIGURE 9.3). Wernicke’s area, located in the left temporal cortex, is involved in language comprehension (whether spoken or signed).

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Together, Broca’s area and Wernicke’s area are sometimes referred to as the language centers of the brain; damage to them results in a serious condition called aphasia, difficulty in producing or comprehending language. As you saw in the Psychology: Evolution of a Science chapter, patients with damage to Broca’s area understand language relatively well, although they have increasing comprehension difficulty as grammatical structures get more complex. But their real struggle is with speech production. Typically, they speak in short, staccato phrases that consist mostly of content morphemes: “Ah, Monday, uh, Casey park. Two, uh, friends, and, uh, 30 minutes.” On the other hand, patients with damage to Wernicke’s area can produce grammatical speech, but it tends to be meaningless: “Feel very well. In other words, I used to be able to work cigarettes. I don’t know how. Things I couldn’t hear from are here.”

As important as Broca’s and Wernicke’s areas are for language, they are not the entire story. A number of neuroimaging studies have revealed evidence of right-hemisphere activation during language tasks, and individuals with damage to the right hemisphere sometimes have subtle problems with language comprehension. These studies indicate that not all language processing is limited to the left hemisphere.

Brain changes also appear to account for the fact that children who are fluent in two languages score higher than monolingual children on several measures of cognitive functioning, including executive control capacities such as the ability to prioritize information and flexibly focus attention (Bialystok, 1999, 2009; Bialystok, Craik, & Luk, 2012). The idea here is that bilingual individuals benefit from exerting executive control in their daily lives when they attempt to suppress the language that they don’t want to use. Research shows that learning a second language produces lasting changes in the brain (Mechelli et al., 2004; Stein et al., 2009). For example, the gray matter in a part of the left parietal lobe that is involved in language is denser in bilingual than in monolingual individuals, and the increased density is most pronounced in those who are most proficient in using their second language (Mechelli et al., 2004). (Some other benefits of bilingualism are noted in the Other Voices box.)

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