Lifespan Development. Tara L. Kuther

Words Known as a Function of Time for Individual Children

      Source: Samuelson & McMurray 2017.

      Fast mapping improves with age and accounts for the naming explosion, or vocabulary spurt—a period of rapid vocabulary learning that occurs between 16 and 24 months of age (Owens, 2016). During this period, infants apply their word-learning strategies to learn multiple words of varying difficulty seemingly at once. Within weeks, a toddler may increase her vocabulary from 50 words to over 400 (Bates, Bretherton, & Snyder, 1988). As shown in Figure 5.9, however, infants vary in the speed of word acquisition, with some showing a rapid increase in vocabulary before others (Samuelson & McMurray, 2017). In addition, although fast mapping helps young children learn many new words, their own speech lags behind what they can understand because young children have difficulty retrieving words from memory (McMurray, 2007). The speed at which young children acquire words during the vocabulary spurt predicts the size of their vocabulary as preschoolers at 54 months of age (Rowe, 2012). That is, children who rapidly expand their knowledge of words during the vocabulary spurt tend to have larger vocabularies in preschool than their peers who acquire new words at a slower pace.

      As children learn words, we see two interesting kinds of mistakes that tell us about how words are acquired (Gershkoff-Stowe, 2002). Underextension refers to applying a word more narrowly than it is usually applied so that the word’s use is restricted to a single object. For example, cup might refer to Daddy’s cup but not to the general class of cups. Later, the opposite tendency appears. Overextension refers to applying a word too broadly. Cow might refer to cows, sheep, horses, and all farm animals. Overextension suggests that the child has learned that a word can signify a whole class of objects. As children develop a larger vocabulary and get feedback on their speech, they demonstrate fewer errors of overextension and underextension (Brooks & Kempe, 2014).

      Two-Word Utterances

      At about 21 months of age, or usually about 8 to 12 months after they say their first word, most children compose their first simple two-word sentences, such as “Kitty come,” or “Mommy milk.” Telegraphic speech, like a telegram, includes only a few essential words. Like other milestones in language development, telegraphic speech is universal among toddlers. Children around the world use two-word phrases to express themselves.

      Language development follows a predictable path, as shown in Table 5.3. Between 20 and 30 months of age, children begin to follow the rules for forming sentences in a given language. Soon they become more comfortable with using plurals, past tense, articles (such as a and the), prepositions (such as in and on), and conjunctions (such as and and but). By 2½ years of age, children demonstrate an awareness of the communicative purpose of speech and the importance of being understood (Owens, 2016). In one experiment, 2½-year-old children asked an adult to hand them a toy. A child was more likely to repeat and clarify the request for the toy when the adult’s verbal response indicated misunderstanding of the child’s request (“Did you say to put the toy on the shelf?”) than when the adult appeared to understand the request, regardless of whether the adult gave the child the toy (Shwe & Markman, 1997).

Table 5.3

      Theories of Language Development

      Over the first 2 years of life, children transform from wailing newborns who communicate their needs through cries to toddlers who can use words to articulate their needs, desires, and thoughts. Developmental scientists have offered several explanations for infants’ rapid acquisition of language. Some explanations emphasize the role of the environment in accounting for language, whereas others emphasize biological factors.

      Learning Theory and Language Development

      Baby Howie gurgles, “Babababa!” His parents encourage him excitedly, “Say bottle; ba-ba!” Howie squeals, “Babababa!” “Yes! You want your ba-ba!” Parents play an important role in language development. They provide specific instruction and communicate excitement about their infants’ developing competence, encouraging infants to practice new language skills. Learning theorist B. F. Skinner (1957) proposed that language, just like all other behaviors, is learned through operant conditioning: reinforcement and punishment. From birth, infants make sounds at random. Caregivers respond to infants’ early utterances with interest and attention, imitating and reinforcing their verbal behavior (Pelaez, Virues-Ortega, & Gewirtz, 2011; Petursdottir & Mellor, 2017). Infants repeat the sounds. Caregivers then reward sounds that resemble adult speech with attention, smiles, and affection. Infants imitate sounds that adults make and repeat sounds that are reinforced. From this perspective, imitation and reinforcement shape children’s language development. The quantity and quality of the parents’ verbal interactions with the child and responses to the child’s communication attempts influence the child’s rate of language development (Hoff, 2015).

      In the view of learning theorists, then, infants learn by observing the world around them and adults encourage and reinforce their language learning. However, critics point out that learning theory cannot account for all of language development because it cannot account for the unique utterances and errors that young children make (Berwick, Chomsky, & Piattelli-Palmarini, 2013). Word combinations are complex and varied—they cannot be acquired solely by imitation and reinforcement. Toddlers often put words together in ways that they likely have never heard (e.g., “Mommy milk”). Young children make grammatical errors, such as “mouses” instead of “mice” or “goed” instead of “went,” that cannot be the result of imitation. If language is learned through imitation, how do young children make grammatical errors that they have never heard spoken? Young children repeat things that they hear (sometimes to parents’ chagrin!), but they also construct new phrases and utterances that are unique. Reinforcement from parents and caregivers is powerful encouragement for children, but language development cannot be completely explained by learning theory alone. Despite wide variations in circumstances, living situations, and contexts, infants around the world achieve language milestones at about the same ages, suggesting a biological component to language development.

      Nativist Theory and Language Development

      Nativist theorist Noam Chomsky (Chomsky, 1959, 2017) argued that language use comprises behavior that is too complex to be learned so early and quickly via conditioning alone. Chomsky noted that all young children grasp the essentials of grammar, the rules of language, at an early age and that the languages of the world have many similarities. The human brain thus has an innate capacity to learn language. Specifically, Chomsky believed that infants are born with a language acquisition device (LAD), an innate facilitator of language that permits infants to quickly and efficiently analyze everyday speech and determine its rules, regardless of whether their native language is English, German, Chinese, or Urdu (Yang, Crain, Berwick, Chomsky, & Bolhuis, 2017). The LAD has an innate storehouse of rules, universal grammar, that apply to all human languages. When infants hear language spoken, they naturally notice its linguistic properties, and they acquire the language. Language, therefore, is a biologically driven cognitive mechanism of brain development that is triggered by exposure to language (Friederici, 2017).

      The nativist perspective can account for children’s unique utterances and the unusual grammatical mistakes they make in speaking because children are biologically primed to learn language and do not rely on learning. However, like learning theory, the nativist perspective offers an incomplete account of language development. Specifically, Chomsky’s nativist perspective does not explain the process of language development and how it occurs (Ibbotson & Tomasello, 2016). Researchers have not identified the language acquisition device or universal grammar that Chomsky thought underlies all languages. In addition, there are more individual differences in language learning and in languages than Chomsky proposed (Dąbrowska, 2015). Moreover, language