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“I can’t believe how quickly Matthew picks up new words. It’s time for us to be more careful about what we say around him,” warned Elana. Her husband agreed, “He’s only 2 years old and he has quite a vocabulary. Who would think that he’d learn so many words so quickly?” By 13 months of age, children begin to quickly learn the meaning of new words and understand that words correspond to particular things or events (Woodward, Markman, & Fitzsimmons, 1994). Most infants of Matthew’s age expand their vocabularies rapidly, often to the surprise of their parents. Infants learn new words through fast mapping, a process of quickly acquiring and retaining a word after hearing it applied a few times (Kan & Kohnert, 2008; Marinellie & Kneile, 2012). At 18 months, infants are more likely to learn a new word if both they and the speaker are attending to the new object when the speaker introduces the new word (Baldwin et al., 1996). Two-year-olds have been shown to be able to learn a word even after a single brief exposure under ambiguous conditions (Spiegel & Halberda, 2011) or after overhearing a speaker use the word when talking to someone else (Akhtar, Jipson, & Callanan, 2001). Between 24 and 30 months, infants can learn new words even when their attention is distracted by other objects or events (Moore, Angelopoulos, & Bennett, 1999).

Description

Figure 5.9 Number of 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).