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At 2 years of age, the average child knows about 500 words; vocabulary acquisition continues at a rapid pace. The average 3-year-old child has a vocabulary of 900 to 1,000 words. By 6 years of age, most children have a vocabulary of about 14,000 words, which means that the average child learns a new word every 1 to 2 hours, every day (Owens, 2015). How is language learned so quickly? Children continue to use fast mapping (see Chapter 5) as a strategy to enable them to learn the meaning of a new word after hearing it once or twice based on contextual association and understanding (Kucker, McMurray, & Samuelson, 2015). Fast mapping improves with age.

Children learn words that they hear often, that label things and events that interest them, and that they encounter in contexts that are meaningful to them (Harris et al., 2011). Preschoolers can learn words from watching videos with both human and robot speakers, but they learn more quickly in response to human speakers (Moriguchi, Kanda, Ishiguro, Shimada, & Itakura, 2011), especially when the speaker responds to them, such as through videoconferencing (e.g., Skype) (Roseberry et al., 2014). Children learn best in interactive contexts with parents, teachers, siblings, and peers that entail turn-taking, joint attention, and scaffolding experiences that provide hints to the meaning of new words (MacWhinney, 2015).

Another strategy that children use to increase their vocabulary is logical extension. When learning a word, children extend it to other objects in the same category. For example, when learning that a dog with spots is called a Dalmatian, a child may refer to a Dalmatian bunny (a white bunny with black spots) or a Dalmatian horse. Children tend to make words their own and apply them to all situations they want to talk about (Behrend, Scofield, & Kleinknecht, 2001). At about age 3, children demonstrate the mutual exclusivity assumption in learning new words: They assume that objects have only one label or name. According to mutual exclusivity, a new word is assumed to be a label for an unfamiliar object, not a synonym or second label for a familiar object (Markman, Wasow, & Hansen, 2003). In one study, young children were shown one familiar object and one unfamiliar object. They were told, “Show me the X,” where X is a nonsense syllable. The children reached for the unfamiliar object, suggesting that they expect new words to label new objects rather than acting as synonyms (Markman & Wachtel, 1988). Similarly, young children use the mutual exclusivity assumption to learn the names of parts of objects, such as the brim of a hat, the cab of a truck, or a bird’s beak (Hansen & Markman, 2009).

At around 5 years of age, many children can infer the meanings of words given the context. They can quickly understand and apply most words they hear.

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By 5 years of age, many children can quickly understand and apply most words that they hear. If a word is used in context or explained with examples, most 5-year-olds can learn it. Preschoolers learn words by making inferences given the context—and inferential learning is associated with better retention than learning by direct instruction (Zosh, Brinster, & Halberda, 2013). Certain classes of words are challenging for young children. For example, they have difficulty understanding that words that express comparisons—tall and short or high and low—are relative in nature and are used in comparing one object to another. Thus, the context defines their meaning, such that calling an object tall is often meant in relation to another object that is short. Children may erroneously interpret tall as referring to all tall things and therefore miss the relative nature of the term (Ryalls, 2000). Children also have difficulty with words that express relative place and time, such as here, there, now, yesterday, and tomorrow. Despite these errors, children make great advances in vocabulary, learning thousands of words each year.