Word Spurt Image

Figure 4.2 Modeling fitted survivor functions for the timing of first words: Estimated function for high levels of maternal responses with affirmations, descriptions, and play prompts at 9 months (upper 10th percentile) and estimated function for low levels of responding with affirmations, descriptions, and play prompts at 9 months (lowest 10th percentile).

children accumulate 50 words in their expressive vocabularies, they experience a sudden acceleration in production, referred to as the vocabulary "spurt" or vocabulary "explosion" (see Bates et al., 1988; Bloom, 1973, 1993, 1998a; Gershkoff-Stowe & Smith, 2004; Reznick & Goldfield, 1992). For example, Bloom (1993) showed that children averaged 51 different words in their productive vocabularies at the time of a vocabulary spurt and that all children in her research reached the 50-word vocabulary mark within 1 month of showing a substantial acceleration in their productive lexicons (Bloom et al., 1996).

In our research, we have also tracked children's language development from first words through the vocabulary spurt, and have documented a sharp incline in word growth around the 50-word mark. Specifically, children produced an average of 5.9 words per month prior to the 50-word mark, as

Vocabulary Spurt

Age in Months

Figure 4.3 The net growth in the number of new words children produced each month between the ages of 9 and 21 months.

Age in Months

Figure 4.3 The net growth in the number of new words children produced each month between the ages of 9 and 21 months.

9 11 13 15 17 19 21

Age in Months

Figure 4.4 The cumulative growth of the number of total words in children's productive vocabularies at each month between the ages of 9 and 21 months.

compared to an average of 39.3 words per month subsequent to the 50-word mark (Tamis-LeMonda et al., 1998). Of course, children varied enormously in both when they achieved the 50-word milestone as well as in their growth rates both before and after this period. Therefore, our focus has been on both average gains across children as well as language growth at the individual level.

To illustrate the phenomenon of the vocabulary spurt, Figure 4.3 presents the net growth in children's language, which referred to the number of new words children produced each month between the ages of 9 and 21 months. Figure 4.4 presents the cumulative plot of the number of total words in children's productive vocabularies each month. As shown in Figure 4.3, there is an overall upward trend in word production across all children (N = 107). Prior to 15 months of age, children added fewer than 10 words to their vocabularies each month; by 18-19 months they added 40-50 words per month to their productive vocabularies on average. However, these average developmental functions (both in terms of net growth, Figure 4.3, and cumulative growth, Figure 4.4) obscure the rapid growth in word production that exists in individual children, and also mask the fact that some children showed little to no improvement in their language over this time frame.

Figures 4.5 through 4.10 present data on individual children to illustrate both the phenomenon of the growth spurt as well as the dramatic individual differences among children in growth over time. Certain children added over 100 words per month to their lexicons (with one acquiring over 200 new words in a month; see Figures 4.5-4.8). Other children hardly added any new words to their productive vocabularies, as illustrated in the individual plots in Figures 4.9 and 4.10. Children such as those depicted in Figures 4.9 and 4.10 had therefore failed to achieve the 50-word milestone by the end of the study (i.e., at 21 months), and showed no acceleration in their word growth.

In addition to children exhibiting quantitative change in language around the middle of the second year, as indexed by impressive gains in the absolute number of words in their productive vocabularies, word growth may be qualitatively different from the more effortful process of language acquisition observed at the start of the second year. At around the 50-word mark, word production is uniformly spontaneous, new words are acquired with greater facility, and words are no longer as transitory as they had been during earlier stages of production. Moreover, once children acquire a substantial number of nouns and verbs in their lexicons, they demonstrate a remarkable capacity to learn novel words from just a few exposures or even just one exposure (e.g., Clark, 1995); this phenomenon has been referred to as "fast mapping" (Carey, 1978), and is demonstrated by children as young as 2 years

Child 1

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