Predicting when a word is acquired

The second more detailed research question regarded the relationship between different factors that have been shown to affect children’s lexicons. This question was targeted by paper II and III.

Paper II investigated the potential effects on Norwegian lexical development from word class, im-ageability, frequency, word length in phonemes and phonological neighbourhood density (PND).

Lexical development was measured by age of acquisition (AoA) and vocabulary size of acquis-ition (VSoA), both calculated from the Norwegian CDI norms (see section4.3). Paper III in-vestigated the effects of word class, frequency, imagebility,subjective AoA and the compound measure complexity index (CI) on Polish and Norwegian CLT scores from monolingual and bi-lingual preschoolers; Polish imageability data are on the stocks,¹ but were not available for the

1M. Łuniewska, personal communication, September 21, 2016.

6.1. Linguistic factors 59 current investigation.

Both these two papers documented significant effects of CDS frequency. In paper II a new CDS frequency list from two Norwegian corpora (Hansen, 2016) was contrasted to frequency in the corpus NoWaC, based on the .no internet domain (Guevara, 2010). CDS frequency correlated with AoA and VSoA within all word classes (social words, common nouns, other nominals, verbs, adjectives and closed-class items) whereas frequency in NoWaC only correlated with the acquisition measures within common nouns and verbs. Regression models indicated that CDS frequency was by far the most important predictor of both AoA and VSoA. The findings in paper III corroborated these results, as frequency predicted performance on word production tasks among monolingual as well as bilingual children. The theoretical implications of these results will be further discussed in section 6.3.2.

In paper II, word length was found to have a significant effect on both AoA and VSoA; words were acquired later the longer they are. This result may seem at odds with paper I finding the first words in Norwegian to be balanced between mono- and disyllabic words. However, these findings do in fact not contradict each other, as they deal with different stages and measures.

Whereas paper I sets out to investigate the very first words, none of which exceed two syllables, paper II goes beyond this scope, dealing with both three- and four-syllable words. The word length effect found in paper II could be an attribute of these longer words being acquired late.

Regarding the measures, word length in syllables (paper I) may correlate with the number of phonemes (paper II), but there is no absolute correspondence between the two: A monosyllabic word with two consonant clusters may consist of more phonemes than a disyllabic word with no consonant clusters and open syllables.

PND did not significantly affect a words’ AoA when word length was controlled for, but a significant effect on VSoA was observed among nominals, with words being acquired earlier the denser their phonological neighbourhoods. Significant effects were expected also when acquis-ition was measured by AoA (see Storkel, 2004). As mentioned in paper II, the reason for this lacking effect could be the way the Norwegian PND measure is designed: Although paper II and Storkel (2004) assume the same general definition of a phonological neighbourhood, Storkel (2004) filtered words by familiarity to better gauge children’s lexical networks: Only words with a familiarity rating of six or more on a seven-point scale counted towards a words’ PND. This approach was not possible for paper II, as familiarity ratings are not available for Norwegian;

a possible substitute is imageability, but this property is only available for about 1600 words (Lind et al., 2015). Another approach is to weight phonological neighbourhoods by frequency (Luce & Pisoni, 1998), diminishing the influence of obscure words. This approach is possible for Norwegian and would be interesting from a usage-based point of view: Taking the strength of each exemplar into account should improve the model of our mental lexicon. Following this train of thought, the best solution for research on lexical development may be to weight PND by frequency in CDS.

60 Chapter 6. General discussion Regarding word class, paper II aligns with previous research; the first words were tightly connected to social interaction, nominals were acquired before predicates, and closed-class items were the latest acquired. There was overlap between the word classes. To exemplify, while some nouns were among the very first words, others were among the very last. This overlap aligns with with Tomasello’s (2003) claim thatany wordmay be acquired given the right social setting.

Paper III corresponded with paper II, as both monolingual and bilingual preschoolers generally scored higher on the CLT noun tasks than on the verb tasks, with an overlap between the two word classes in item difficulty.

Imageability was also studied in both these papers, but with somewhat conflicting results.

On the one hand, paper II found that when the potential effects of word class and imageability were analysed through a regression model, only imageability had a significant effect on acquis-ition (with words being easier to acquire the more imageable they are). On the other hand, in paper III, word class seems to overshadow imageability, which only affected the performance of monolinguals, not bilinguals, and only within the more difficult tasks, that is, within production tasks, among verbs, and within late acquired words. The paper noted that ceiling effects could be the reason for the lack of imageability effects within the easier tasks, but this cannot explain why imageability did not have an impact on the bilingual children’s CLT performance, since there was in fact less of a ceiling effect in this group.

Why, then, do these two papers come to different conclusions regarding which of these two factors is the most important predictor of lexical development? In addition to the ceiling effect noted for CLT, I can see three potential explanations, concerning age range, data size and tool design. The age range was 0–3 for paper II and 3;5–5;11 for paper III, and the reason for the differing conclusions could hence be that imageability plays a more important role in the very beginning of lexical development than for older preschoolers. However, note that imageabil-ity effects has been reported beyond the scope of early language development, for instance in picture-naming tasks (Bates et al., 2001) and word learning simulations (Gillette et al., 1999) involving adult participants. Thus, we would indeed expect imageability to affect CLT results.

Concerning the data size, the investigation of imageability effects in paper II included data from 6500 children and imageability scores on 447 nouns, verbs and adjectives, while that in paper III involved 66 children and imageability scores on 128 nouns and verbs, divided between two task types (comprehension and production). Hence, paper II allowed for a far more thorough investigation of the relationship between imageability, word class and acquisition than paper III.

Regarding the design of the tools involved, any word may be included in a checklist, whereas a picture-based tool is by necessity biased towards words of which meanings can easily be de-picted; in the development of CLT, a cross-linguistic naming study was used to ensure that the target words are easy to elicitate from pictures (Haman et al., 2015). Thus, the lack of a robust imageability effect in paper III may be a result of a strong bias towards highly imageable target words. As I will return to in the next section, the choice of a picture-based design may also

6.1. Linguistic factors 61 contribute to the observed differences in performance between nouns and verbs. To sum up, it is plausible that imageability does in fact influence 3–5-year-olds’ performance on picture-based lexical assessment tools, but that this influence is not detectable in paper III, due to ceiling ef-fects, insufficient data, high imageability values among the CLT target words, or a combination thereof.

There are several reasons to expect imageability to influence lexical development and pro-cessing. As mentioned in section 2.1.2, Taylor (2002) suggests that one of the properties that separate words on thebasic level in a taxonomy from words above this level, is that they are highly imageable. Thus, many highly imageable words may be easily acquired in virtue of being basic level words. Gillette et al. (1999) observed that the most readily identified nouns in their experiment were indeed basic level whole objects. Importantly, though, Gillette et al. (1999) argued that the imageability effect is not a result of how words are conceptualised in the mental lexicon, but an attribute of the communicative situation, along the lines of Tomasello’s (2003) social-pragmatic approach:

The child word learner at the earliest stages of language exposure is limited to the in-formation provided by the observeable extralinguistic contingencies. If observation provides the sole information base, then nouns labelling concrete nominal categor-ies should be eascategor-iest to acquire. Moreover, verbs like throw and come will be easier to acquire than want and know even if the learner has equal conceptual access to physical-action categories and mental-state categories.

(Gillette et al., 1999, p. 154)