Vocabulary in 2-Year-Olds with Autism Spectrum Disorder: a Magnified Verb Problem?

Background:

Research suggests that verbs are harder for typically developing children (TDC) to learn than nouns (Gentner, 1982). Anecdotal evidence also suggests that children with autism spectrum disorders (CASD) have particular difficulty learning verbs. However, studies of early vocabulary composition in CASD are conspicuously absent from the literature. Exploring verb acquisition in this population is crucial because verbs are the gateway to grammar (Fernald, Perfors & Marchman, 2006). Verb referents are less perceptually available than words for concrete objects, which makes support from social and grammatical cues even more crucial to their acquisition, and may likewise place CASD at a learning disadvantage (Maguire et al., 2006; Parish-Morris et al., 2007).

Objectives:

What is the trajectory of verb acquisition in 2-year-olds with and without ASD? We hypothesized that because CASD experience limited access to the social and grammatical cues that scaffold verb learning, their lexicons would reflect a larger discrepancy in the proportion of nouns versus verbs than language-matched TDC.

Methods:

Vocabulary data for 24- to 30-month-old CASD with at least one word in their vocabularies according to parent report (mean age=27.41, N=75) was collected using the MacArthur Communicative Development Inventory Words and Gestures form (Dale & Fenson, 1996; possible nouns=184, possible verbs=55). A TDC group was constructed from a nationally normed sample  (mean age=13.5, N=657) and matched to the CASD group on the number of nouns+verbs in the lexicon (M=83.78). Proportions (verbs/nouns+verbs) were calculated for CASD at 24 and 26-30 months, and for TDC groups using monthly norms from 11-16 months, which resulted in 12 sets of proportions (Dale & Fenson, 1996).

Results:

Preliminary analyses revealed that both groups had proportionately fewer verbs than nouns in their early lexicons, as expected (Bornstein et al., 2004; Swenson et al., 2007). Total number of nouns+verbs was highly correlated with age for TDC (r=.99, p<.001) but not for CASD (r=.31, p=n.s.). Using the complete CASD data set, we explored whether a verb discrepancy is most apparent in CASD with larger vocabularies. Results revealed a significant negative relationship between vocabulary size in CASD and verbs/verbs+nouns (r=.22, p<.05), suggesting that as the vocabularies of CASD get larger, the disparity between nouns and verbs grows (i.e., the proportion of verbs in the vocabulary decreases as total number of nouns+verbs increases). This relationship was not present in TDC (r=.46, p=n.s.; Tardif, 2006).

Conclusions:

This is the first study to demonstrate that 2-year-old CASD have more difficulty with verb acquisition than language-matched typically developing peers. Future studies will explore why the vocabulary composition of CASD differs from typically developing children. What characteristics differentiate CASD who acquire a cadre of verbs from those who do not, and what types of verbs are more readily learned? Research on verb acquisition in CASD has the potential to inform intervention, and will elucidate the specific mechanisms that underlie verb learning in both typically and atypically developing children.