A short-term longitudinal study on the development of agreement in Italian: Syntactic configurations and non-word repetition as predictors of grammatical development in monolingual, bilingual and children with DLD

We investigate grammatical learning in Italian through a multipoint assessment over a short period (3 months), aiming to evaluate the potential of a longitudinal measure to differentiate between typical bilingual development and Developmental Language Disorder (DLD). Seventy-five children aged 4-6 years were tested and assigned to three groups: monolingual typically developing (TD), bilingual TD, and monolingual DLD. Participants completed standardised measures of language development at screening (nonword repetition, NWR; and sentence comprehension, BVL_Comp) and a forced-choice grammatical task measuring accuracy on agreement structures at Time 1 (T1) and 3 months apart at Time 2 (T2). No overall significant between-group differences were found in longitudinal change. However, within-group analyses revealed important diagnostic patterns. For bilingual TD - but not for children with DLD - longitudinal improvement in grammatical accuracy correlated positively with NWR scores obtained at screening, confirming NWR's predictive value for short-term grammatical growth. Within the DLD group, a significant divergence emerged between subtypes: children with Expressive DLD showed greater improvement than those with Receptive/Expressive DLD, who continued to exhibit persistent morphological difficulties. Theoretically, our findings confirm group- differences in the acquisition of agreement structures (Subject - Verb > Clitic - Past Participle) and demonstrate that children's sensitivity to agreement morphology serves as a reliable proxy for overall grammatical development given the positive correlation with standardised BVL_Comp scores. Although longitudinal changes in grammatical accuracy over 3 months were modest, subgroup patterns highlight the diagnostic potential of fine-grained, short-term longitudinal measures in distinguishing between profiles of language impairment and bilingual development.