Agreement is a morphosyntactic dependency which is sensitive to the hierarchical structure of the clause and is constrained by the structural distance that separates the elements involved in this relation. In this paper we present two experiments, providing new evidence that Italian-speaking children with Developmental Language Disorder (DLD), as well as Typically Developing (TD) children, are sensitive to the same hierarchical and locality factors that characterise agreement in adult grammars. This sensitivity holds even though DLD children show accrued difficulties in more complex agreement configurations. In the first experiment, a forced-choice task was used to establish whether children are more affected in the computation of S-V agreement when an element intervenes hierarchically or linearly in the agreement relation: DLD children are more subject to attraction errors when the attractor intervenes hierarchically, indicating that DLD children discriminate between hierarchical and linear configurations. The second experiment, also conducted through a forced-choice task, shows that the computation of agreement in DLD children is more 'fragile' than in TD children (and also in children with a primary impairment in the phonological domain), in that it is more sensitive to the factors of complexity identified in Moscati and Rizzi's (2014) typology of agreement configurations. To capture the agreement pattern found in DLD children, we put forth a novel hypothesis: the Fragile Computation of Agreement Hypothesis. Its main tenet is that DLD children make use of the same grammatical operations employed by their peers, as demonstrated in Experiment 1, but difficulties increase as a function of the complexity of the agreement configuration.
Keywords: Developmental Language Disorder; Specific Language Impairment; morphosyntax.