Unlike in languages such as Italian and Spanish, whose grammars license empty subjects, in adult English subjects may be dropped only in certain pragmatic contexts, i.e. in answers to questions, diary contexts, etc. Thus, English subject drop can be considered a pragmatic phenomenon. As is well-known, young normally developing English-speaking children up to age 3;0 drop subjects at rates varying between 25% and 70% (Hyams, 1992; 1996). Abstracting away from null subjects that co-occur with Root Infinitives, which we do not discuss here, this phenomenon can be explained by the lack of the pragmatic rule that governs subject drop in adult English. If this is true, two predictions follow from our hypothesis formulated in the first paragraph: a) English-speaking children with SLI older than 3;0 never drop the subject unless it is pragmatically allowed, but b) the same children do make errors w.r.t. purely syntactic properties of subjects, namely subject-verb agreement, and nominative case assignment.
An investigation into the speech of 17 English-speaking children with SLI all followed for 4 years between the ages of 3;11 and 8;7 and an MLU range of 2.1 – 8.1 (Tallal, Curtiss and Kaplan, 1988) suggests that a) English-speaking children with SLI of 4 years and older do NOT lack the pragmatic rule that governs subject drop in English, contrary to younger normally developing children; b) syntactically, the children with SLI (at least the year 1 children) make errors comparable to normally developing children of around 3 years old, indicating that they are in the same grammar developmental stage, and c) pragmatic principles develop as a function of age, rather than as a function of grammar developmental stage.
Concluding, our results provide support for a dissociation between the Pragmatic System and the Grammar, each developing at its own pace, and for the hypothesis that the Pragmatic System of children with SLI is not impaired itself.