Abstract
This action research investigates Persian learners’ clustering acquisition of overt obligatory subjects and PRO in infinitival clauses in English as L2. In L1 acquisition research, the correlations between superficially unrelated linguistic phenomena are analyzed in terms of clustering effects. For instance, in German L1 acquisition, there is evidence for a clustering acquisition of subject-verb agreement and the decrease of (incorrect) null subjects (Clahsen and Hong, 1995). The developmental connection between these two phenomena in L1 acquisition has been interpreted as clustering appearance of syntactic properties of non-pro condition. Thus, the present research will report on a grammaticality judgment test ( GJT) investigating the clustering appearance of obligatory overt subjects and PRO in infinitival clauses in 60 Persian learners of English divided into three proficiency levels, each level consisting of ten early starters and ten late starters. Our main finding is that the two phenomena covary in the Persian learners indicating that properties of overt obligatory subjects and PRO in infinitival clauses are acquired through parameter resetting, rather than separately. Moreover, we observed no difference between the performance of early and late starters with respect to clustering acquisition of the above linguistic variables.

Keywords: clustering effects, developmental correlations, finite clauses, infinitival clauses, non-finite clauses, null subject, obligatory overt subject, parameter (re)setting , parametric variation, PRO

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