Investigating deaf students' knowledge of Persian syntax: further evidence for a critical period hypothesis.

The present study aims to investigate syntactic deficits in 13 Iranian deaf students aged between 17 and 21 years. Four tests in the form of sentence-recognition and sentence-completion were administered to examine their knowledge of verb inflection, derivational morphology, word order, and prepositions. A between-category analysis of errors indicated a significant dissociation between categories, most notably between verb inflection and derivational morphology and between word order as the category with fewest errors and the three others. On theoretical grounds, the fact that subjects have not acquired much syntax even after years of learning seemed to strengthen the significance of acquiring syntax and morphology in the early years.