Acquiring regular and irregular inflection in a language with verb classes

The dual mechanism model claims that only rule-generated inflections are productive; that is, can be applied to words borrowed from other languages or derived from other grammatical categories (e.g. verbs derived from nouns or adjectives). Productivity, high generalisability and insensitivity to type or token frequency effects are intertwined properties of rule-generated inflections across languages (Pinker, 1991). We tested this cross-linguistic prediction in two experiments investigating Italian children's spontaneous performance with the past definite (Experiment 1) and their elicited performance with the past definite and the past participle (Experiment 2). Our findings show that performance profiles with productive and unproductive inflections cannot be "categorically distinguished". The phonologically transparent morphological patterns exhibited by an unproductive verb class are high in generalisability when children make errors with root change verbs. The morphological patterns exhibited by a semi-productive class, which in Italian is consistently used with verbs derived from adjectives, are more correctly applied when forms are high in token frequency.