Deriving the Complementarity Effect: Relativized Minimality in Breton agreement

Breton φ-agreement is characterized by the Complementarity Effect, which allows pro-dropped but not lexical DPs to control φ-agreement. We contrast verbal and prepositional systems: a lexical DP co-occurs with the root form of a preposition, but with a 3rd.sg. (frozen agreement) form of a verb. We argue that frozen agreement arises through φ-relativized locality: the Breton vP independently shows nominal properties, and thus intervenes for agreement. The φ-probe of T Agrees with the vP for 3rd.sg. rather than the vP-internal subject. In the prepositional system on the other hand, lexical DPs occur with bare stems and φ-inflection spells out affixed pronouns. The mechanics predict that in verbal constructions where the subject originates outside the vP, it is local enough to control the agreement of T, which correctly yields Have under a prepositional analysis as the sole verb immune to the Complementarity Effect. Finally, we propose a typology of Complementarity Effects in agreement depending on the interaction of intervention (frozen agreement) and syntactic incorporation past the intervener