A feature principle for partial agreement

An Optimality Theoretic account of partial agreement (agreement with just one of multiple conjuncts) presented here explains typologically interrelated properties of partial agreement. For example, in languages such as Moroccan and Lebanese Arabic, partial agreement is optional, incompatible with anaphor binding, and limited to non-collective predication; in languages such as Welsh and Standard Arabic, it is obligatory, compatible with anaphor binding and free of semantic constraint on predication. These correlated properties are explained through the interaction of constraints requiring agreement with an NPʹs concord features and a distinct set of constraints requiring agreement with index features. Partial agreement is possible only when the optimal type of agreement (index agreement in some languages, concord agreement in others) cannot be satisfied by the conjoined phrase as a whole. Conjoined NPs lack concord features, so a constraint requiring agreement with concord features will oblige agreement with a conjunct that bears concord features. Conjoined NPs may lack index features (if they encode an exclusively non-collective referent set), but this lack of index will also entail partial agreement. Which of the conjuncts a verb agrees with is determined by alignment constraints favoring the conjunct that is linearly closest to the agreement target.