The aim of this paper is to present a theory that explicitly characterizes patterns of summative agreement. What I call summative agreement is a peculiar agreement pattern observed in right-node raising and left-node raising constructions in languages such as Basque, Dargwa, English, German, and Russian. In these languages, when a verb takes two subject NPs that are both singular, the verb can unexpectedly appear in a form that agrees with a plural subject. The following is an English example of this phenomenon, discussed in Postal (1998): The pilot claimed that the first nurse, and the sailor proved that the second nurse, were spies. The phenomenon of summative agreement is of considerable theoretical significance, because it contradicts all currently available theories of agreement, as well as all currently available theories of right-node raising and left-node raising, as far as I am aware. The theory that is proposed in this paper builds on my own theory of right-node raising and left-node raising, presented in Yatabe (2001), and is based on the view that agreement results from a non-lexical constraint that regulates under what circumstances a domain object can be merged with other domain objects by the compaction operation.
(S. Yatabe, "A linearization-based theory of summative agreement in peripheral-node raising constructions," in Jong-Bok Kim and Stephen Wechsler, eds., Proceedings of the 9th International Conference on Head-Driven Phrase Structure Grammar, CSLI, Stanford, 2003, pp. 391-411.)