[QUOTE=Colibri]
When the antecedents differ, then something called agreement by proximity can prevail. The verb agrees with the noun closest to it. This is most common when the nouns joined by “or” differ in number:
Either John or his brothers need to sign the contract.
Either John’s brothers or Sally needs to sign the contract.
I would say your sense that the second instance above sounds better is due to agreement by proximity of the pair of names joined by or.
See here for more on subject/verb agreement.
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I see what you’re saying about the last one, where you’ve got a plural subject followed by a singular subject, but I don’t like it. That’s why I think it makes logical sense to treat the conjunctive subject as a plural rather than a singular. The oppositive conjunction or implies that there’s only one option that the case will ultimately apply to, so why not treat them as one? Your example of “John’s brother’s” and “Sally” supports my point, since the oppositive conjunction can link both plural and singular subjects. Resorting to the plural instead of the singular solves this issue.
Here’s another example:
Either John or I are going first.
vs.
Either John or I am going first.
See, I think the first one sounds better to my ear, since the verb agrees with “John or I” as a compound subject. I know I’d say “John or I are,” since it’s the more inclusive rather than the one that happens to agree with the second part of the plural subject. Further, if you take one of the options out of the sentence, you’ll need to reconjugate the verb; you can’t say “I are going first” or “John are going first.”
I also have to disagree with Frylock about word proximities being relevant to the conjugation of verbs regardless of what’s logical. I realize that that’s increasingly common in modern usage, but I take issue with it. Verbs are supposed to agree with their subjects; it works better that way. That said, is “The majority is…” correct, or is “The majority are…” correct? A majority is, by definition, more than one part, but we consider it as a whole, so I’d conjugate it in the singular.
The British seem to treat such subjects as plural, saying things like “Deep Purple are on tour,” where the tendency in America is to say “Deep Purple is on tour.” However, we Americans tend to treat this rule less logically than the British, because we tend to conjugate in the plural according to plural-sounding band names, i.e. “The Beatles are on tour.” We’d never say “The Beatles is on tour” even though “The Beatles” represents a single group; this is because, to the American ear, “The Beatles” sounds plural (and besides, half of them are dead.)