When a sentence uses a parenthetical plural, should the rest of the sentence treat it as singular or plural?

Consider the following sentence:

We assume that the individual(s) possesses some general knowledge of the rules of football.

Is "possesses" correct there? Should it be "possess"? Is the rule more complex than always using one or the other?

Solution 1:

There are at least two references that say the verb should be singular.

"... when an 's' or 'es' is added in parentheses to a subject or subjects, the verb should be singular because the 's' or 'es' is parenthetical. [For example,] The name(s) of the editor(s) of the book in reference 2 is unknown." APStyleManual

"When -s or -es is added parenthetically to a word to express the possibility of a plural, the verb should be singular. ..." AMAManualOfStyle

I have seen none that say the verb should be plural.

Solution 2:

It is merely customary, in sentences involving parentheses, to write them so that even if everything within parentheses is ignored, the sentence is still readable and the meaning does not change much. In this case, dropping the "(s)" would result in "individual", so "possesses" has been used accordingly.

Actually, to adhere perfectly to the convention (so that the text is readable even if no parentheses are ignored), one may instead write

We assume that the individual(s) possess(es) some general knowledge of the rules of football.

so that the sentence can be read either with both parenthetical parts dropped or with both included, and remain grammatical. But including too many parentheses is considered poor style, as the result is usually awkward to read. Also, unlike this sentence, it's not always possible to satisfy both "branches" (of whether the part within parentheses is read or not), so some compromise is often inevitable anyway — and the choice that is often made is to be consistent with the text without the parentheses (or else whichever option is more likely, depending on the writer).

Solution 3:

When I am writing such a sentence, I make them match the parenthetical plurals wherever they occur, if possible:

We assume that the individual(s) possess(es) some general knowledge of the rules of football.

This is not always possible, and in those cases I prefer to recast the sentence. :)