From Johnson, nice deconstruction of the arguments against singular "they" as an anaphor.Â
If singular they has deep historical precedent, then it is dispositive on the sub-question of what is traditionally correct. In this case, liberal descriptivists and conservative prescriptivists can sing a happy song in harmony. Descriptivists note that nearly everyone uses singular they, at least in speech. Prescriptivists can relax in the knowledge that Chaucer, Shakespeare, the King James translators, Swift, Byron, Austen, Goldsmith, Thackeray, Shaw, Herbert Spencer and others used it. In collecting these examples, the "Merriam-Webster Dictionary of English Usage" notes that these are not "lapses" by the greats. They are the regular pattern, many centuries old. The "prohibition" of singulartheyis only two centuries old. This simply should not be a controversy.
What about the fear of confusion? We should avoid even technically correct usages if they are ambiguous. Mr deBoer says that singular they is "very unlikely to produce confusion". I'd go beyond "very unlikely". It's always possible to create a fake sentence in isolation that might be ambiguous. "Many professors make every student buy their own books." Hm, are the students forced to buy their own books (rather than being allowed to use library copies)? Or are the professors, in their vanity, requiring students to buy the books that the professors wrote?
This example, which took me a while to come up with, is genuinely ambiguous. But such statements, already rare, are never said in a vacuum, the entire utterance of a speaker who walks up to you, says it, and walks away. Instead, this will be appear in a context like "It's a shame that universities don't put multiple copies of required books in the library where poorer students can get at them for free. Many professors make every student buy their own books." Or "Professors shouldn't abuse their discretion to push their own academic theories or line their own pockets, but many professors make every student buy their own books." I think it is nearly literally impossible for singular they to be confusing in an actual conversation or in a longer piece of writing. Readers are invited to try to construct such an example to prove me wrong. And that still doesn't prove singular they ungrammatical.Richard told John he loved his wife is ambiguous, but grammatical. And as above, context (a bit of knowledge of Richard's and John's love lives) will almost certainly save the day.