Grammar, along with its close relatives usage and style, is a common cause of pearl-clutching in some circles. I associate it with prescriptivists, myself. Those who cannot conceive of the correctness of anything other than what they know themselves to be “correct,” for varying degrees of that word. These folks also often conflate grammar with usage and style, which is not the best understanding. The latter two items are closely entwined with grammar, certainly, but they are not the same, nor can they be understood in the same way. Grammar is a set of rules. Usage is a set of guidelines. Style is a different set of guidelines about mechanics, mostly: when to capitalize, when to italicize, how to write initialisms or acronyms, where to place punctuation (when there is no grammatical guidance already in place). Both usage and style also vary with the English being considered. I’ve written here and elsewhere about the differences between American and British English. The grammar is the same; the usage and style vary.
The latest kerfuffle has been about using that to refer to people. A number of vocal participants hold that it is wrong to do so. I do not regret to say it is not. It is grammatically correct, and it always has been.
That is not to say that everyone must use it! If who sounds better to you, use who. If you like using that for classes of people (say, people that wear glasses), but you like who for known individuals (the piano teacher who wears glasses), that is your choice and there is nothing amiss. What I am saying, and what the grammar texts and usage manuals will tell you, is that there is nothing ungrammatical about using that to refer to people.
Because usage guidance changes over time (I am thinking in particular of “singular they” used for nonbinary individuals), I pulled some usage manuals off my shelf to see what has been said over the years about who’s a who and who’s a that. Writing in 1965, Theodore M. Bernstein in The Careful Writer says: “Which normally refers to things, who to persons, and that to either persons or things. The point is elementary and needs no elaboration.”
Roy Copperud, in American Usage and Style: The Consensus (1980), gathered information from several usage manuals and boiled it down for us. (If you want to know precisely which ones, comment on this post and I’ll set them down for you.) “Two critics consider that freely interchangeable with who in restrictive clauses where it fits smoothly, ‘The man that was walking in the park yesterday.’ Fowler prefers who for particular persons (you who) and that in generic references (a man that), but concedes there is a strong tendency to use who out of politeness. . . . That for who is sometimes objected to, but the objection has no basis.” I will add “in grammar” to that last clause.
In The Merriam-Webster Dictionary of English Usage (1989) we find this: “In current usage, that refers to persons or things, which chiefly to things and rarely to subhuman entities, who chiefly to persons and sometimes to animals.” (The example of a “subhuman entity” is a banshee.) “That is definitely standard when used of persons.”
And last, I looked in Dreyer’s English: An Utterly Correct Guide to Clarity and Style. Mr. Dreyer, copy chief of Random House, says: “I don’t know why violation of this nonrule” (that a person must be a who) “flips some people out, but it does, and they can get loudly cranky about it.” Yes, a person can be a that. He cites Gershwin’s “The Man That Got Away.” (If you don’t already follow Benjamin on Twitter, I suggest you correct that pronto. You’ll learn something, I guarantee it. What it will be, I can’t say, but you’ll learn something.)
I’ve already tweeted Bryan A. Garner’s statement on the subject, which is that maintaining a person can only be a who is “a silly fetish.”
So, it seems clear that the experts in the field concur: A person can be either a who or a that. Both are grammatically correct, and neither is inherently preferable. Usage varies, and there are reasons a writer may prefer one over the other, but one cannot say—without inviting argument (or a blog entry)—that using that for people is incorrect.