The Oxford English Dictionary traces singular they to refer to an unnamed person back to the medieval romance William and the Werewolf in 1375:
"Hastely hiȝed eche... þei neyȝþed so neiȝh... þere william & his worþi lef were liand i-fere."
In modern English that’s, "Each man hurried... till they drew near... where William and his darling were lying together."
"Eche on in þer craft ys wijs." ("Each one in their craft is wise.") — Wycliffite Bible, Ecclus. 38:35 (1382)
"If... a psalme scape any person, or a lesson, or els yt they omyt one verse or twayne." — William Bonde, Pylgrimage of Perfection, iii. sig. IIIiiiiv (1526)
"Upon which every body fell a laughing, as how could they help it?" — Henry Fielding, The History of Tom Jones, a Foundling (1749)
"Almost anyone under the circumstances would have doubted if [the letter] were theirs, or indeed if they were themself—but to us it was clear."
— Emily Dickinson, using the singular "theirs" and "themself" in a letter dated Sept. 24, 1881
"A person can't help their birth." — Rosalind, in William Makepeace Thackeray, Vanity Fair (1848)
"Nobody in their senses would give sixpence on the strength of a promissory note of the kind." — Lord Landsdowne in 1910, quoted in The Liberal Magazine (1914)
“She kept her head and kicked her shoes off, as everybody ought to do who falls into deep water in their clothes.”
— C.S. Lewis, using singular "their" in The Voyage of the Dawn Treader, published 1952
In formal writing, don't use singular they to refer to a person whose gender is unknown.
Instead, reword for agreement in number.
CMOS 17 doesn't prohibit singular they as a substitute for the generic he in formal writing but recommends avoiding it.
A pronoun should generally agree in number with the antecedent. Singular they is acceptable in limited cases when alternative wording is overly awkward or clumsy, but rewording is preferable.
Instead of:
Each student is expected to choose their topic.
Reword:
Students are expected to choose their topic.
Each student is expected to choose a topic.
Learn more about adjusting your language to be inclusive at MyProunouns.org.
Just as the major style guides are clear on what to do with singular they in formal writing (that is, reword your sentence so you don't need the singular they), they're also clear that when someone who is non-binary gender, agender, genderfluid, genderqueer, trans, or otherwise gender-nonconforming uses the singular they pronoun for themself, you should also use singular they to describe them, even in formal writing!
Using an individual's chosen pronouns is affirming and respectful. Conversely, using binary pronouns for someone who uses non-binary pronouns, such as singular they, can be alienating and dismissive. For the non-binary person, this can feel marginalizing and even as though they are being erased altogether.
Writers should follow the personal pronoun choices of individuals they write about, if their preferences are known, and editors should respect those preferences.
APA supports the choice of communities to determine their own descriptors. Thus, when people use the singular they as their pronoun, writers should likewise use the singular they when writing about them.
A writer (or speaker) may use they to refer to a specific, known person who does not identify with a gender-specific pronoun such as he or she. "A person’s stated preference for a specific pronoun should be respected."
In stories about people who identify as neither male nor female or ask not to be referred to as he/she/him/her: Use the person’s name in place of a pronoun, or otherwise reword the sentence, whenever possible. If they/them/their use is essential, explain in the text that the person prefers a gender-neutral pronoun. Be sure that the phrasing does not imply more than one person.
Remember that AP style is intended for journalists and so is meant to be used in newspaper articles. It is concerned with clarity in writing that will be read by a large audience and must be understood by many people from different backgrounds.
