Due to the quirks of language evolution, English went centuries without a pronoun that would accurately—and not awkwardly—describe a person of unknown gender. Sure, you could say "he" or "she," but that would give gender where there was none, thus leading to confusion and errors. You could also use combinations such as "he or she," but those are awkward to say—and still leave out gender-nonconforming individuals. There was also "one," but that would lead to awkward and repetitive sentences.
That left only "they"; for centuries, grammarians railed against the singular use of "they," arguing that it should only be used in its original, plural sense. However, people have long used "they" in singular form as part of informal speech, and in recent decades, it has become increasingly common in written English as well. The tide began shifting in the 2000s with the increasing social awareness of gender-nonconforming individuals, and just in the past 5 years, many authorities have officially given their stamps of approval to the singular "they" as a way of filling this gap in English.
The History Of Pronouns Referring To A Single Unknown Individual
Throughout much of English history, when a writer needed to refer to a person of unknown gender, they would use "he" as a default. This was the dominant form even though it was biased—and often inaccurate. The English grammar authorities of earlier eras were, of course, almost exclusively men, so they saw no real problems with treating the default individual as male. Sentences such as this were common:
Each new college student was warned that he should be devoted to his studies.
Sure, even as late as the 19th century, most college students were men, but even then, this use of "he" was sometimes incorrect—and, more importantly, implied to any women that they were not considered college material. This implication is pernicious, though it took a while for it to incite any resistance.
By the mid-20th century, people were beginning to seek alternatives to the biased use of "he" as a default. The most commonly used alternative was "he or she"; however, as you can see from the following example, this combined pronoun sounded awkward, especially in multiples:
Each new college student was warned that he or she should be devoted to his or her studies.
Even at the time that this method was most common, many authorities recognized that it was not ideal due to this awkwardness. Other alternatives arose, but none caught on. For instance, some advocated for alternating male and female pronouns when the gender was unknown, but this led to weirdness such as
Each new college student was warned that he should be devoted to her studies.
Others proposed a shorter combined form, "s/he"; however, the slash is strange, and the other pronoun forms aren't as easily abbreviated. There's no good short alternative to "his/her," for instance. There is also the problem of how to read such combinations aloud. The following example is strange when spoken:
Each new college student was warned that s/he should be devoted to his/her studies.
None of this experimentation led to any consensus. Most style guides said to use "he or she" when necessary... but they also advised against using this form too often (due to the awkwardness). The general advice was to recast sentences using plural pronouns whenever possible, as in
All new college students were warned that they should be devoted to their studies.
This is a good solution in many cases, but some singular pronouns cannot easily be made plural. In this example, there can only be one "winner," so there is no way to use the plural "they":
Regardless of whether Jon or Nancy is elected, the winner should keep his or her promises.
That brings us to the 21st century and the rise of the singular "they." There are now two common forms of the singular "they," both of which we will discuss below:
Generic: To refer to a single person of unknown gender
Specific: To refer to a known individual who does not use "he" or "she" pronouns
What Is The Rule, And Why Can You Ignore It?
Despite all the failed attempts above, grammarians continued to argue against using the singular "they." This rule was not without its logic, either. The word "they" already carried a tremendous weight in English, as it was the only plural pronoun. Regardless of whether you were pluralizing "he," "she," or "it," you had to use "they"—leading to many ambiguity problems. Consider this example:
The teachers told the boys, who had drums, and the girls, who had cymbals, that they were too loud.
The pronoun "they" in that example could refer to the boys, the girls, the boys and the girls, the drums, the cymbals, or even the teachers. Such ambiguity issues are common with "they," so it is understandable that a grammarian would want to prevent "they" from taking on even more meanings.
However, despite this reasonable concern, there remained a clear need for a better solution to the "he or she" problem. In addition, as the 21st century dawned, awareness was increasing regarding the proper treatment of nonbinary and other gender-nonconforming people. The simplest solution was to allow for "they" to be used in the singular. After all, people had been using "they" in this way in their informal speech for centuries without causing much confusion. (This usage dates back to the 14th century!)
Individuals have suggested numerous other singular gender-neutral pronouns—"ze" is my favorite—but none have yet become widespread. Instead, people have just begun using "they" in the singular in more contexts. This usage became so common that it eventually forced grammarians to adapt. In recent years, even some very formal style guides have approved of the singular "they":
Oxford Dictionaries recognizes the singular "they" as well, though it notes that some authorities still criticize its use.
APA Style added both the generic and the specific uses of the singular "they" to the 7th edition of its highly popular manual in 2019.
Chicago Style is a bit more standoffish; it allows the specific singular "they" (to refer to someone who does not identify as male or female). However, it still recommends avoiding the generic singular "they" (to refer to an unknown individual). I should note, however, that Chicago admits that this usage is becoming more accepted, and its most recent edition (the 17th) is from 2016, which makes it a bit outdated given the rapid pace of change on this issue.
Since 2017, AP Style has advised that both the generic and specific "they" are acceptable "in limited use." It still recommends rephrasing when possible to avoid the generic "they," but it recognizes the need for it in some cases, and it endorses the specific "they" when appropriate.
Some authorities still discourage the singular "they," of course. However, all agree that the singular "they" is rapidly gaining acceptance, even in formal writing. I would expect this trend to continue. Indeed, I expect the singular "they" to be more or less fully accepted by 2025. Anyone who uses it today need not fear that it will quickly become outmoded.
When Should I Still Avoid Using The Singular "They"?
The most obvious reason to avoid this usage is because your style guide says not to use it. Always follow the rules in your style guide (if you are using one). In addition, if you are writing formally, I'd recommend minimizing the use of the generic version, at least for now. Try rephrasing a sentence using plurals, as described above; if that is not reasonable, however, the singular "they" is fine.
For the specific "they," always use an individual's preferred pronoun. Because many gender-nonconforming individuals prefer other pronouns (such as "ze"), "they" is not always appropriate, even when "he" and "she" are incorrect. If you are unsure what a person prefers, ask them (if possible).
You might also want to avoid using the singular "they" when doing so would require the use of a "self" pronoun. There is no consensus regarding the correct "self" form for the singular "they": Both "themselves" and "themself" have adherents, but neither is a perfect solution. (The former sounds awkward in the singular, and the latter is not yet an accepted word.) In such cases, consider rephrasing without pronouns:
Roshi opted to donate the coat to the clothing drive rather than take it for themself. Roshi opted to donate the coat to the clothing drive rather than keep it.
Finally, as noted above, "they" has great potential for ambiguity, so do not use the singular "they" unless the antecedent is clear. For instance, consider this slightly modified version of the earlier example:
The deans warned each new college student that they should be devoted to their studies.
In this example, "they" could refer to not only the "student" but also the "deans," so it is best to rephrase without the pronouns:
The deans warned each new college student to study dutifully.
Do you have any questions about how to properly use the singular "they"? If so, please let me know in the comment or via email (email@example.com). I'd love to help! If you don't want to worry about this issue, you can of course always have me edit your work—I'll make sure that you use the best pronouns.