From OWL – Purdue Online Writing Lab:
Linguistically, pronouns are words used to refer to people by replacing proper nouns, like names. A pronoun can refer to either a person talking or a person who is being talked about. Common pronouns include they/them/theirs, she/her/hers, and he/him/his.
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The English language does not have a gender-neutral third-person singular personal pronoun, but in recent years they has gained considerable traction in this role. They has been officially recognized as correct by several key bodies such as the Associated Press and the Chicago Manual of Style.
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Historically, the OWL has had resources on gender inclusive language that mainly focus on incorporating women into general language—for instance, using “he or she” or just “she” as the pronoun for a general subject, rather than always defaulting to “he”. Now, the conversation on gender inclusive language has expanded further to include people whose genders are neither male nor female (e.g., gender-nonconforming, gender-neutral, genderfluid, genderqueer, or nonbinary individuals, though this list is not exhaustive). In basic terms, this means that he and she are not sufficient to describe the genders of all people, because not all people are either male or female. As such, the phrase “he or she” does not cover the full range of persons.
The alternative pronoun most commonly used is they, often referred to as singular they. Here’s an example:Someone left his or her backpack behind. → Someone left their backpack behind.
Since we don’t know the gender of the person who left their backpack behind, we use they to include all genders as possibilities for that mystery person. In addition to being respectful of people of all genders, this makes the sentence shorter and easier to say. In fact, almost all of us use this language on a regular basis without even thinking about it.
Link to the rest at OWL – Purdue Online Writing Lab
As a reminder, PG doesn’t always agree with everything he posts on TPV. He has, however, been a regular user of the singular they and their for a long time, dating back to a simpler age when there were just two genders.