They / them / their has been used with singular indefinite pronouns and determiners for a long time: the OED records it happening in the 16th Century!

However, in the 18th Century grammarians fought against this, saying that we needed formal agreement in tenses. This was still considered the norm into the 20th Century and was reiterated by Fowler (1926) and Gowers (1965).

Yet the pendulum began to swing the other direction again. By 1989 Webster’s English Usage noted that the singular usage of they / them / their was common everywhere from mass-circulated newspapers to academic publications. Eagleson (1995), New Oxford (1998), Canadian Oxford (1998) and Merriam-Webster (2000) all either noted it was happening or approved of it.

Burchfield (1996) lamented that the trend was probably “irreversible” and even today it is common to find some Style Guides recommend strategies to avoid using they / them / their as singular.(1)

Now exactly why this happens is harder to answer. Languages are constantly changing, but in the late 16th Century we saw some evolution in the role of pronouns and cases. Ye was used as the subject of a clause, while you was the object. (Ye have not chosen me; I have chosen you.)

But this distinction was breaking down already in Shakespeare and by the 17th Century grammarians made the two words interchangeable. In another hundred years ye had been relegated to literary and pompous texts. You also took over the singular roles of thou and thee. What happened with you is a similar process to what has happened with they.

My main sources for this were the Shorter Oxford English Dictionary and The Cambridge Guide to English Usage.

(1) If you’re curious about avoidance strategies, here are some common ones:

“He or she”

“He/she”

“s/he”

“he” alternating with “she”

“it”

“they” but turn the whole sentence plural

“you”

Only use abstract nouns

Repeat words that identify people