Books

Singular they has been part of the English language since the 14th century.

seriously, when i see questions like OP, my instinct is to be like 'because you're a dumbass'.

but usually it's because their a prescriptive, and they think their use of their language at this particular time, is and always was the only legitimate form of it. and all past or future uses, including their own, are faulty and wrong.

god i remember being in like 5th or 6th grade and arguling about the colloqual use of 'they' with my teachers and getting failed. then in college i got 'lectured' about using 'he' as a pronoun for a singular person was sexist and awful you must use 'he/she' no matter how stupid and weird that looks. and now if you don't use 'they' you're awful because he/she is binary exclusivity or something.

and i bet in 10 years it will be different, yet again, and people will tell you whichever of the previous pronouns or singular pronoun you use are 'highly problematic' because it's not whatever what people have deemed to be 'least problematic'. as if it really matters that fucking much, especially compared to the actual broader themes of this novel.

Bodine (1975) – Androcentrism in Prescriptive Grammar

Excerpt of AI-generated findings:

Though published in 1975, Bodine’s landmark paper relies heavily on field data and structural analyses of English third-person singular sex-indefinite pronouns conducted during the late 1960s (specifically citing frameworks from Postal, 1969). Bodine categorizes this phenomenon into two main types:

• Sex-unknown: e.g., "Who dropped their ticket?"

• Mixed-sex, distributive: e.g., "Anyone can do it if they try hard enough."

Bodine notes that despite two centuries of intense prescriptive efforts by educators to enforce the generic "he", singular "they" remained the dominant colloquial and written choice for nonspecific referents throughout the 1960s.

Actual abstract from that article:

Abstract

This paper demonstrates that prior to the beginning of the prescriptive grammar movement in English, singular ‘they’ was both accepted and widespread. It is argued that the prescriptive grammarians' attack on singular ‘they’ was socially motivated, and the specific reasons for their attack are discussed. By analogy with socially motivated changes in second person pronouns in a variety of European languages, it is suggested that third person pronoun usage will be affected by the current feminist opposition to sex-indefinite ‘he’ – particularly since the well-established alternative, singular ‘they’, has remained widespread in spoken English throughout the two and a half centuries of its ‘official’ proscription. Finally, the implications of changes in third person singular, sex-indefinite pronouns for several issues of general interest within linguistics are explored. (Language change, sex roles and language, language attitudes, language planning, prescriptive grammar, pronouns.)