Jessica Love in The American Scholar:
According to modern-day grammar books, “they” as a singular, gender-neutral pronoun (e.g., I saw someone, but I don’t think they saw me) is incorrect, since a plural pronoun cannot describe a singular referent. And so we have settled on the generic “he.” This is, we are told, the way things have always been—good enough for Jonathan Swift or Jane Austen.
Except that what was in fact good enough for Swift and Austen was “they.” As Ann Bodine argues in her 1975 article “Androcentrism in Prescriptive Grammar,” prior to the 19th century, “they” was commonly—and uncontroversially—used as a generic singular pronoun. Grammarians were the ones who inserted the generic “he” into English about 200 years ago in an effort to improve the language.
Bodine is skeptical that such logical improvements are either improvements or logical. She points out that although “they” does not agree with a singular, gender-neutral referent by the single feature of number, “he” also does not agree with its singular, gender-neutral referent by the single feature of gender. “A non-sexist ‘correction,’ ” she writes, “would have been to advocate ‘he or she,’ but rather than encourage this usage the grammarians actually tried to eradicate it also, claiming ‘he or she’ is ‘clumsy,’ ‘pedantic,’ or ‘unnecessary.’ Significantly, they never attacked terms such as ‘one or more’ or ‘person or persons,’ although the plural logically includes the singular more than the masculine includes the feminine.”
More here.