I learned this really interesting fact from the Genderqueer Identities blog here. This was floating around Twitter quite a bit a while ago, so you might have already seen it.
When you were in grade school and were first learning correct grammar, you were probably taught that if you didn't know the gender of someone, you should refer to that person as "he or she" (or possibly just "he"), instead of "they" because "they" could imply that there is more than one person. "They" is grammatically incorrect to refer to a single person (even though we all tend to use it when speaking).
"He or she" is actually a fairly new grammatical rule; anyone who has done any sort of gender studies will probably have learned that through a good chunk of modern history, "he" was often used in place of saying "one". The masculine is always the default, the baseline. That has only recently (in the last few decades) been put on more even footing by people now saying "he or she", since automatically referring to anyone, gender unknown, as "he" is incredibly sexist.
YOU CAN BLAME THE VICTORIANS FOR THIS.
According to the blog author's class textbook (Reflect and Relate, Spender, 2nd Ed., 1990, page 187):
"before the 1850s, people commonly used 'they' as the singular pronoun for individuals whose gender was unknown—for example, “the owner went out to the stables, where they fed the horses”.
"In 1850, male grammarians petitioned the British Parliament to pass a law declaring that all gender-indeterminate references be labeled 'he' instead of 'they'. Since that time, teachers of English worldwide have taught their students that 'they' used as a singular pronoun is 'not proper.'"
(I've added quotation marks around the pronouns in question in the quotation to make it slightly easier to read).
This brings up a lot of interesting Victorian gender stuff, without question. But one thing that I'd like to clarify is this: what does (or did) Parliament have to do with grammatical rules? Does (or did) Parliament stand in as an official governing body over an official English language much in the was that L'Académie française does for the French language or the Accademia della Crusca does for the Italian language? Actually, according to Wikipedia, there are tons of language academies, but I don't see one for English.
Anyone have any information? Is this still in practice? Can I write to my local MP and get a law passed that would imprison people who say "supposubly" (supposedly), "Febuary" (February), "libary" (library) or "pitcher" (picture)?