What’s wrong with the following sentence?
Each student must have their own book.
If you said it is grammatically incorrect because of pronoun agreement, you’re correct. Each is singular, but their is plural. You might be tempted to correct this sentence like so:
Each student must have his own book.
But now the sentence is politically incorrect or, more specifically, it is using sexist language. It is no longer acceptable to assume all students, or all readers for that matter, are male. How then, can we revise the sentence so that it is both politically and grammatically correct? One possible solution is shown below:
Each student must have his or her own book.
This solution corrects the error in pronoun agreement in a politically acceptable manner. Some may argue that his or her is an awkward, cumbersome construction, but non-discriminatory language trumps eloquence, especially in writing communities that value equality and social progress. (Some argue–from a feminist perspective–that we should use Each student must have her book for the same length of time that the male form was used.)
The his or her solution has met a new challenge, however. We’re now aware, based on the experiences of the intersex and transgender communities, that not everyone falls discretely into one gender or another. Thus this solution is now considered genderist. A possible solution to this is to reword the sentence to use a plural subject and a plural possessive pronoun:
All students must have their own books.
Another solution, which is gaining momentum within the writing community, is to employ gender-neutral pronouns such as ze and hir, in which case our sentence would read:
Each student must have hir book.
Because of all the difficulties caused trying to achieve pronoun agreement with a genderist language, and the frequency with which pronoun disagreement is appearing in print, it’s now become more or less acceptable to use Each student must have their own book.
For more about this topic, read the New York Times article, “All Purpose Pronoun.”