The "Singular They"
/Many of the language tenets that the purists and the snobs and the sticklers criticize are in fact perfectly logical according to the grammar of English. One example being so-called “singular they,” as in: everyone return to their seats. A number of the purists would claim this has a grammatical error: namely, the clash of concord between the plural pronoun “they” and the singular antecedent “everyone.” But in fact it would be the purists that are wrong and error-makers are right—because singular “they” has a long history in English, including Shakespeare and Jane Austen. And if, in fact, you analyze the semantics of it, it is not at all illogical, because “they” in that context is not in fact a plural pronoun but rather a bound variable.
STEVEN PINKER