Freddie de Boer, a part time blogger and full time linguistics postgrad, has written a post exploring what he terms “Pronoun X”:
We have this problem in English: we’re lacking a particular pronoun, the third person gender-neutral singular. The conventional way around this is to use their: “every student picked up their paper.” But this usage drives prescriptivist grammarians crazy, as “every” is singular, which we can tell from how “student” inflects as singular. (It’s “every student,” after all, not “every students.”) The typical advice is to instead us “his or her” in place of their. That’s a technically satisfying answer, but as anyone who actually uses English knows, it’s imperfect: it sounds clunky…
Leaving the linguistic debate to Freddie, I think you can tell a lot about a person’s psychology from whether they use “his or her” or “their”. People who use the former, caring greatly about the technically correct grammar, are more likely to be anal, rigidly organized, dyspeptic and eager to look clever; whereas people who use the latter, caring more about what sounds right, are more likely to be relaxed, creative, and comfortable in their own skin.
Of course there are many exceptions to this scheme: myself, for instance. I tend to use ‘their’, but do so because, well, who wants to be perceived as a ‘his or her’ person? Not me, that’s for sure!