I'm happy to report having just discovered that Wikipedia has a lengthy discourse on this very subject: Wikipedia on "Singular they"

The most interesting things I learned:

1) "Singular they" has a long history in the English language, e.g.,

Caesar: "No, Cleopatra. No man goes to battle to be killed." / Cleopatra: "But they do get killed". —

Shaw, Caesar and Cleopatra (1901) I would have everybody marry if they can do it properly. — Austen, Mansfield Park (1814)

(I will admit that I myself find the Austen sentence awkward, but not the Shaw sentence.) (I also am fully aware that dialogue in novels is not a good guide to how to write correctly, as its goal is to capture how people speak, not reflect how people write.)

2) Some prescriptive grammatical authorities find the usage acceptable, but most don't. For example, 82% of The American Heritage Dictionary of the English Language usage panel rejected the usage (though that means 18% did not).

3) Wonder of wonders, there has been a study very much on point. I very much encourage anyone interested to read it. But I've summarized some of the key findings (hopefully accurately) below.

a) Sentences involving use of "singular they" are processed as efficiently as uses of, say, singular "he" when the antecedent is stereotypically male (e.g., truck drivers). "Singular they" is processed more quickly than is a singular pronoun which does not match the antecedent (e.g., using "she" when referring to a generic truck driver). This matches Andrew's intuitions.

b) When the antecedent is specific, as opposed to generic (e.g., a sentence about "that truck driver" as opposed to truck drivers in general), uses of singular they are processed less efficiently than use of the gendered pronoun that matches the stereotype (e.g., "he" for "that truck driver," and "her" for "that nurse").

Of course, this study should be taken with a grain of salt. First of all, as many people pointed out, we are lawyers-to-be, not ordinary people. Just because the average American is not impeded in her reading of a sentence where singular they is used, does not mean that people in our future audience (professors, judges, colleagues) are not - the average American will read a sentence containing singular they just as efficiently as one containing "he" or "she," but Eben and many of us will not, because of our special attention to precise writing. The study would no doubt have come up with different results if, say, the members of the American Heritage usage panel were the test subjects.

However, if the goal is writing that is not distracting, alternating "he" and "she" can cause trouble anytime the referent is stereotypically male or female, and the pronoun does not match.

The study did yield a finding that was opposite to my prediction: "neutral" antecedents (like, "The runner") take "he" or "she" equally well. I'm not sure if follow-up studies have been done, but it would seem that this experimental methodology could ferret out which nouns are stereotypically gendered and which are truly neutral. Are there nouns that are not quite as gender-stereotyped as "truck driver," but not quite as neutral as "runner?" What about "doctor," "lawyer," "artist," "hair stylist?"

The study also found that in sentences like "Anybody who litters should be fined $50, even if he/she/they cannot see a trashcan nearby..." the singular they is actually the pronoun that leads to fastest reading times. Again, I would imagine that the same result would not hold for people very attuned to prescriptive grammar; but it is worth noting that the test subjects here were university students, not street rabble.

-- MichaelBerkovits - 21 May 2008