Consider the sentence:
You didn't leave the dog in the car, did you?
In spoken English, this statement may be given with a rising intonation or a falling one. If the former, it suggests that leaving the dog in the car is a bad thing, and might even suggest incredulity and consternation on the part of the person asking.
In the latter case, when the sentence ends with a falling intonation, the speaker probably believes that the dog should have been left in the car, and that the person being addressed fell short. It amounts to an accusation.
Now, given that question tags are always questions, it seems they ought to be punctuated with a question mark. But in written form, especially dialogue, it feels to me that question tags meant to be spoken with a falling intonation might get by with just a period:
You didn't leave the dog in the car, did you.
I've tried Web searches but haven't gotten close to a set of search terms that point me toward an appropriate source. Anybody know of a definitive answer to this question?
Further Reflection
One of the reasons I ask this is that any declarative statement may be changed into a question by means of a rising intonation at the end.
You left the dog in the car.
becomes a question if your voice modulates upward at the end. In written English, it is customary to show that by means of a question mark:
You left the dog in the car?
I include this information because @FumbleFingers asserts that "punctuation may not be used to differentiate [someone's] two intonations." Yet clearly there are cases in which punctuation is used in precisely that way (though in this case involving the opposite modulation from what I'm suggesting). So I wonder if it might not be possible to move the needle in the other direction.
If not, why not?
Note: I almost accepted my own answer to this question, but retracted it. If someone comes up with a better one I'll certainly consider awarding that one the checkmark. I think this question touches on an important concept, despite the scant attention it has received.
This just in, 1 Nov 2020
I read the following in an article by author Stephen King in the Washington Post this morning.
[I asked] who she was voting for in the [2016 presidential election]. I expected her to say Hillary Clinton, because I stupidly assumed that, as a woman, Annie would love to see a woman president, but also because the polls, in Maine and in the other 49, made it clear that Donald Trump was going down, buried under a landslide.
“Trump,” she said.
I was shocked. I think I said, “You’re joking.”
She gave me a look that said, Surprised you, didn’t I.
[Italics by King, boldface emphasis by me]
Best Answer
The Chicago Manual of Style, 15th Edition (2003) has this entry under "Exclamation Point":
If we take this guidance seriously, it seems to me, then for like reasons we ought to find it acceptable for a question that is essentially a statement to end with a period.
Consider this lyric from Lisa Germano's song "Bad Attitude": "But if life was easy, you wouldn't learn anything, now would you." I certainly wouldn't criticize a writer for complying with the standard approach of ending that statement with a question mark—and in fact I believe that Ms. Germano does use that punctuation. Nevertheless, given that her more-speaking-than-singing voice drops by more than an octave between "now" and "would you," I wouldn't think it misleading to end the sentence with a period.
In addition, Chicago 15 has an entry under "Question Mark" for what it calls "courtesy questions":
That gives us two instances in which a widely influential U.S. reference work endorses using punctuation other than a question mark to end a phrase otherwise structured as a question.
I tend to agree with Robusto that strict adherence to the rule requiring all statements that are laid out in a form that would normally identify them as questions to end in question marks prevents writers from indicating, as they otherwise might, whether the intonation of the speaker's voice is rising or falling. The bad aspect of any widespread effort to differentiate intonation by punctuation is that it invites countless instances where authors choose punctuation mismatched to their intended intonation, and readers—newly alert for clues to intonation in the punctuation—consequently misinterpret the sentence.