Having commented in an idle moment on what and/or means and why we have it, I started to receive email from people solemnly informing me that they were native speakers but in their variety of English or had only the exclusive meaning, where the disjuncts can't both be true. In other words, these are people who think that in their variety of English, if I say If Gordon Brown or the Pope is in the USA today I'll eat a copy of Strunk and White, I do not have to eat a copy of that disgusting little book The Elements of Style: I luck out on the grounds that (as it happens) both of them are in the USA today.
I hate to sound dogmatic, but my correspondents are actually wrong about their own native language.
What misleads them is that in many contexts (not all) the use of or instead of and strongly suggests the speaker has the exclusive meaning in mind; but the exclusive meaning is just a special case (holding where P or Q is true but P and Q happens to be false). The suggestion is only a defeasible one, of the type that Grice called a conversational implicature. My correspondents mistake it for an entailment, and then reify their mistake and insist on it.
The fact is that to check the condition if Gordon Brown or the Pope is in the USA today holds, you have to check the truth value of "Gordon Brown is in the USA today" and check the truth value of "The Pope is in the USA today", and if you get true as the result under either of those checks, then the condition holds. That is, I'm saying it comes out true under all circumstances except where neither Gordon Brown nor the Pope is in the USA today. I claim that is exactly the right theory of the semantics of the sentence in question.
But I will never find the eloquence to convince my correspondents that they are wrong.
I took the financial risk of offering one of them, a middle-aged gentleman whose initials are H.H., dinner at a superb restaurant with the Language Log Senior Writers' Entertainment Account credit card paying the bill. I added that the offer could only be taken up by someone who was over 21 or had two H's in his name. Believe it or not, the man would not back down, and staunchly insisted that the sentence "sounds wrong unless the words or both are added at the end"; so he won't ever get that dinner! If people will pass up a free dinner and an evening of conversation with a senior writer at Language Log just to defend a fallacious claim about their language, there will be little any mode of persuasion can do, I fear.
If you are open to rational persuasion and you want to see an argument that English or is not exclusive, an argument that could keep you out of jail, I will provide one. It is adapted from an argument that was independently developed in 1977 by Gerald Gazdar (who briefly presents it in pages 78-83 of his 1979 book Pragmatics: Implicature, Presupposition, and Logical Form) and Jeff Pelletier (see "Or" in Theoretical Linguistics 4: 61-74). It involves considering the conditions under which the denial of a disjunction would be true.
Imagine that a woman is under oath and being considered as a prospective juror in the trial of a crooked businessman, and the counsel for the defense asks her, "Have you ever met the defendant or done business with his company?" (the idea being to elicit a possible reason for excluding her from the jury). And suppose she answers "No."
Under the theory that A or B has the exclusive meaning (roughly "either A only or B only, but not both A and B"), the negation of A or B entails that some number other than exactly one out of A and B fails to hold. But the only available numbers are zero and two. So the negation, under the exclusive-or theory, is equivalent to ((not A) and (not B)) or (A and B)), as you can easily check if you know a little propositional logic.
Suppose it now turns out that the woman has a long history of intimate relations with the defendant and constantly does business with his company. What is going to happen to her if the court finds out?
She will go to jail for perjury, that's what. And she will not be able to wriggle out of it by saying that although she had met the defendant and done business with his company she technically did not meet the condition "either has met the defendant or has done business with the defendant's company" because or was used by the defense counsel. The court would reject that as nonsense.
And this is not a case where the law is an ass. The court would be right to throw her in the slammer. English or really is inclusive, not exclusive, in its basic literal meaning. If you have both met the defendant and done business with his company, the right answer to "Have you ever met the defendant or done business with his company?" is "Yes." Remember that. Trust Language Log and stay out of jail.
Update (April 17, 5:10 a.m.): Ray Girvan points out far more dangerous mistakes waiting to be made by those who have the wrong theory of what or means:
Another example in a safety-critical situation: the National Blood
Service's screening questions for blood donors (see
http://www.blood.co.uk/pages/c11xclud.html). For example:
"Have you ever had jaundice or hepatitis?"
"Have you ever been given money or drugs for sex?"
They'd take a dim view of anyone who answered "no" because they'd had
both jaundice and hepatitis, or been given both money and drugs for
The "Who can't give blood" section has similar examples
He's absolutely right. If you are in the position of having accepted both a couple of thousand dollars and a line of cocaine for agreeing to participate in sexual activity with Eliot Spitzer, then the right answer to "Have you ever been given money or drugs for sex?" is Yes. In fact don't even bother to go down to the blood drive office and waste their time, OK?