Does anybody have any conjectures as to why this quirk is so common? For an example, see this TED talk by Kevin Slavin.
The standard grammatical explanation of this is that it’s a variant of constructions like:
What the reason is is that she’d just returned from Guatemala.
These are quite standardly grammatical, analogous to e.g.
What I know is that capuchins are a kind of monkey.
This construction acts in some respects as a fixed idiom, with slightly different connotations from plain old “The reason is that she’d just returned…”, and as such, it’s started to evolve independently. In particular, it’s developed the variant which omits the what, which occurs frequently enough that descriptive linguists happily accept it as grammatical, though slightly nonstandard.
Those different connotations are subtle; the following is my subjective impression, but if someone can find a proper corpus-based analysis of them, that would be better.
The form “The problem is that I don’t know why he’s angry.” can be the first mention of the fact that there’s a problem; it puts focus on this assertion. Contrastingly, “[What] the problem is, is that I don’t know why he’s angry.” is typically used when the listener/reader is already aware that there’s a problem; it emphasises the delineation of precisely what the problem is, possibly in contrast to other things it could be:
The problem isn’t that they’re stupid. What the problem is, is that they’re overspecialised.
(Mark Liberman also discusses the “The X is, is” construction on Language Log, and partially disagrees with this standard analysis, linking to an alternative proposed explanation. I’ve not read the linked paper, I’m afraid, and the standard analysis makes sense to me, so I’m leaving it at this for now.)
There is an interesting study here on spoken repetition that asks, "Are Word Repetitions Really Intended by the Speaker?" Repeated words fall under the category of speech disfluencies, which are:
Spontaneous speech contains various disﬂuencies such as ﬁllers, self-repairs, and repeated words. These disﬂuencies seem to reﬂect problems in speech production. When speakers cannot formulate an entire utterance at once, or when they change their minds about what to say, they may suspend their speech and produce ﬁllers or replace words they have already produced.
The "quirk" of repeating the "is" in spontaneous speech is used to "fill in" or repair a sentence as you are forming it. If you listen to recordings of other speech, there are usually other fillers like repeated words, or "ummm". It is not a quirk that is specific to any one person, but rather it affects most (if not all) spoken word.
I think the point is (is) that there's a distinction between a preliminary noun-clause ending in "is", as in the first example below and which is correct, and a preliminary noun-clause that is complete prior to the iteration of the word "is", as in the second example below. My sense is that this section category is incorrect.
- What the message is, is that ...
- The message is (is) that ...
I'd also note that this mannerism seems more prevalent in, if not exclusive to, American English. I'm not sure what that might indicate, though.