Here's a sentence I just wrote: "The reason Taylor reminds you of Clinton and the reason it was especially noticeable last night is because both men are echoing Elvis."John -- my son, John Althouse Cohen -- responds:
Should "is" be "are"? I think it would sound ridiculous, but rule-based thinking would say yes.
I think it can only be "is," even if you're using "rule-based thinking." "Are" would just be comical. It would remind me of the scene in "After Hours" where Griffin Dunne says:The ice of my "rule-based thinking" is slowly melting.
So I march right in there to apologize, but she'd already killed herself. I was too late. He was about to give me the money, when all of a sudden, his phone rang. His girlfriend killed herself tonight. Is that a coincidence? No, because the same girl who I came downtown to see was dead, too. That's because they're the same person. They're both dead.
The point is: He was joking by using the plural and saying, "They're both dead." You can't use "both" in reference to one person. And in your sentence, there's only one reason; it just serves two functions.
Here's another analogy: "George Harrison was the guitarist and singer in 'Here Comes the Sun.'" By inverting that, you automatically get, "The guitarist and singer in 'Here Comes the Sun' was George Harrison." It would be absurd to use "were" instead of "was" in either of those sentences. It doesn't matter whether "George Harrison" comes first in the sentence; the grammar is still the same. "...are George Harrison" is just as ungrammatical as "George Harrison are..."
Incidentally, you should change "because" to "that." Look up "the reason is because" in any usage book.