There’s a long-running debate over whether the construction reason why is acceptable. Critics generally argue that why essentially means reason, so saying reason why is like saying reason twice. Saying something twice is redundant, and redundancy is bad; ergo, reason why is bad. This is really a rather bizarre argument. Reason is a noun; why is usually an interrogative adverb. They do cover some of the same semantic space, but not the same syntactic space. Does this really make the construction redundant? Defendants generally admit that it’s redundant, but in a harmless way. But rebutting the critics by calling it “not ungrammatical” or saying that “redundancy is not inherently bad” is a pretty weak defense. However, that defense can be strengthened with the addition of something that has been missing from the discussion: an examination of the syntactic role of why in such constructions.
Nearly every discussion on reason why that I’ve ever seen—including Merriam-Webster’s Dictionary of English Usage and Garner’s Modern American Usage—leaves out this very important syntactic component. The only exceptions that I’ve seen are this post on the Grammarphobia blog and this one on Daily Writing Tips, which both mention that why is a conjunction. The writers at Grammarphobia argue that reason why is not actually redundant because of why’s syntactic role, but Mark Nichol at Daily Writing Tips seems much more confused about the issue. He says that even though reason why has been around for centuries and only came under fire in the twentieth century, he’ll continue to avoid it in his own writing “but will forgive the combination when I am editing that of others” (how magnanimous). But he doesn’t understand why reason why is okay but reason is because is not, because both why and because are conjunctions.
I won’t get into reason is because here, but suffice it to say that these are very different constructions. As I mentioned in my previous post on relative pronouns and adverbs, why functions as a relative adverb, but it appears almost exclusively after the word reason. (To be clear, all relative pronouns and adverbs can be considered conjunctions because they connect a subordinate clause—the relative clause—to a main one.) In a phrase like the reason why this is correct, why connects the relative clause this is correct to the noun it modifies, reason. Relative pronouns refer to a noun phrase, while relative adverbs refer to some kind of adverbial phrase. As with any relative clause, you can extract a main clause out of the relative clause by replacing the relative pronoun or adverb and doing a little rearranging (that’s the man who I met > I met the man), though with relative adverbs you often have to add in a function word or two: the reason why this is correct > this is correct for this reason. This is pretty obvious when you think about it. A phrase like the reason why this is correct contains another clause—this is correct. There has to be something to connect it syntactically to the rest of the phrase.
In defending the construction, Gabe Doyle at Motivated Grammar compares it to the redundancy in The person who left their wet swimsuit on my books is going to pay. This is actually a more apt comparison than Mr. Doyle realizes, because he doesn’t make the connection between the relative pronoun who and the relative adverb why. He argues that it is just as redundant as reason why (and therefore not a problem), because who means person in a sense.
But as I said above, this isn’t really redundancy. Who is a relative pronoun connecting a clause to a noun phrase. If who means the same thing as person, it’s only because that’s its job as a pronoun. Pronouns are supposed to refer to other things in the sentence, and thus they mean the same thing. Why works much the same way. Why means the same thing as reason only because it refers to it.
So what about reason that or just plain reason? Again, as I discussed in my last post on relative pronouns and adverbs, English has two systems of relativization: the wh words and that, and that is omissible except where it functions as the subject of the relative clause. Thus we have the option of saying the reason why this is correct, the reason that this is correct (though that sounds awkward in some instances), or just plain the reason this is correct (again, this is occasionally awkward). The Cambridge Grammar of the English Language also mentions the possibility the reason for which, though this also sounds awkward and stilted in most cases. But I suspect that many awkward plain reasons are the result of editorial intervention, as in this case I found in the research for my thesis: There are three preliminary reasons
why the question of rationality might make a difference in the context of Leibniz’s thought.
It’s important to note, though, that there are some constructions in which why is more superfluous. As Robert Lane Greene noted on the Johnson blog, sometimes why is used after reason without a following relative clause. (Mr. Greene calls it a complement clause.) He gives the example I’m leaving your father. The reason why is that he’s a drunk. The why here doesn’t really serve a syntactic function, since it’s not introducing a clause, though the Oxford English Dictionary calls this an elliptical construction. In essence, the why is serving as a placeholder for the full relative clause: I’m leaving your father. The reason why (I’m leaving him) is that he’s a drunk. It’s not strictly necessary to delete the why here, though it is generally colloquial and may not sound right in formal writing.
But this is by no means a blanket injunction against reason why. I think the rule forbidding reason why probably arose out of simple grammatical misanalysis of this relative construction, or perhaps by broadening a ban on elliptical reason why into a ban on all instances of reason why. Whatever the reason for the ban, it’s misguided and should be laid to rest. Reason why is not only not ungrammatical or harmlessly redundant, but it’s a legitimately correct and fully grammatical construction. Just because there are other options doesn’t mean one is right and the rest are wrong.