A couple years ago, frequent commenter and friend of the site Vance Maverick left a comment linking to a mysterious sign located in San Francisco’s Mission District:

The sign was brought up as part of a brief discussion of the construction the reason why, and whether it ought to be replaced with the reason that. When the building the sign was on passed to new owners, they appeared to answer this question by modifying the sign:

But was it right to remove the why?* What’s the beef with the reason why, and ought it to be the reason that?

You probably already know the argument against the reason why, because it’s the same hoary argument trotted out for so many grammatical constructions that have, for whatever reason, earned the irritated attention of prescriptivists. I’m speaking, of course, of the great grammatical bogeyman of redundancy. A quick pair of examples:

Both the reason is because and the reason why have something very basic in common: they’re entries for the category of the redundancy category.”

“if you say ‘The reason why…’ it’s like saying the word ‘reason’ twice.” [Tarzan and Jane’s Guide to Grammar, 2005]

But so what? Why is redundancy bad? Well, you might say that it’s inefficient. But communication is a noisy system, whether you’re talking in a windy area, or reading an email through a smudged screen, or talking to a somewhat distracted interlocutor. In addition to these external sources of noise, the language itself adds some noise, in the form of lexical and structural ambiguities (e.g., the possible meanings of Time flies like an arrow). Adding redundant information is the rational thing to do if you expect the noise levels to be high enough that some information will be lost, and in almost every linguistic situation, that’s the case.

For many non-linguistic problems, redundancy is already regarded as a logical solution. Suppose you give me the first 15 digits of your credit card number (please do). Then I can tell you what the final digit is going to be, because the last digit is completely determined by applying an algorithm to the first 15 digits. Why not just use 15 digits, then? Well, because writing down a sequence of numbers is an easy task to screw up. By including the check digit in the 16th spot, most minor transcription errors can be caught before the transaction is started. Yeah, it’s redundant, but it’s rational redundancy.

Language has similar reasons to use rational redundancy. In an example relevant to my daily life, UCSD’s campus is positioned under the flight path of planes landing at the local Marine base. Sometimes in the middle of a sentence the engine noise becomes too loud for someone I’m talking to to hear what I’ve said. It would be absurd to refuse to repeat myself because the second time is redundant.

Now a second example: explaining a complicated concept. In academic papers, you’ll often see someone state a point, and then immediately follow it up with “That is to say”, followed by a re-statement of the argument. If you got the argument the first time, the second explanation might be unnecessary, but because some people might not have gotten it, it’s worth re-iterating.

That is to say, redundancy is not inherently bad in language.** Every agreement marker in a language is in some sense redundant. For instance, if I want to compliment some bears in Spanish for their strength, I might say:

(1) Ustedes son osos fuertes (“You-plural are bears strongs“)

Each of the four words in that sentence are marked as plural. Shouldn’t it be sufficient to only label one word as plural? This is a little bit of a special case, because the redundancy is required by the grammar. But the truth is that redundancy is common even in places where the grammar doesn’t demand it. An obvious example:

(2) The person who left their wet swimsuit on my books is going to pay.

This sentence would be fine as the person that, and would be less redundant (we already know it’s a human being that the relative clause is modifying), but no one complains here. In fact, this is the preferred version according to many prescriptivists.

Now, all of this goes to show that some redundancy is okay, but it doesn’t directly address whether this particular redundancy is okay. We can surely agree that there are unacceptable redundancies, like the old department of redundancy department. But unacceptable redundancies have something else wrong with them. It’s not that they’re merely redundant; it’s that they’re redundant and longer, or redundant and confusing, or redundant and awkward.

The reason why (and similarly, the person who) is only redundant. It’s actually shorter than the alternative the reason that, and it’s neither confusing nor awkward. At absolute worst, it’s stylistically unpleasant, and even that’s in the eye of the beholder.

One last thing, and something that should tell you that the redundancy point is off the mark, is that the reason why is both common and venerable. Both Google Books N-grams and the Corpus of Historical American English have the reason why being consistently more common than the reason that for the last 200 years. And the first example of the reason why in the Oxford English Dictionary dates back to 1533:

“He couth fynd na resson quhy he aucht nocht to helpe þe romane pepill to recovir þe land.”

[Google Books N-grams results for "the reason why" and "the reason that"]

Summary: Are people telling you that the reason why is redundant and therefore unacceptable? They’re wrong; there’s nothing inherently unacceptable about redundancy, and the reason why has been standard for centuries.

*: The story of the sign is revealed here.

**: Nor in other areas; architectural structural redundancy prevented significant damage to the Empire State Building when a plane crashed into it in the 40s.