Redundancy

GABE DOYLE of Motivated Grammar (motto: "Prescriptivism must die!"), is sticking up for "the reason why". The phrase is usually criticised by sticklers as redundant. Mr Doyle makes an in important counterargument: redundancy is a core feature of language. Every language has features that are not required for comprehension, but reinforce something that is also conveyed another way. In the Spanish phrase las tres casas blancas, "the three white houses", every single word is marked for plurality. "Three" would suffice to let the listener know we aren't talking about one house (and the number would be all that's required in languages that don't mark plurality on nouns and adjectives), but Spanish grammar requires the rest of the markers.  And redundancy is a feature, not a bug, of many noisy systems: no city or state name is needed if the US Postal Service has a ZIP code. But if one digit of the code is illegible, the city and state name will make sure your letter gets there anyway. The right amount of redundancy is that which gets the signal through the noise without waste.

But Mr Doyle makes another argument for "reason why": it's simply been more prevalent than "reason that" in books, according to Google's N-gram search. 

But here Mr Doyle's argument stumbles. He seems to be thinking of noun phrases with "the reason [why/that]" plus a complement clause, as in

The reason why I'm leaving your father is his drinking

versus

The reason that I'm leaving your father is his drinking.

He's right that the "that" version feels more awkward, and is less common.  (But of course both "why" and "that" can be left out here: The reason I'm leaving your father.)

I'm thinking of a different kind of noun phrase, one without the complement clause. 

I'm leaving your father. The reason why is that he's a drunk.

This is much better expressed as

I'm leaving your father. The reason is that he's a drunk.

And it turns out that "the reason why is" is much less common than "the reason is."

(A search for "the reason why is that" versus "the reason is that" turns up a similar picture.)

Mr Doyle is right that "the reason why" is old—he gives a citation from 1530. And it's not ungrammatical. But it's not "venerable", as he says, given that English authors overwhelmingly prefer "the reason is".  Descriptivism means accurately describing all kinds of language.   This Johnson thinks, as the original Johnson did, that describing and prescribing are not mortal enemies. (And others from Johnathon Owen to Bryan Garner to John McIntyre agree.)  I would describe "the reason why is" as rare in good writing, and I would prescribe, as a teacher or an editor, that writers seeking my approval not use it.  

And don't get me going on "the reason why is because"...