Many well-meaning writers and editors condemn “the reason why” and “the reason is because” for the crime of redundancy. But that stance (or, at least, part of it) is shaky. “The reason is because” has no supporters, but “the reason why,” despite also being idiomatic, is ubiquitous even among highly respected writers.
Yes, “the reason why” and “the reason is because” are redundant — guilty as charged. In place of “I want to know the reason why you took my book,” one can write “I want to know the reason (that) you took my book,” “I want to know why you took my book,” or “I want to know your reason for taking my book.”
Instead of “The reason is because I thought it was mine,” one can write, “The reason is that I thought it was mine,” “The reason is, I thought it was mine,” “I took it because I thought it was mine,” or, simply, “I thought it was mine.” (“Because I thought it was mine” is acceptable in informal usage.)
“The reason why” has been used frequently throughout the history of Modern English as well as that of Middle English — all the way back to the 1200s. (However, “the reason is because” has no such pedigree.) Only in the twentieth century did prescriptivist grammarians begin to urge writers to, whenever possible, use “the reason that” (or one of the other alternatives mentioned above).
I will continue to avoid combining reason and why in my own writing but will forgive the combination when I am editing that of others — and, of course, it is correct when reason is a verb, rather than a noun, as in “to reason why” — and I will not tolerate “the reason is because” in any form. However, I’m puzzled by why one is accepted and the other isn’t.
In “the reason why,” why is a conjunction linking the noun reason to the phrase “you took my book.” (Equivalent usage includes the phrases “the place where” and “the time when.”) But because is a conjunction, too. And though language maven Bryan A. Garner approves of “the reason why” yet condemns “the reason is because,” a sample sentence in the entry for because in Merriam-Webster’s Collegiate Dictionary, “The reason I haven’t been fired is because my boss hasn’t got round to it yet,” amuses me.
The dictionary uses the condemned redundancy in its example of usage of because. But it’s not the sentence that prompts my mirth; it’s the name of the source of the sample sentence: E. B. White, coauthor of the revered writing guide The Elements of Style.