Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
reason is because
This locution has long been in use in Standard English; it has also long been criticized, the explanations for the harsh judgments varying from redundancy to bad grammar. But some reputable authors continue to write it, and some Standard users continue to say it at nearly all Conversational levels. Edited English and many of those who write for its editors, however, continue to prefer the reason is that. Some Standard users never say the reason is because, others use it even though they’ve been taught otherwise, and many never notice it in their speech. Best advice: avoid it at Planned, Oratorical, and Formal levels, and especially in Edited English, or risk the censure of conservatives.
