"〜ちゃう" is often simply described as the casual/contracted/etc. form of "〜てしまう", with no further explanation given. This would seem to imply that in terms of meaning they are identical, with only politeness/formality considerations determining which is used.
However, there is evidence that this is not the case. Although Google hits are an extremely inexact measure, these are the rough figures for a few searches (all in quotation marks):
- 教えちゃいました = 60,000 hits
教えてしまいました = 1,600,000 hits
教えちゃってください = 28,000 hits
教えてしまってください = 3 hits
食べちゃってください = 1,150,000 hits
食べてしまってください = 1,800,000 hits
報告しちゃってください = 1,150,000 hits
報告してしまってください = 0 hits
感動しちゃってください = 600,000 hits
感動してしまってください = 0 hits
インストールしちゃってください = 230,000 hits
- インストールしてしまってください = 645,000 hits
60k vs 1,600k is a big difference in relative terms, but even 60k hits implies a perfectly acceptable form. However, hit counts of 0 and 3 are quite striking and suggest that the forms are basically never used, even though the equivalent 〜ちゃってください forms are.
So why exactly is it this? Why do some 〜てしまってください vs 〜ちゃってください pairs return such lopsided hit counts, while others do not?
(Note: I'm looking for answers with a bit of rigor, ideally expressing a general principle, and certainly going beyond "It just feels more natural for some words" or similar.)
Edit: Couple of clarifications:
The past tense (〜ました) example above is basically for context, I don't think it needs explication. I only noticed this phenomena with the 〜てください contexts. If it does or doesn't happen with other contexts, that would probably make up part of a good answer!
The Google numbers are terrible, okay, point taken. It still looks to me like the possible number of hits for 〜てしまってください is either "virtually none" or "dozens to hundreds", with not much in between and no obvious relation to 〜ちゃってください numbers. If I've just chosen bad examples and there is actually a subtle gradient I'm just not seeing, proof of this would constitute a good answer. (In particular if anyone wants to search a proper corpus that would be great.)