Mirror, Mirror

The title text to this xkcd comic describes a famous trick question:

"When you look at words in a mirror, how come they're reversed left to right but not top to bottom? What's special about the horizontal axis?"

The answer is fairly obvious (they're reversed left to right because that's how you flipped the text around in order to face it towards the mirror), if slightly counter-intuitive, because it is easy to confuse the idea of a reflected image with a reversed image. It's also easy to prove that the premise of the question is incorrect - write a word on a sheet of paper, and then turn it top to bottom to face the mirror. Oh hey, the word is now upside-down, but not remotely flipped from left to right!

The question of what's special about the horizontal axis needs to be taken separately from the mirror. Or rather, why do we feel compelled to turn a sheet of paper or a book from left to right when turning it towards a mirror, rather than top to bottom, diagonally or any other angle out of the three hundred and sixty available? 

I thought it might something to do with the way we read horizontally, and therefore someone accustomed to reading top-to-bottom would turn the paper in that direction instead. My father contended that, since our feet are stuck to the ground by gravity, we are accustomed to move along the horizontal axis.  If you're in a dressing room and want to take a look at the back of your outfit, you twist yourself left or right about your waist - it never occurs to you to bend forward or backward until you can see your reflection, possibly because you're simply not flexible to do that anyway.

It's a bit funny though, how we don't even stop to think about such things. Or about astronauts in zero-gravity environments who might have a different perspective on things.