A few years ago somebody had this theory that a human face had a good side and a darkside. So the experiment was you take a picture full face, straight on evenly lit and print in say 8x10. Then you flop the negative and print another one. Lets call the normal one Print A and the flopped one Print B. Then you take and cut each one in half right down the nose.
Next you take the right half away from Print A and put it on the right side of Print B while doing the same with Print B, putting the right half on Print A.
So what you now have is Print A with only one side of the face making up the whole face and Print B is the same.
So the theory is that on one you look like a bad person and the other you look like your good self.
It's easier to do than it is to explain.
I don't think the theory was ever proven to be correct.