Which way should be "on" for a switch?

Most of us use switches every day in various forms. And although there are sometimes general norms in countries as to which direction is "on", there is variation between countries.

So when developing a product with a switch in it, which direction makes the most sense for "on", and why?

I'm not talking about other ways of showing that they will be on or off, I'm more talking about whether "on" is up, down, left, or right. Any research on this would be appreciated.