Obviously, we are more honest when someone (or a security camera) is watching us, but studies have actually shown that if any depiction of an eye is in view, even if it is cartoonish or nonhuman, it makes people less likely to cheat or to behave immorally.
Put the bong down until the article is over.
In one experiment, all a professor had to do to drastically influence the actions of her colleagues was change the clip art on a piece of paper. They did the test in a teachers lounge, where the staff enjoyed a coffee/tea station that ran on the honor system. Teachers were welcome to help themselves, but a notice posted near the station asked users to pay for their coffee in the honesty box.
A picture of a cartoon eye was placed at the top of the reminder notice, and the amount of money left in the honesty box tripled.
Just to be sure it wasn't a coincidence, the next week the eye was replaced with a flower. Contributions went back to normal. Somehow, the subtle reminder of being watched made people way more honest.
Putting this poster in your bedroom will not get you laid.
But we've not only been programmed to fear the all-seeing eye, we have also been warned since childhood that otherworldly, omnipresent forces are also watching us, all the damn time. Whether it's Santa knowing our sleeping patterns or God himself hovering over our every move like a holy hawk, many of us were told that an invisible something was watching and keeping score. As a result, even if you're an atheist, any reference to God seems to make you more generous with your money and more moral in general.