Equity premium puzzle

The equity premium puzzle is a mystery about why stocks are more attractive to investors than investments like bonds. Basically, it's a puzzle because investors should expect to get what's called a "risk premium" when they choose to invest in stocks - that means they should make more money on their investment than they would if they chose something safer like a bond. But when economists look at the numbers, they find that the risk premium investors get from stocks is bigger than it should be. This means that investors are getting more money back from their stock investments than they should, and economists can't figure out why!