The equity premium: A puzzle (Mehra and Prescott (1985)) lays out the basic problem with equity returns:
Restrictions that a class of general equilibrium models place upon the average returns of equity and Treasury bills are found to be strongly violated by the U.S. data in the 1889–1978 period. This result is robust to model specification and measurement problems. We conclude that, most likely, an equilibrium model which is not an Arrow-Debreu economy will be the one that simultaneously rationalizes both historically observed large average equity return and the small average risk-free return. ...Historically the average return on equity has far exceeded the average return on short-term virtually default-free debt. Over the ninety-year period 1889-1978 the average real annual yield on the Standard and Poor 500 Index was seven percent, while the average yield on short-term debt was less than one percent. The question addressed in this paper is whether this large differential in average yields can be accounted for by models that abstract from transactions costs, liquidity constraints and other frictions absent in the Ar~ow-Debreu set-up. Our finding is that it cannot be, at least not for the class of economies considered. Our conclusion is that most likely some equilibrium model with a friction will be the one that successfully accounts for the large average equity premium.
I've summarized this for my grandmother by saying that the most basic economic models explain the high equity premium by saying people are very risk adverse but if they are that risk adverse, other things make no sense, like how much people charge to do dangerous jobs.
In the 30 years since, much ink has been spilled on the subject of just what is causing the equity premiums in the historical record. Indeed, there is a huge book, the Handbook of the Equity Risk Premium that just summarizes these results. I've read much of this book and I've come away a bit confused. There are so many explanations each capable of explaining much of the puzzle that the puzzle itself appears overdetermined. That is, if all the (or even several popular ) explanations are true, the question isn't "why is the premium is so large?" but rather "why is the premium so small?".
What are we to conclude about this? All the explanations jointly can't be true, some are contradictory. For example, the peso and reverse-peso explanation says long run returns are likely lower than historical returns in the USA, while preference based explanations of the equity premium need an actual premium to explain. However, once we've eliminated the contradictory explanations, what then? What does one conclude about the individual explanations, the literature as a whole, and the underlying phenomenon?