This is a resource page on the equity premium, or equity premium puzzle.
The bottom line is that for more than fifty years the US equity markets have provided returns that are substantially larger than US fixed income markets --- even after observed risk is taken into account.
This empirical fact is now relied upon by many investors, most notably pension fund managers. Naturally, given that this is a"lesson learned," one wonders if the future will indeed replicate the past.
Resources:
Equity Risk Premium Forum (2001).
What Risk Premium Is "Normal"? Robert D. Arnott and Peter L. Bernstein