From The New York Times
Vanguard created a model portfolio divided equally between stocks and bonds, and compared the returns in periods of economic expansion and recession. It found that “the average real returns of such a portfolio since 1926 have been statistically equivalent regardless of whether the U.S. economy was in or out of recession.”
The average returns (after inflation) was found to be over 5%. Of course to use this study to build a portfolio, you have to assume future returns will match historical returns.