I was just reading this learned article in the Financial Analysts Journal
"Stocks for the Long Run? Sometimes Yes, Sometimes No"
Analyzing historical data for 19th century US stock and bond returns, this study challenges Jeremy Siegel’s “Stocks for the Long Run” thesis. Evidence indicates that 20th century US asset returns cannot be generalized out of sample.
"Abstract
When Jeremy Siegel published his “Stocks for the Long Run” thesis, little was known about 19th-century stock and bond returns. Digital archives have made it possible to compute real total return on US stock and bond indexes from 1792. The new historical record shows that over multi-decade periods, sometimes stocks outperformed bonds, sometimes bonds outperformed stocks and sometimes they performed about the same. New international data confirm this pattern. Asset returns in the US in the 20th century do not generalize. Regimes of asset outperformance come and go; sometimes there is an equity premium, sometimes not.
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