There are two major driving forces of stock-market returns, according to John Hussman: valuations and investor sentiment.
"Put simply, we estimate that the S&P 500 faces the same prospect of full-cycle loss and return-free risk as it did in 1929, 2000, and 2007.
The S&P 500, in its current form since 1957, fell more than 46% from 2000-2002 and more than 52% from 2007-2009.
Hussman FundsAs for investors sentiment — or what Hussman calls "market internals" — he uses a proprietary measure of the uniformity of investor behavior.
Predicted in April 2007 that the S&P 500 could lose 40%, then it lost 55% in the subsequent collapse from 2007 to 2009.