One way to reduce investment risk is diversification, but that requires that the investments don't correlate. In the market downturn of 2008 and 2009 many investors learned that a portfolio of domestic stocks, international stocks and bonds did not provide as much diversification as they had thought. This Seeking Alpha article
reviews what can be added to a portfolio to provide more diversification. Some of the non-correlating investments mentioned include gold, TIPS, emerging market bonds and REITs.