Here is an interesting chart comparing the correlation of various asset classes to the S&P 500 over the last 5 years. Chart is from Richard Bernstein Advisors, found via The Reformed Broker.
A negative correlation between two asset classes means that they tend to move in opposite directions. While long-term US Treasury bonds have been the most strongly uncorrelated, it is also worth noting that intermediate-term US Treasuries (5-7 years) were nearly as uncorrelated. Of course, this is the past and correlations can and will change.
Still, this would seem like good news for people who hold a “total” bond fund like the Vanguard Total Bond Market Index Fund (VBMFX, BND) or iShares Barclays Aggregate Bond Fund (AGG) as these contain ~70% US government bonds and have an intermediate average maturity. You want your bonds to serve as a hedge against stock movements, and they did over the past 5 years while still maintaining positive returns (~4% annualized for AGG, ~15% annualized for S&P 500).