Typical investors pay most of their attention to the stock market. After all, stocks offer the chance to see the biggest returns over time, with the prospect of earning life-changing wealth if you're fortunate enough to identify successful companies early in their histories.

The 10-year-old bull market in stocks has come as a pleasant surprise to many investors who had to endure the pain of the financial crisis. But what's been even more shocking for many on Wall Street is the fact that the bond market has continued to produce such strong returns. A long-term trend that's taken interest rates on Treasury bonds from double-digit percentages in the early 1980s to their current low levels has produced impressive results for those who've kept a bond allocation in their investment portfolios.

As you can see below, the bond market has held its own since 2014, and even though the roughly 50% total return of the iShares 20+ Year Treasury ETF (TLT 0.62%) hasn't quite been able to match the 65% that the SPDR S&P 500 ETF (SPY 0.94%) has produced over the past five and a half years, the margin is a lot closer than many might have expected.

Bar chart showing returns on bonds and stocks over the past five years

Data source: Morningstar. Chart by author.

Bond prices are sensitive to interest rates, and moves in rates have been quite volatile recently. Throughout most of 2017 and 2018, the Federal Reserve moved aggressively to raise interest rates, and that showed up in the negative returns that long-term bonds had in 2018. However, as you can see, the stock market actually performed worse last year due to fears that the long economic expansion might come to an end.

Moreover, the recent reversal in the direction of interest rates has led to a rebound for bonds. Even though stocks have outperformed the bond market slightly so far in 2019, that outperformance hasn't been enough to let the S&P 500 catch up to the performance of long-term Treasurys since the beginning of 2018.

Looking ahead, some believe that the Federal Reserve might well have to cut interest rates in the near future, especially if the global economy continues to falter. That would potentially produce even more gains for bonds -- and fly in the face of those who've argued for years that bonds would eventually bring big losses for their investors.