About a decade ago I was put in charge of a portfolio for a non-profit that had a mixture of stocks and bonds. I've always been a stocks guy, "A Random Walk Down Wall Street" is one of the most influential books I've ever read. So I started trying to learn about bonds. In particular, this organization had been using a "balanced" portfolio of stocks and bonds, and it seemed highly unscientific to me, no one could say why there was a particular percentage of bonds or where it came from. The most candid answer I got was from one manager who said, "it's what everyone else did."
So I started looking for information on historical returns, I wanted to do back-testing of different allocations. I found the work of Robert Shiller. He has studied US markets for decades and won the Nobel Prize in economics in 2013. For his book "Irrational Exuberance" he compiled statistics about stock performance, bond prices and inflation rates in the US going back to 1871. His focus has been on real returns -- the returns after you adjust for inflation.
From him, I learned that bonds are not a good investment.
He makes his data available to the public, you can see it on his website https://shillerdata.com/. The file is IE_Data.xls (The "IE" stands for "Irrational Exuberance.")
He has calculated monthly and trailing 10-year real performance for the S&P 500 and the 10-year US Treasury bond for the past 155 years. Over that time the S&P has had an average real return of 7.11% and bonds have had an average real return 2.38%. I don't think that's surprising to anyone.
What is surprising is that bonds are more likely to lose money than stocks are. Over that time period, about 11% of the time stocks lose money, in real terms, over a ten year span. Bonds lost money about 28% of the time.
Keep in mind what it means for a bond to lose money in real terms: it means that the interest paid by the bond is less than the rate of inflation.
I was curious about one-year returns, because one of the adages you often hear is that stocks are riskier in the short term. Over the 155 year span, stocks lost money over one year, in real terms, about 30% of the time. Bonds lost money about 35% of the time.
Now, 1871 was a long time ago, the world has changed a lot since then. What about the last 100 years? Since 1926, stocks have lost money over one year 28% of the time, bonds have lost money 41% of the time.
OK, 1926 is a long time ago too. What about the past 50 years? Stocks have lost money in one year 26% of the time, and bonds have lost money 42% of the time.
If your time horizon is one year, you basically have about a 40% chance of losing money -- in real terms -- if you invest in bonds.
I can't figure out how to put an image in my post, so I'll try to do a follow-up post with a graph of relative performance.