Mark Thoma called my attention to a Vox blog post by Rachel and Smith, who are at the Bank of England. The post is a condensed version of this Bank of England working paper, which assembles a lot of evidence on low real interest rates, and takes a global perspective.
In the blog post, the key point is that simple, static, savings/investment logic helps us think about what is going on. To see what they're getting at, here's their Figure 4:
In the United States, we can tell a story similar to the above figure, in that real interest rates have fallen, and if we look at investment rates, we get:this paper by Hamilton et al.) the real interest rate on government debt has fallen substantially since the 1980s.
But, we could argue that the relevant real rate of return that applies to the first chart is the real rate of return on capital. Conveniently, as it turns out, Gomme, Ravikumar, and Rupert have measured rates of return on capital from the NIPA (national income and product accounts) data. Here's one of their charts:
As I argue in the slides I link to in this post, I think it's more useful to think about asset markets in explaining low real interest rates. For the U.S., it's important that we consider the market for U.S. Treasury debt, and what makes demand high and supply low in that market. That then has implications for monetary policy and inflation that I think are interesting. See the slides for the details.