https://en.wikipedia.org/wiki/Global_saving_glut https://en.wikipedia.org/wiki/Global_saving_glut The key insight is that savings and investment opportunities

https://en.wikipedia.org/wiki/Global_saving_glut https://en.wikipedia.org/wiki/Global_saving_glut The key insight is that savings and investment opportunities are two sides of a single market. Demand and supply. Savings grow reliably and opportunities do not, so demand for investment opportunities outstrips supply of investment opportunities. Therefore, the opportunities get more expensive = lower rate of return.