Real interest rates have been dropping since the 1980s, even though returns on investments have stayed steady. By using a model that looks at factors like labor force and productivity growth, researchers found that the decrease in interest rates can be explained by a change in how risky people think future productivity growth is. This means that the amount of debt people can take on has increased, which matches the rise in total debt seen since the financial crisis. This challenges the idea that lower interest rates are due to people paying off debt.