The article examines how changes in U.S. real output growth and inflation volatility have evolved over time from 1870 to 2009. The researchers found that output growth volatility is mainly influenced by permanent shocks, while inflation volatility is driven by temporary shocks. They also discovered that temporary shocks are linked to changes in aggregate demand, while permanent shocks are associated with aggregate supply. The study suggests that aggregate supply played a significant role in output volatility fluctuations, especially before World War I.