The article examines what causes fluctuations in business cycles, focusing on the impact of technology and demand shocks on productivity. By analyzing industry-level data from 1960 to 2005, the researchers found that demand shocks play a significant role in driving aggregate fluctuations, while technology shocks are industry-specific and have minimal impact at the aggregate level. They also discovered that the decrease in productivity cyclicality in the U.S. after the mid-1980s can be attributed to a reduction in the size of demand shocks.