The nature of firm growth

Only half of all startups survive past the age of ve and surviving businesses grow at vastly di erent speeds. Using micro data on employment in the population of U.S. businesses, we estimate that the lion's share of these differences is driven by ex-ante heterogeneity across fi rms, rather than by ex-post shocks. We embed such heterogeneity in a fi rm dynamics model and study how ex-ante differences shape the distribution of fi rm size, "up-or-out" dynamics, and the associated gains in aggregate output. "Gazelles" - a small subset of startups with particularly high growth potential - emerge as key drivers of these outcomes. Analyzing changes in the distribution of ex-ante fi rm heterogeneity over time reveals that gazelles are driven towards extinction, creating substantial aggregate losses.