Growth Shocks and the Herfindahl-Hirschman Index for Capitalization-Weighted Equity

This article explores the relationship between economic growth shocks and market concentration within capitalization-weighted equity indices, using the Herfindahl–Hirschman Index (HHI) as a measure of concentration. Deploying a constant-growth discounting framework, we demonstrate how market stress amplifies concentration and drives a shift in equity style performance as capital flows into large-cap growth firms that are perceived as safer during downturns. Conversely, economic recovery and inflation reduce concentration, creating opportunities for value stocks and small-cap stocks to rebound. Empirical analysis of historical events—including the dot-com bubble, the 2008 financial crisis, and the COVID-19 pandemic—illustrates the cyclical nature of market concentration driven by changes in earnings dispersion and growth sensitivity. The results show how growth and value stocks react differently to economic shocks and inflationary periods. By understanding these dynamics, practitioners can refine their portfolio strategies, time style rotations, and anticipate shifts in market leadership tied to broader economic conditions. The article’s findings also provide a practical tool for interpreting HHI trends and their implications for asset allocation in an unpredictable market environment.