Market Concentration

This card shows how much of the total value of the US stock market is held by its largest companies: the top tenth of NYSE-listed firms by size, plus any Nasdaq or other stocks that large. Today that is roughly the top 5% of listed firms, not the top 10 stocks. The bucket held about 70 firms in 1930 and holds about 170 today. A high reading means the market's fortunes rest on a small group of giants, so a stumble by a few of them moves the whole index. The share peaked at 75% in 1932 and reached 73% at the end of 2000, and set a record of about 78% in late 2025. It is not a timing signal. In 2000 the share kept rising until December, nine months after the market top, because small stocks fell even further than the giants. The rating ranks the latest reading against every month since 1926, with a higher share rated as higher risk. The data comes from CRSP via Ken French's size portfolios. The long fall through the 1960s and 1970s mostly reflects AMEX stocks joining the database in 1962 and Nasdaq stocks in 1972, which added many smaller firms, not the market becoming less concentrated. Ken French publishes each month's figures about two months later.