According to @KobeissiLetter, US stock market concentration is near record territory, with the S&P 500 Herfindahl-Hirschman Index around 195, among the highest readings on record (source: @KobeissiLetter). The source explains that HHI measures how market value is distributed across all 500 constituents and that higher readings signal greater concentration and dependence on a smaller set of names (source: @KobeissiLetter). For SPX traders, this implies index performance and volatility are more driven by moves in top-weighted mega caps, a direct consequence of the metric definition cited by the source (source: @KobeissiLetter).