Nearly 60% of the MSCI USA Index's value — approximately USD 38 trillion — comes from companies that weren't public 50 years ago.

“That's an astounding number,” says Abhishek Gupta, executive director of MSCI Research & Development and co-author of the firm's recent research examining the anatomy of U.S. equities.

The transformation has unfolded over decades, rather than through a single wave of initial public offerings. Companies that listed in the 1980s and 1990s each account for nearly one-fifth of the index’s weight as June 2026, while those that listed in the 2000s and 2010s each contribute roughly another tenth.

For investors, the finding underscores the importance of indexes that evolve with the market, capturing newly public companies as they become part of the investable opportunity set.

Read the research https://www.msci.com/research-and-insights/paper/the-anatomy-of-us-equities