Two years ago, Professor of Finance Hank Bessembinder stirred up the market with his study showing all the wealth created by U.S. stocks is the result of gains in a small group of companies. His update hasn't changed the situation.

In this article published July 17, 2019, on Bloomberg:

It is historically the norm in the U.S. and around the world that a few top-performing companies have great influence over how the market does overall. It’s the norm and I expect it to be the case in the future.

– Hank Bessembinder, professor of finance and Francis J. and Mary B. Labriola Endowed Chair in Competitive Business