Index funds and active management - Warren Buffett is a vocal critic of active management.

Towards his later life, particularly following the global financial crisis of 2007-8, Buffett became an increasingly vocal critic of active management, i.e., mutual funds and hedge funds. 

Buffett is skeptical that active management and stock-picking can outperform the market in the long run, and has advised both individual and institutional investors to move their money to low-cost index funds that track broad, diversified stock market indices. 

Buffett said in one of his letters to shareholders that "when trillions of dollars are managed by Wall Streeters charging high fees, it will usually be the managers who reap outsized profits, not the clients."

In 2007, Buffett made a bet with numerous managers that a simple S&P 500 index fund will outperform hedge funds that charge exorbitant fees. By 2017, the index fund was outperforming every hedge fund that had made the bet against Buffett by a significant margin.