Nvidia stock accounts for more than a third of the S&P 500's gains this year—and it's a big risk for investors, top economist saysNvidia stock accounts for more than a third of the S&P 500's gains this year—and it's a big risk for investors, top economist says - Fortune

The extreme concentration of returns in the S&P 500 makes investors more vulnerable to single headlines that impact the stock, a top economist said.