India, Oct. 6 -- The S&P 500 is more dependent on three stocks than ever before. Nvidia, Apple and Microsoft now together account for more than 21% of the S&P 500, according to data from Creative Planning.

This means a very large part of the index's performance is tied to just these three technology companies. Their combined weight in the S&P 500 has reached a record level, showing how concentrated the US stock market has become.

Nvidia, Apple and Microsoft are now far more important to the index than the biggest companies were in previous decades. The only comparable example came in the mid-1980s, when IBM, AT&T and ExxonMobil together accounted for 13.4% of the S&P 500, according to the data cited by Yahoo Finance.

The current concen...