Friday's S&P 500 and Nasdaq-100 rebalance may reflect concerns over concentration risk

cnbc.com:

It's arguably the biggest stock story of 2023: a small number of giant technology companies now make up a very large part of big indexes like the S&P 500 and the Nasdaq-100. 

Five companies (Apple, Microsoft, Amazon, Nvidia and Alphabet) make up about 25% of the S&P 500. Six companies (Apple, Microsoft, Amazon, Nvidia, Alphabet and Broadcom) make up about 40% of the Nasdaq-100. 

The S&P 500 and the Nasdaq are rebalancing their respective indexes this Friday. While this is a routine event, some of the changes may reflect the concerns over concentration risk. 

Now that the CPI and the Fed meeting are out of the way, these rebalances are the last major «liquidity events» of the year, corresponding with another notable trading event: triple witching, or the quarterly expiration of stock options, index options and index futures. 

This is an opportunity for the trading community to move large blocks of stock for the last gasps of tax loss harvesting or to position for the new year. Trading volume will typically drop 30%-40% in the final two weeks of the year after triple witching, with only the final trading day showing significant volume.

All of this might appear of only academic interest, but the big move to passive index investing in the past 20 years has made these events more important to investors. 

When these indexes are adjusted, either because of additions or deletions, or because share counts change, or because the weightings are changed to reduce the influence of the largest companies, it means a lot of money moves in and out of mutual funds and ETFs that are directly or indirectly tied to the indexes. 

Standard & Poor's estimates that nearly $13 trillion is directly or indirectly indexed to the S&P 500. The three