S&P 500, Nasdaq-100 are getting an update. Trillions depend on who's in and who's out

Traders work at the New York Stock Exchange on Dec. 17, 2024.The S&P 500 and the Nasdaq-100 indexes get new members added and old ones kicked out on Friday in a quarterly rebalancing mostly based on companies' adjusted market capitalization. Years ago, these index reconstitutions had very little impact on trading. Not anymore. The explosion of index-based ETFs has changed all that. Today, roughly $16 trillion is indexed to the S&P 500. The largest Nasdaq-100 ETF (QQQ), has $321 billion in assets, and the In ...