Do index funds drive the price of their underlying stocks? Do the index funds instantly purchase shares of their individual stocks when their own shares are bought or do they wait until end of day?
To answer your first question: Yes, index funds absolutely drive the prices of their underlying stocks, though often through "hidden" market mechanics rather than direct buying.
Whether they buy "instantly" depends entirely on whether you are buying an ETF (like VOO) or a Mutual Fund (like VFIAX).
1. Do they drive the price?
Yes, but it's a "tail wagging the dog" scenario. This happens through three main channels:
The Arbitrage Effect: When thousands of people buy an ETF, its price starts to rise slightly above the value of the actual stocks it holds (this is called a "premium"). Professional traders (Authorized Participants) notice this and instantly buy the underlying stocks and sell the ETF to pocket the difference. This buying pressure pushes the stock prices up.
Inclusion Rallies: When a stock is added to a major index (like S&P 500), index funds must buy it. This "forced" demand often causes a massive price spike before the stock even joins the index.
Price Correlation: Because index funds buy and sell all stocks in the "basket" simultaneously, stocks in the same index tend to move together more closely than they would based on their individual company performance.
2. When do they buy the shares?
The timing depends on the vehicle you are using.
Why "End of Day" is standard for Mutual Funds
Mutual funds calculate their Net Asset Value (NAV) once per day after the market closes. If you place an order at 10:00 AM, the fund won't actually "buy" your portion of the stocks until the end of the day because they need to see the total sum of all investors' orders to trade efficiently in bulk.
How ETFs handle "Instant" buying
ETFs don't actually buy the stocks every time you buy one share. Instead, Authorized Participants (APs) act as the middleman. They watch the supply and demand. If there is a massive surge in buying at noon, the AP will go into the market, buy the underlying stocks right then, and "hand" them to the ETF provider in exchange for new ETF shares. This keeps the prices perfectly synced in real-time.
Note: Because of this "batched" vs. "real-time" difference, Mutual Funds are generally better for long-term automated investing, while ETFs offer more control for people who care about the specific price at 10:30 AM versus 3:55 PM.
