How Index Funds Actually Work

An index is a list of securities that follow a set of rules. The S&P 500 index, for example, is a list of 500 large US companies selected by a committee based on size, liquidity, and profitability. The rules determine who is on the list, not a manager's opinion.

An index fund is a portfolio designed to replicate the performance of an index. It buys the securities on the list in proportion to their size. No research, no picks, no bets.

Market Cap Weighting

Most major indexes are market-cap weighted. This means larger companies get a bigger slice of the fund.

In a total market fund:

• Apple (market cap ~$3T) gets a larger weight
• A $5B regional bank gets a tiny weight

This is intentional. The market is constantly voting on what each company is worth. Market-cap weighting lets the market's collective judgment determine the allocation, not any individual's prediction.

The Index Rebalances Itself

When a company grows, its weight in the index increases automatically. When a company shrinks or goes bankrupt, it exits. You never have to decide when to sell a failing company -- the index does it for you, mechanically, when the market has already priced in the decline.

This means the index automatically concentrates your money in what the market believes is most valuable, and reduces it in what the market has already punished. Not perfect, but disciplined.

The Simple Truth

You do not need to understand everything about how indexes work to benefit from them. The key insight is this: an index fund captures the market's return by owning everything and then getting out of the way.

No manager trying to be clever. No research costs baked into your fees. No behavioral errors from someone trying to time the market. Just ownership of the economy, held cheaply, for as long as you keep it.