What an Index Fund Tracks and Why It's Cheap

Imagine a librarian whose only job is to keep the shelves arranged in exactly the same order as a master catalog printed somewhere else. When the catalog adds a book, she adds it. When the catalog drops one, she drops it. She does not browse, does not recommend, does not have opinions about which books deserve more space. The shelves do whatever the catalog does. An index fund is, more or less, that librarian — and the catalog is an index.

An index is a published list of securities and the weights they are assigned. The S&P 500 lists roughly five hundred large U.S. companies, weighted by their market capitalization — the share price times the number of shares outstanding. The MSCI Emerging Markets index lists thousands of companies across dozens of countries, weighted by a similar but adjusted formula. The index itself owns nothing. It is a recipe, maintained by a committee or a rules document, that says: here is what a portfolio shaped like this asset class would look like.

An index fund is a pooled investment vehicle whose manager attempts to hold securities in the same proportions the index specifies. If Apple is 7 percent of the S&P 500 this morning, the fund tries to make Apple 7 percent of its holdings. When the index committee adds a company, the fund buys it. When a company is removed, the fund sells it. The manager is not asking whether Apple is overvalued or whether the next addition is a good business. That question has been ruled out of scope.

This is where the cost story begins. Running an actively managed fund requires analysts who study companies, portfolio managers who decide what to buy, traders who execute those decisions, and the research infrastructure underneath all of it. Those salaries are paid out of the fund's assets, in the form of an annual fee called the expense ratio. A typical actively managed U.S. equity fund charges somewhere between 0.5 and 1 percent of assets per year. An index fund tracking the same market often charges 0.03 to 0.10 percent. The difference is not a discount; it reflects a different production process.

Four structural features keep index fund costs low. First, the investment decisions are outsourced to the index provider, so the fund needs few analysts. Second, turnover — the rate at which holdings are bought and sold — is low, because the index itself changes only modestly each year, and every trade incurs commissions, bid-ask spreads, and tax consequences. Third, index funds tend to be large, and many costs of running a fund are roughly fixed, so a bigger asset base spreads them thinner per dollar invested. Fourth, the strategy is transparent and replicable, which has driven intense price competition among providers; an investor choosing between two S&P 500 funds is choosing between near-identical products, and providers have responded by cutting fees toward zero.

It is worth being precise about what cheap does and does not mean. A low expense ratio does not guarantee a good outcome. The fund will fall when its index falls, and a market-cap-weighted index concentrates exposure in whatever companies have grown largest, which is a real choice with real consequences. Tracking error — the gap between the fund's return and the index's return — is small but not zero, and is itself partly a function of how the fund handles dividends, rebalancing, and securities lending. The cheapness is a feature of the production method, not a promise about the result.

What the index fund offers, then, is a particular trade. The investor gives up any chance that a skilled manager will outperform the index, and in exchange receives the index's return minus a very small fee. Whether that trade is attractive depends on how much one believes active managers can reliably beat their benchmarks after costs — a question with a substantial empirical literature behind it. The mechanics of the fund itself, though, are almost boringly simple. Someone publishes a list. The fund holds the list. The fee is small because holding a list is not, in the end, very expensive work.