What an index fund does
An index fund holds a basket of stocks or bonds that mirrors a specific market index — a published list of companies weighted by size or other rules. When you buy shares in an index fund, you own a tiny piece of every holding in that basket. The fund's value rises and falls with the index it tracks, so your return matches the market's return minus the fund's expenses.
The fund manager does not pick individual winners. Instead, they buy and hold the exact securities in the index, in the same proportions. If the S&P 500 index includes Apple at 7% of its total value, the fund holds Apple at roughly 7% of its assets. When the index changes — a company is added or removed — the fund rebalances to match.
You do not own the index itself. You own shares of the fund, which is a separate legal entity that holds the securities on your behalf. The fund charges a fee (called an expense ratio) to cover the cost of buying, holding, and tracking those securities.
Key Takeaways
- Index funds hold all or most of the securities in a published index, so your return tracks that index minus the fund's expense ratio.
- The fund manager rebalances automatically when the index changes, without trying to beat the market or time trades.
- You own shares of the fund itself, not the individual stocks or bonds inside it, and the fund holds those securities in a custodial account.
- Different index funds track different indexes — the S&P 500, the total U.S. stock market, international stocks, bonds, or combinations — so the fund you choose determines which market you are exposed to.
- The expense ratio is the only cost you see directly; it is deducted from the fund's value each year and affects your return.
How the fund buys and holds securities
When you deposit money into an index fund, the fund manager uses that cash to purchase the securities listed in the index. For a large index like the S&P 500, this means buying shares of 500 large U.S. companies. The fund holds these securities in a custodial account — a legal structure that separates the fund's assets from the fund company's own money.
The custodian (usually a bank or trust company) keeps the securities safe and handles the paperwork. You do not receive individual stock certificates or bond documents. Instead, you receive a statement showing how many shares of the fund you own and what that stake is worth on any given day.
When companies in the index pay dividends, the fund collects that cash. You can choose to reinvest those dividends (buy more fund shares) or receive them as a payment. Most index fund investors reinvest automatically, which compounds growth over time.
Why the fund's value changes
The fund's share price moves when the market value of the securities inside it changes. If the S&P 500 index rises 2% in a day, an S&P 500 index fund's share price also rises roughly 2% (minus a tiny daily portion of the expense ratio). If the index falls, the fund falls with it.
The fund does not try to avoid losses or time the market. It straightforward holds the index and lets price movements happen. This is different from an actively managed fund, where a manager buys and sells individual securities to try to beat the index.
The fund's value also reflects the expense ratio, which is deducted continuously. A fund with a 0.03% annual expense ratio costs about $3 per year for every $10,000 invested. This fee is built into the fund's daily price — you do not write a separate check.
How rebalancing keeps the fund aligned with the index
When a company is added to or removed from the index, or when the index's weighting rules change, the fund must buy or sell securities to stay aligned. This rebalancing happens automatically according to the index's published rules, not based on a manager's judgment.
For example, if a company grows so large that it would represent more than the index allows, the index removes some of that company's weight and shifts it elsewhere. The fund sells a portion of that holding and buys more of the underweighted securities. This keeps the fund's composition matching the index's composition.
Rebalancing also happens when you and other investors add or withdraw money. If many people deposit cash, the fund buys new securities proportionally across all holdings. If many people withdraw, the fund sells proportionally. This keeps existing shareholders' stakes aligned with the index.
The difference between index funds and individual stock ownership
When you own an individual stock, you own a direct claim on that company's earnings and assets. You receive voting rights and can sell whenever you choose. When you own an index fund, you own a share of a fund that owns many companies. Your voting rights (if any) belong to the fund, not to you directly.
An index fund spreads your money across dozens, hundreds, or thousands of securities, so a single company's failure does not wipe out your investment. An individual stock concentrates your risk in one company. Index funds are simpler to manage — you make one purchase decision instead of many.
Index funds also have lower turnover, meaning the fund buys and sells less frequently than an active manager would. Lower turnover means lower trading costs and fewer taxable events in taxable accounts, which can improve your after-tax return.
How index funds compare to actively managed funds
An actively managed fund employs a manager or team to research companies and decide which securities to buy and sell. The manager tries to beat the index by picking winners and avoiding losers. An index fund does not try to beat the index — it straightforward holds the index.
Active managers charge higher expense ratios because they employ analysts and traders. Index fund expense ratios are typically much lower because the fund straightforward follows a published rule. Over long periods, the lower costs of index funds often result in better returns than active funds, even before accounting for the manager's skill.
Active funds can outperform in any given year, but studies show that most do not beat their index consistently over 10 or 20 years. An index fund's return is predictable: it will match the index minus the expense ratio, with no surprises.
What happens when you sell your shares
When you sell shares of an index fund, you receive cash equal to the fund's current share price times the number of shares you sell. The fund does not have to sell its underlying securities to pay you — it uses cash from other investors' deposits or from dividends and interest it has collected.
If the fund does need to sell securities to meet large withdrawals, it sells proportionally across all holdings to maintain the index alignment. This does not affect your return — you straightforward receive the market value of your stake on the day you sell.
In a taxable account, selling at a profit triggers a capital gains tax. In a tax-advantaged account like a 401(k) or IRA, you do not owe tax when you sell, though you may owe tax later when you withdraw the money.
Frequently Asked Questions
Can an index fund go to zero?
An index fund can decline sharply if the index it tracks declines sharply — the 2008 financial crisis saw many stock index funds lose 50% or more. But an index fund cannot go to zero unless every company in the index fails simultaneously, which is extremely unlikely for broad indexes like the S&P 500. Bond index funds carry different risks, including interest rate risk and credit risk.
Do I own the companies in an index fund?
You own a share of the fund, which owns the companies. You do not own the companies directly. The fund holds the securities in a custodial account, and you have a claim on the fund's value. If the fund is liquidated, you receive your proportional share of the proceeds after expenses are paid.
What if the index fund manager makes a mistake?
Index funds are designed to minimize manager discretion, so mistakes are rare. If the fund buys the wrong securities or holds them in the wrong proportions, it will underperform the index. Most index funds track their indexes very closely — the difference is usually less than 0.1% per year. If a fund consistently lags by more than its expense ratio, that is a sign of poor execution.
How often does an index change?
It depends on the index. The S&P 500 changes when companies are added or removed, which happens several times per year. The total U.S. stock market index changes more frequently because it includes thousands of companies. Bond indexes change when bonds mature or are added to the market. The fund rebalances automatically whenever the index changes, without waiting for your approval.
Can I lose money in an index fund?
Yes. If the index declines, the fund declines. Stock indexes can fall 20%, 30%, or more during recessions and market downturns. Bond indexes can fall when interest rates rise. The longer you hold the fund, the more time you have to recover from declines, but short-term losses are always possible.