Index funds are a type of mutual fund, not a separate category

An index fund is a mutual fund that tracks a specific market index — a pre-set list of stocks or bonds. The S&P 500 index fund, for example, holds the same 500 large-company stocks that make up the S&P 500 index, in the same proportions. When you buy shares of an index fund, you own a piece of all those holdings at once.

The key difference between an index fund and other mutual funds is how the fund manager decides what to buy. An index fund follows a rule: match the index. A traditional mutual fund manager picks individual stocks or bonds based on their own research and judgment, trying to beat the index. Both are mutual funds — both pool money from many investors and hold a basket of securities — but they use different strategies.

Think of it this way: all index funds are mutual funds, but not all mutual funds are index funds. An index fund is a specific approach to building a mutual fund portfolio.

Key Takeaways

  • Index funds are mutual funds that automatically hold the same stocks or bonds as a published market index, rather than relying on a manager to pick individual holdings.
  • Index funds typically charge lower fees than actively managed mutual funds because they require less research and decision-making.
  • You can find index funds that track the S&P 500, the total stock market, international stocks, bonds, or other indexes.
  • Index funds and actively managed mutual funds can coexist in the same investment account — many investors own both.

How index funds work inside a mutual fund structure

A mutual fund is a legal container that holds investments on behalf of many shareholders. An index fund uses that same container but fills it according to a formula. When the S&P 500 index adds or removes a company, the index fund automatically adjusts its holdings to match. The fund manager's job is to keep the fund aligned with the index, not to predict which stocks will outperform.

This mechanical approach means index funds can operate with smaller teams and lower overhead. Because the fund is not paying for extensive research departments or frequent trading decisions, the annual fee — called the expense ratio — is usually much lower. An S&P 500 index fund might charge 0.03% per year, while an actively managed large-cap stock fund might charge 0.50% or more.

You buy and sell index fund shares the same way you would any mutual fund: through a brokerage account, often with no transaction fee. The fund's value rises and falls with the index it tracks.

Index funds versus actively managed mutual funds

The main trade-off is predictability versus the possibility of outperformance. An index fund will match its index's returns, minus the small fee. You know roughly what you are getting. An actively managed mutual fund aims to beat its benchmark index, but this requires the manager to make correct decisions about which securities to hold. If the manager succeeds, you earn more; if they do not, you earn less — and you pay higher fees either way.

Over long periods, most actively managed mutual funds do not beat their index benchmarks after fees are subtracted. This is why many investors choose index funds: the fees are lower, and the results are predictable. But some investors prefer actively managed funds because they believe certain managers have genuine skill, or because they want someone actively making decisions about their money.

Both types are mutual funds. Both are regulated the same way. The choice between them is a strategy decision, not a category decision.

Common types of index funds you will encounter

Stock index funds track equity indexes. The S&P 500 index fund holds 500 large U.S. companies. Total stock market index funds hold thousands of U.S. companies of all sizes. International stock index funds track companies outside the United States. Each follows its index mechanically.

Bond index funds track bond indexes. A total bond market index fund might hold thousands of government and corporate bonds. These funds work the same way as stock index funds — they hold whatever the index holds, in the same proportions.

Target-date index funds combine multiple indexes in a single fund. A target-date 2050 fund might hold a mix of stock and bond index funds, automatically shifting toward more bonds as 2050 approaches. The fund itself is a mutual fund, and the holdings inside it are also index funds.

Why fees matter more in index funds

Because index funds aim to match their index, not beat it, the expense ratio becomes the main factor in performance. A 0.03% fee means you keep 99.97% of the index's returns. A 0.50% fee means you keep 99.50%. Over decades, that difference compounds significantly.

Some index funds charge more than others even though they track the same index. A fund from a large provider might charge 0.03% to track the S&P 500, while another might charge 0.10% for the same index. Over 30 years, that 0.07% difference can reduce your total return by several percentage points. When comparing index funds, the expense ratio is the first number to check.

Index funds in a diversified portfolio

Many investors use index funds as the core of their portfolio and add other holdings around them. You might own an S&P 500 index fund for large U.S. companies, an international stock index fund for exposure outside the United States, and a bond index fund for stability. Together, these three index funds give you broad diversification with low fees.

Some investors mix index funds and actively managed mutual funds. They might use index funds for broad market exposure and actively managed funds for specific sectors or strategies they believe in. Since both are mutual funds held in the same account, there is no technical barrier to combining them.

Where to find index funds

Index funds are offered by most major brokerages and mutual fund companies. Vanguard, Fidelity, Schwab, and iShares all offer index funds tracking the same major indexes, though with different names and slightly different fees. You can compare them by looking up the fund's ticker symbol and expense ratio on your brokerage platform.

Index funds are available in regular taxable brokerage accounts, retirement accounts like IRAs and 401(k)s, and education savings accounts like 529 plans. The mutual fund structure is the same regardless of the account type.

Frequently Asked Questions

Can an index fund underperform its index?

Yes, slightly. The fund's return will lag the index by roughly the amount of its expense ratio, because the fee is subtracted from returns. A fund tracking the S&P 500 with a 0.05% expense ratio will return about 0.05% less than the index itself. This is normal and expected.

Do index funds pay dividends?

Yes. If the stocks or bonds in the index pay dividends, the index fund collects them and distributes them to shareholders. You can usually choose to reinvest dividends automatically or receive them as cash.

Is an index fund safer than an actively managed mutual fund?

Safety depends on what the fund holds, not whether it is index-based or actively managed. A bond index fund is generally less volatile than a stock index fund, regardless of management style. An index fund and an actively managed fund holding the same type of securities carry similar risk.

Can I lose money in an index fund?

Yes. If the index the fund tracks declines in value, the fund's value declines too. Index funds are not protected against market downturns. However, they are protected against poor manager decisions, because the manager is straightforward following the index.

What is the difference between an index fund and an ETF?

An ETF (exchange-traded fund) is a different legal structure that often tracks an index, but a mutual fund is also a legal structure. Many ETFs track indexes, and many mutual funds track indexes. The difference is how they trade — ETFs trade like stocks throughout the day, while mutual funds trade once per day after market close. Both can be index-based or actively managed.