Index funds are a type of mutual fund, but not all mutual funds are index funds

An index fund is a mutual fund built to track a specific market index — a preset list of stocks or bonds. The S&P 500 index fund, for example, holds the same 500 companies in the same proportions as the S&P 500 index itself. A mutual fund that is not an index fund is actively managed: a fund manager picks which stocks or bonds to buy and sell, trying to beat the market.

Both are mutual funds — both pool money from many investors and hold a basket of securities. The difference is in the strategy. Index funds follow a formula. Active funds follow a person's judgment. This matters because it affects what you pay, how often the holdings change, and what returns you might expect.

Key Takeaways

  • Index funds are mutual funds that track a published market index by holding the same securities in the same weights as that index.
  • Active mutual funds are managed by a fund manager who buys and sells holdings to try to outperform the market.
  • Index funds typically charge lower fees because no manager is making daily decisions about what to buy and sell.
  • Both index funds and active mutual funds are regulated the same way and both hold diversified baskets of securities.

How index funds work inside a mutual fund structure

A mutual fund is a legal container. It collects money from investors, buys securities with that money, and divides ownership into shares. An index fund uses that container to hold a fixed set of securities that match an index.

When you buy a share of an S&P 500 index mutual fund, you own a tiny piece of all 500 companies in that index, in the exact proportion they appear in the index. If Apple makes up 7% of the S&P 500, your fund holds 7% Apple. The fund manager does not decide this — the index does. When the index changes (when a company is added or removed), the fund adjusts automatically.

This is different from an active mutual fund, where the manager decides which 50 or 100 or 500 stocks to hold based on their research and judgment. The manager buys when they think a stock will rise and sells when they think it will fall.

Why fees are lower for index funds

Index funds charge lower fees than active mutual funds because they require less work. An index fund manager is not researching companies or making buy-and-sell decisions. They are following a formula. The fund's expense ratio — the annual cost you pay — typically ranges from 0.03% to 0.20% per year for a large index fund.

Active mutual funds pay for research teams, analysts, and a manager's salary. Their expense ratios often run 0.50% to 1.50% or higher per year. Over decades, that difference compounds. A 1% annual fee on a $100,000 investment costs you $1,000 per year. A 0.10% fee costs $100. Over 30 years, that gap can mean tens of thousands of dollars in your pocket instead of the fund company's.

Index funds and active funds in the same mutual fund family

Most large mutual fund companies offer both index and active funds. Vanguard, Fidelity, and Schwab all sell index mutual funds and active mutual funds. You choose which one to buy based on your preference and strategy.

A mutual fund company is not one fund — it is a company that runs many funds. Vanguard runs the Vanguard S&P 500 Index Fund (index-based) and also the Vanguard Growth Fund (actively managed). They are separate funds with separate holdings, separate fees, and separate performance records. Choosing between them is a decision about strategy, not about which company to use.

Tax treatment and regulatory status

Index mutual funds and active mutual funds are regulated the same way by the Securities and Exchange Commission (SEC). Both must disclose their holdings, their fees, and their performance. Both are required to act in the investor's interest. Both can be held in regular taxable accounts or in retirement accounts like IRAs and 401(k)s.

Index funds often produce fewer taxable events because the holdings change less frequently. Active funds buy and sell more often, which can trigger capital gains taxes in taxable accounts. This is another reason index funds can cost less over time, even before you look at the fee difference.

When you might choose an active fund instead

Some investors prefer active mutual funds because they believe a skilled manager can outperform the market. This is a judgment call, not a fact. Historical data shows that most active managers do not beat their index over long periods, but some do. If you believe a particular manager has an edge, you might choose their active fund despite the higher fees.

Active funds can also focus on specific strategies — socially responsible investing, dividend-paying stocks, or small-cap growth — that may not have a corresponding index fund. If you want a specific approach that an index does not cover, an active fund may be your only option within the mutual fund structure.

The relationship between index funds and ETFs

Index funds exist as mutual funds, but they also exist as exchange-traded funds (ETFs). An ETF is a different legal structure that holds the same kind of basket of securities. Many index strategies are available in both formats. The S&P 500 exists as an index mutual fund and as multiple index ETFs.

The difference between an index mutual fund and an index ETF is not about the index — it is about how you buy and sell them. Mutual funds trade once per day at the end of the day. ETFs trade throughout the day like stocks. For most long-term investors, this difference does not matter much. Both track the same index and charge similar fees.

Frequently Asked Questions

Can an index fund underperform its index?

Yes, slightly. An index fund charges fees and has small costs for buying and selling securities. These costs mean the fund returns slightly less than the index itself. A fund tracking the S&P 500 might return 9.5% when the index returns 10%. The difference is the fund's expense ratio and trading costs.

Do index funds ever change what they hold?

Yes, but only when the index changes. If a company is added to or removed from the S&P 500, the fund adds or removes it. This happens a few times per year. An active fund might change holdings dozens of times per year based on the manager's decisions.

Is an index fund safer than an active fund?

Not necessarily. Both hold diversified baskets of securities, so both spread risk across many companies. An index fund is not safer — it is just different. It gives you the market's return, while an active fund bets that its manager can beat the market. The safety question is about diversification, not about index versus active.

Can I hold an index mutual fund in a retirement account?

Yes. Index mutual funds can be held in IRAs, 401(k)s, and other retirement accounts. Many people use index funds as the core of a retirement portfolio because of the low fees and broad diversification.