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

An index fund is a mutual fund built to track a specific market index — a pre-set list of stocks or bonds. A mutual fund, by contrast, is any pooled investment where your money sits alongside other investors' money, managed by a fund company. So every index fund is a mutual fund, but not every mutual fund is an index fund. The difference lies in how the fund manager decides what to buy.

Most mutual funds employ a manager who picks individual stocks or bonds, trying to beat the market. Index funds do the opposite: they simply buy all (or a representative sample of) the stocks in a chosen index and hold them. The S&P 500 index fund, for example, buys shares in roughly 500 large U.S. companies in the same proportions as the index itself. The fund manager's job is to track the index accurately, not to outperform it.

This distinction matters because it affects what you pay, how often the fund changes, and what returns you can realistically expect. Understanding which type you own — or are considering — changes how you should think about its role in your overall investment picture.

Key Takeaways

  • Index funds are mutual funds that track a published market index, while actively managed mutual funds employ managers who pick individual holdings to try to beat the market.
  • Index funds typically charge lower fees because they require less active decision-making and less frequent trading than actively managed funds.
  • An index fund's performance will closely match its index minus fees, while an actively managed fund's performance depends on the manager's stock-picking skill.
  • Both index funds and actively managed mutual funds hold your money in a pool with other investors and are regulated by the same rules.

How fees differ between the two approaches

Index funds almost always cost less to own than actively managed mutual funds. Because an index fund simply replicates a published index, the fund company does not need to pay analysts to research stocks or traders to constantly buy and sell. The fund manager's job is largely automated: buy the index holdings, rebalance when the index changes, and keep costs down.

Actively managed mutual funds, by contrast, employ teams of analysts and portfolio managers whose salaries and research costs get passed to you through higher fees. These funds also trade more frequently, which generates transaction costs. The average actively managed U.S. stock mutual fund charges roughly 0.5% to 1% per year in fees, while index funds often charge 0.03% to 0.20% per year. Over decades, that difference compounds significantly.

Both types of funds disclose their fees in a document called the prospectus, which you can request from the fund company or find on their website. The fee is expressed as an expense ratio — the percentage of your investment that goes to fund costs each year.

Performance: what index funds typically deliver

An index fund's performance will track its index closely, minus the fund's fees and any small tracking error. If the S&P 500 index rises 10% in a year and your S&P 500 index fund charges 0.10% in fees, you can expect roughly a 9.9% return (before taxes). This predictability is by design.

Actively managed funds aim to beat their benchmark index, but most do not. Research consistently shows that over 10-year periods, the majority of actively managed stock funds underperform their index counterparts after fees. Some managers do outperform for a time, but identifying which ones will do so in the future is difficult. Past performance does not predict future results.

This does not mean actively managed funds are always the wrong choice — some investors value the potential for outperformance, or they believe a particular manager has genuine skill. But the odds are stacked against it, which is why index funds have become the default choice for many investors, especially those building long-term retirement savings.

When the fund buys and sells holdings

Index funds rebalance only when the underlying index changes. If a company is added to or removed from the S&P 500, the fund buys or sells to match. This happens infrequently — perhaps a few times per year — so index funds generate minimal trading activity and minimal tax consequences for you.

Actively managed funds trade constantly. A manager might sell a stock because they believe it is overvalued, or buy one because they think it is about to rise. This frequent trading can trigger capital gains taxes inside the fund, which are passed to shareholders. In a taxable account (not a retirement account), this tax drag can eat into your returns.

In a tax-sheltered account like a 401(k) or IRA, the tax difference matters less because gains are not taxed annually. But in a regular brokerage account, the tax efficiency of index funds is a real advantage.

What both types have in common

Index funds and actively managed mutual funds are both regulated by the Securities and Exchange Commission (SEC) under the same rules. Both must disclose their holdings, fees, and risks in a prospectus. Both pool your money with other investors, so you own a fractional share of the entire fund portfolio. Both can be held in retirement accounts or taxable accounts. And both can be bought through a brokerage account or, in some cases, directly from the fund company.

The main operational difference is the investment strategy — one is passive (tracking an index), the other is active (trying to beat an index). But from a legal and structural standpoint, they are the same animal.

Index funds available in different market segments

Index funds exist for nearly every major market segment. You can find index funds that track the S&P 500 (large U.S. companies), the total U.S. stock market, international stocks, bonds, real estate investment trusts (REITs), and combinations of these. Some index funds track narrow segments — like technology stocks or emerging markets — while others track broad, diversified indexes.

The same is true for actively managed funds. You can find actively managed funds focused on growth stocks, dividend-paying stocks, bonds, or mixed portfolios. The choice between index and actively managed is separate from the choice of which market segment to invest in.

When building a portfolio, many investors use a mix: index funds for core holdings (because they are cheap and reliable) and perhaps one or two actively managed funds if they want to take a chance on a manager they believe in. Others use index funds exclusively.

Frequently Asked Questions

Can I lose money in an index fund?

Yes. An index fund's value rises and falls with the market it tracks. If the S&P 500 drops 20%, an S&P 500 index fund will also drop roughly 20% (minus fees). Index funds are not may provide investments. The advantage is that you own a diversified basket of companies, so you are not betting on a single stock.

Do index funds ever change what they hold?

Yes, but rarely and only when the underlying index changes. If a company is added to or removed from the index, the fund buys or sells to match. This happens a few times per year on average. Actively managed funds, by contrast, change holdings constantly as the manager makes new decisions.

Is an index fund safer than an actively managed fund?

Not necessarily. Both rise and fall with market conditions. Index funds are considered more predictable because you know exactly what you own and what you will pay in fees. But "predictable" does not mean "safe" — market downturns affect both equally. The real advantage of index funds is lower cost and tax efficiency, not safety.

Can I buy an index fund directly from the fund company?

Yes, most large fund companies allow direct purchases. You can also buy index funds through a brokerage account. Either way, you will need to open an account and fund it with money. Some funds have minimum investment amounts, though many have lowered or eliminated these in recent years.

What if I want to sell my index fund shares?

You can sell any time the market is open. The price you receive is the fund's net asset value (NAV) at the end of that trading day. If you own the fund in a taxable account, selling may trigger a capital gains tax. If you own it in a retirement account like an IRA, you can sell without immediate tax consequences.