Key Takeaways
- Index funds passively track a market index rather than trying to outperform it.
- Because they require less active management, index funds typically carry lower fees than actively managed funds.
- Broad-market index funds offer built-in diversification across hundreds or thousands of securities.
- Like all investments, index funds carry risk — their value can fall along with the market.
- Index funds are commonly used in retirement accounts like 401(k)s and IRAs for long-term investing.
Index Fund
An index fund is a type of investment fund designed to mirror the performance of a specific market index — such as the S&P 500 or the Dow Jones Industrial Average. Instead of a manager hand-picking stocks, the fund simply holds the same securities in the same proportions as the index it tracks. This passive approach means the fund rises and falls alongside that slice of the market.
Index funds can be structured as mutual funds or exchange-traded funds (ETFs). The key distinction from actively managed funds is that they follow a rules-based, buy-and-hold strategy rather than relying on manager discretion to select securities.
The Basic Idea: Following the Market, Not Fighting It
Most people who hear about investing eventually encounter a fundamental debate: should you try to beat the market, or simply match it? Index funds are built on a straightforward answer — match it.
When you invest in an index fund, your money is pooled with other investors' money and used to purchase the same holdings as a particular market index. If the index includes 500 large U.S. company stocks, the fund holds those same 500 stocks in roughly the same proportions. If one company's share rises to represent a larger slice of the index, the fund adjusts accordingly.
This passive approach stands in contrast to active management, where a portfolio manager decides which stocks to buy and sell in hopes of outpacing the index. That strategy involves more human judgment — and more costs. If you'd like a broader foundation on what investing means in the first place, see our introduction to investing.
~$15T
U.S. assets in index funds and ETFs
According to the Investment Company Institute, index funds and ETFs have attracted trillions of dollars in U.S. investor assets, reflecting a broad shift toward passive investing over recent decades.
0.05%
Typical expense ratio for broad-market index funds
Many broad-market index funds now carry expense ratios below 0.10%, compared to averages exceeding 0.60% for actively managed equity funds, according to Morningstar industry data.
Why Fees Matter More Than Most Investors Realize
One of the most consistently cited advantages of index funds is their cost. Because no team of analysts is researching individual securities or executing frequent trades, the operational costs of running an index fund are significantly lower than those of an actively managed fund.
These costs are passed on to investors as an expense ratio — the annual percentage of your investment taken as a fee. A broad-market index fund might carry an expense ratio of 0.03% to 0.20%, while some actively managed funds charge 1% or more.
That gap may sound small, but over decades, the compounding effect of lower fees can translate into a meaningful difference in your ending balance. A dollar saved in fees each year is a dollar that stays invested and potentially grows alongside the market.
Check the Expense Ratio Before You Invest
When evaluating any index fund, locate its expense ratio in the fund's prospectus or fact sheet. Even among index funds, fees can vary. Comparing expense ratios across funds tracking the same index helps you understand the real cost of each option. A lower expense ratio means more of your investment return stays with you over time.
Diversification: Built In by Design
Diversification means spreading your investments across many different assets so that a poor performance in one area doesn't devastate your entire portfolio. Index funds offer this almost automatically.
A fund tracking the S&P 500, for instance, gives investors exposure to approximately 500 companies across industries including technology, healthcare, finance, consumer goods, and energy. A fund tracking a total stock market index may hold thousands of securities.
This breadth means a single company's bad news — an earnings miss, a scandal, a product failure — has a limited impact on your overall position. The fund absorbs the hit across a wide base of holdings.
What Index Funds Don't Do
It's important to understand what you are — and are not — getting with an index fund. These funds do not aim to outperform the market; they aim to match it. In years when the overall market declines, an index fund tracking that market will decline too.
There is no protection built in against broad market downturns. Investors who need their money in the short term may be exposed to significant risk. Financial planners typically suggest building an adequate emergency reserve before committing money to market investments. Our article on emergency funds and investing explores this relationship in more detail.
Index funds also vary considerably depending on what they track. A fund following a narrow sector — say, technology companies only — carries different risk than one tracking the entire U.S. stock market. Knowing what an index fund actually holds is an essential first step before investing.
For a deeper look at how index funds are packaged and traded, the comparison of mutual funds and ETFs explains the structural differences. And if you're weighing different long-term fund options, our guide to target-date funds offers a useful contrast.
This article is for general informational and educational purposes only. It does not constitute personalized financial, investment, or tax advice. Consult a licensed financial adviser before making any investment decisions based on your individual circumstances.