Key Takeaways
- Index funds track a market index rather than relying on active stock selection by a fund manager.
- Because they require less active management, index funds typically carry lower fees than actively managed funds.
- A single index fund can hold hundreds or thousands of individual securities, providing broad diversification instantly.
- Index funds do not eliminate investment risk — their value rises and falls with the market.
- They are widely used inside retirement accounts like 401(k)s and IRAs for long-term, hands-off 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 total U.S. stock market. Instead of a manager picking individual stocks, the fund simply holds all — or a representative sample — of the securities in that index. When the index goes up, the fund goes up; when it falls, the fund falls too.
Index funds can be structured as mutual funds or exchange-traded funds (ETFs). Both follow passive management strategies, meaning portfolio changes occur only when the underlying index itself changes.
What an Index Fund Actually Holds
When you invest in an index fund, you are essentially buying a small slice of every company or bond included in a particular market index. If the fund tracks the S&P 500, your money is spread across roughly 500 large U.S. companies — from technology firms to healthcare providers to consumer goods companies. If it tracks a total bond market index, you hold a broad mix of U.S. government and corporate debt.
This structure gives investors instant diversification. Rather than betting on a single company's success, you participate in the collective performance of an entire segment of the economy. For a plain-language overview of the underlying assets index funds hold, see our investing terminology guide.
~90%
Active large-cap funds underperforming S&P 500 over 20 years
According to S&P Dow Jones Indices SPIVA U.S. Scorecard data, roughly 90% of actively managed large-cap U.S. funds underperformed the S&P 500 over a 20-year period.
< 0.10%
Typical expense ratio for broad market index funds
Many widely held index funds carry annual expense ratios well below 0.10%, compared to 0.50%–1.00% or more for actively managed mutual funds.
$7T+
Assets held in U.S. index mutual funds and ETFs
The Investment Company Institute has reported that index funds — both mutual fund and ETF structures — collectively hold trillions of dollars in U.S. investor assets, reflecting their mainstream adoption.
Why Passive Management Matters
Most traditional mutual funds are actively managed, meaning a portfolio manager and research team regularly buy and sell securities in an attempt to outperform the market. That research and trading activity costs money — which gets passed on to investors as higher fees, known as the expense ratio.
Index funds are passively managed. The fund only buys or sells when the underlying index itself changes (for example, when a company is added or removed from the S&P 500). This dramatically reduces operating costs, keeping expense ratios low — often a fraction of a percent per year.
Lower costs matter significantly over time. Because of how compounding works, even a small difference in annual fees can compound into a meaningful gap in final portfolio value over decades. To see how compounding shapes long-term outcomes, read our article on compound interest and wealth growth.
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 a difference of 0.50% annually may seem small, but compounded over 20–30 years it can represent a meaningful reduction in your final balance. Lower is generally better for long-term, passive strategies.
What the Performance Record Shows
A well-documented pattern in investment research is that most actively managed funds, over long time horizons, do not consistently outperform their benchmark index after fees are deducted. This finding — supported by data from sources such as S&P Dow Jones Indices' SPIVA reports — is a primary reason passive investing receives so much attention from financial educators and researchers.
That said, index funds are not a guarantee of gains. They rise and fall with the market, and investors in an S&P 500 index fund experienced significant losses during recessions such as 2008–2009 and the early months of 2020. Past market recoveries do not guarantee future ones. Understanding this reality upfront is essential — index funds work best as a long-term strategy, not a short-term one.
“The stock market is a device for transferring money from the impatient to the patient.”
— Warren Buffett, Chairman and CEO of Berkshire Hathaway, widely cited investor
How to Think About Index Funds as a Beginner
For someone new to investing, index funds are frequently cited as a logical starting point — not because they are without risk, but because they offer simplicity, diversification, and cost efficiency in a single vehicle. You do not need to research individual companies or predict which sectors will outperform.
Index funds are available inside many employer-sponsored retirement accounts such as 401(k)s, as well as individual retirement accounts (IRAs) you open on your own. Many investors contribute to them on a regular schedule — a habit often called dollar-cost averaging, which you can explore in our comparison of dollar-cost averaging vs. lump-sum investing.
If you're considering how to begin, our guide on starting to invest with a small amount of money walks through foundational steps, including account types and contribution habits. And if you've encountered beliefs like "you need to time the market to succeed," our piece on common investing myths addresses those directly.
This article is for general informational and educational purposes only and does not constitute personalized financial or investment advice. Consult a licensed financial adviser before making investment decisions suited to your individual circumstances.