Key Takeaways
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. Rather than having a fund manager actively select individual stocks, an index fund simply holds the same securities — in the same proportions — as the index it tracks. This passive approach means the fund rises and falls roughly in line with the broader market segment it represents.
Index funds are available as both mutual funds and exchange-traded funds (ETFs). The key structural difference is that ETFs trade on an exchange throughout the day like individual stocks, while traditional index mutual funds are priced once per day after markets close.
How an Index Fund Actually Works
An index is simply a list of securities chosen according to a defined set of rules — for example, the largest 500 publicly traded U.S. companies by market capitalization. An index fund's job is to hold those same securities in roughly the same proportions, so that its performance closely matches the index.
Because no one is trying to predict which stocks will outperform, the fund doesn't need a large team of analysts making active decisions. This passive structure is the defining characteristic of index funds and the reason they tend to carry lower costs than actively managed alternatives.
The primary cost investors encounter is the expense ratio — an annual percentage fee deducted from the fund's assets to cover operating expenses. Even small differences in expense ratios can have a meaningful effect over long time horizons, which is one reason costs receive significant attention in personal finance discussions.
Over 50%
Share of U.S. fund assets in passive strategies
According to Morningstar data, passive funds — primarily index funds — surpassed actively managed funds in total U.S. assets under management for the first time in 2024, reflecting a long-term shift in investor behavior.
~0.05%
Typical expense ratio for broad index funds
Many broad market index funds carry expense ratios well below 0.10%, compared to actively managed funds that can charge 0.50% or more, according to industry cost data tracked by Morningstar.
500+
Companies in a typical S&P 500 index fund
An S&P 500 index fund holds shares of approximately 500 large U.S. companies across sectors including technology, healthcare, financials, and consumer goods, providing wide market-segment coverage in a single fund.
Diversification by Design
One of the most frequently cited properties of index funds is built-in diversification. When a fund tracks an index containing hundreds of companies across multiple industries, a single poor-performing stock has a relatively limited effect on the overall fund value. This structural spread of risk is what makes index funds a common starting point in conversations about diversification in investing.
However, diversification does not eliminate risk. An index fund that tracks U.S. large-cap stocks will still decline when that segment of the market declines. Investors who want exposure across asset classes — domestic stocks, international stocks, bonds — typically look at multiple funds rather than relying on a single one.
For a broader grounding in how savings and investing fit together, the fundamentals of savings and investing covers the full landscape in accessible terms.
Active vs. Passive: Understanding the Distinction
The contrast between actively managed funds and passively managed index funds is central to many investing conversations. Active fund managers attempt to select securities that will outperform the market. This approach involves research, analysis, and frequent trading — all of which generate higher operational costs that are passed on to investors through higher expense ratios.
Research from academic and industry sources has consistently shown that a majority of actively managed funds underperform their comparable index benchmarks over long periods, after fees are accounted for. That finding has driven sustained interest in passive index investing, though it is important to note that past research patterns do not guarantee future results, and individual fund performance varies.
“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 and advocate of low-cost index investing for most individual investors
Index funds are not inherently superior in every context — they will never outperform their benchmark, either — but the cost and simplicity argument has resonated broadly with everyday investors.
Where Index Funds Fit in a Financial Plan
Index funds are not a complete financial strategy on their own. Before committing money to any investment, many financial educators suggest first addressing foundational needs. Building an emergency fund, for instance, is typically discussed as a prerequisite — see our piece on emergency fund vs. investing trade-offs for a detailed look at that decision.
Index funds can be held in taxable brokerage accounts or inside tax-advantaged retirement accounts. Understanding how account types interact with investment choices matters — for example, the tax treatment of index fund gains differs between a standard account and a Roth IRA. If you're considering opening an investment account for the first time, a guide for people starting from zero walks through the basics of what an account is and how to approach it.
For comparison, some savers also consider money market accounts, which sit between a savings account and an investment — useful context when thinking about where different dollars belong.
This article is for general informational and educational purposes only and does not constitute personalized investment, tax, or financial advice. All investing involves risk, including the possible loss of principal. Please consult a qualified financial professional before making decisions about your own financial situation.