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An index fund is a mutual fund or exchange-traded fund designed to track the returns of a specified market index. Instead of asking a manager to pick securities expected to beat the market, an index fund generally follows a rules-based benchmark.
What is a market index?
A market index measures the performance of a basket of securities. Examples include indexes representing large U.S. companies, small companies, bonds, industries, or international markets. You cannot invest directly in an index; an index fund provides indirect exposure by trying to track it.
How index funds track a benchmark
Investor.gov explains that some funds hold all securities in the index, while others hold a sample. Funds can also use derivatives in some strategies. The goal is to approximate index performance before fund expenses, not to guarantee an exact match.
Market-cap weighting
Many indexes weight companies by market capitalization. Larger companies then represent a greater percentage of the index. Other indexes may be price-weighted, equal-weighted, factor-based, or use specialized rules.
Understanding the weighting method matters because it determines where the fund’s exposure is concentrated.
Why index funds can be lower cost
Passive strategies often require less security-selection research and trading than active management, which can reduce costs. But Investor.gov explicitly warns that not all index funds have lower expenses than actively managed funds.
Tracking error
An index fund can underperform its benchmark because of fees, transaction costs, sampling, cash holdings, taxes, and other implementation effects. That gap is called tracking difference or can contribute to tracking error.
Diversification depends on the index
A broad total-market index can hold thousands of securities. A narrow sector or thematic index may be highly concentrated. The fact that a product is an index fund tells you how it is managed, not how diversified it is.
Index mutual fund vs. index ETF
An index strategy can be packaged as a mutual fund or ETF. The investment objective may be similar, while trading, pricing, tax mechanics, minimums, and transaction costs can differ.
Risks to understand
- Market risk in the securities held.
- Concentration risk from the chosen index.
- Tracking error.
- Fees and trading costs.
- Currency or country risk for international funds.
- Interest-rate and credit risk for bond index funds.
Questions to ask before investing
- Which index does the fund track?
- How is the index constructed and weighted?
- What is the expense ratio?
- How closely has the fund tracked its benchmark?
- What are the largest holdings and sector exposures?
- How does the fund fit your goal and time horizon?
Continue learning
Visit the Investing & Markets hub and read ETF vs. Mutual Fund.
Sources & methodology
This article is educational and does not recommend a specific fund or strategy.
EconomicHQ standard
Sources, dates, and methodology matter.
EconomicHQ prioritizes primary sources for consequential financial and economic claims and identifies reporting periods for changing information. This article is educational and does not provide individualized financial, investment, tax, legal, or credit advice.