Substantive update: October 3, 2026. Removed claims of predictable performance and clarified that index funds can lose money, differ in diversification and carry different costs.
An index fund aims to follow a specified index. The word “index” describes an investment approach—not a guarantee of broad diversification, low risk or a positive return.
Index fund and ETF are not synonyms
An index fund can be a mutual fund or an exchange-traded fund. An ETF describes a fund structure whose shares trade on an exchange; ETFs can follow indexes or use active strategies. Investor.gov explains the distinction in its guides to index funds and ETFs.
A fund may hold all the securities in its benchmark, a sample, or use other permitted techniques. Its actual return can differ from the index because of costs and how it implements the strategy. You buy a fund that seeks to track an index; you do not buy the abstract index itself.
Start with what the index owns
A broad-market equity index and a narrow single-industry index are different exposures even if both funds use passive management. A market-capitalization-weighted index can have substantial weight in a few large companies. Several funds can also own the same underlying shares.
Before comparing returns, write a one-sentence description of the benchmark: asset type, market, selection rules and weighting method. Then check the fund’s current holdings and largest positions. If you cannot explain how the exposure complements what you already own, another fund may add complexity without adding meaningful diversification.
Compare the full cost, not just a label
Not every index fund is cheaper than every actively managed alternative. Read the current prospectus and fee table. The fund’s ongoing expense ratio is only one possible cost; account charges, advisory fees, trading costs and taxes may also matter.
For a simple illustration, a 0.05% annual expense ratio applied to a constant $10,000 balance is $5; a 0.50% ratio is $50. Actual fund expenses accrue against changing assets, so this is a scale comparison, not a prediction of the exact charge on a statement. It excludes all other costs and investment returns.
The SEC’s guide to investment fees explains why recurring costs matter over time. A higher fee is not evidence that a product will earn a higher return.
A five-question comparison checklist
- What exactly does the benchmark include, exclude and overweight?
- Does this fund add a needed exposure, or duplicate holdings elsewhere?
- What are its ongoing expenses and the costs of owning it in my account?
- How does its actual performance compare with the appropriate benchmark over the same period and on the same distribution basis?
- Could I tolerate a substantial decline without needing to sell to meet a near-term expense?
Index funds inherit risks from what they hold. Tracking an equity market downward is still successful tracking, but it is a loss for the investor. Diversification can reduce concentration risk; it cannot eliminate market losses or make returns predictable.
Use our fee-comparison tools to explore costs and our portfolio hub to think about the role of each holding. This guide is education, not a recommendation for a particular fund or allocation.