An index fund is a pooled investment—either a mutual fund or an ETF—that aims to match the performance of a specific market index, such as the S&P 500, rather than trying to beat it. Instead of a manager picking stocks, the fund simply holds the same securities as the index, in similar proportions.
Index funds matter because decades of data show that most actively managed funds fail to consistently outperform their benchmark index after fees, making low-cost index investing a common core strategy for long-term investors.
How index funds actually work
The fund's manager builds a portfolio that mirrors the index's composition—for an S&P 500 index fund, that means holding roughly 500 large US companies weighted by market capitalization. As the index adds, removes, or reweights companies, the fund adjusts to match, a process called rebalancing to the benchmark.
Because there's no active research team trying to pick winners, operating costs are low, which is passed on to investors as a low expense ratio, often between 0.02% and 0.10% annually for popular broad-market funds.
Index funds vs actively managed funds
Actively managed funds employ analysts who select securities they believe will outperform, and typically charge higher fees—often 0.5% to 1.5% annually—to cover that research. Studies consistently show that over 10+ year periods, the majority of actively managed US stock funds underperform their benchmark index after fees.
Index funds won't ever beat their index (they'll actually slightly trail it due to fees), but they also won't dramatically underperform it, which appeals to investors who prioritize predictability and low cost.
What makes a good index fund
A good index fund tracks a broad, well-constructed index closely (low 'tracking error'), charges a low expense ratio, and comes from a reputable provider with sufficient assets under management to remain stable and liquid.
It's also worth checking how the index itself is built. A market-cap-weighted index like the S&P 500 will naturally concentrate more money in its largest companies, so two funds tracking 'the US market' can still have meaningfully different sector exposure depending on the underlying index rules.
Taxes and trading mechanics for index funds
Index mutual funds and index ETFs are both taxed the same way on gains and dividends, but they differ in how those taxable events arise. Because index mutual funds sometimes need to sell holdings to meet shareholder redemptions, they can occasionally distribute taxable capital gains even if you didn't sell any shares yourself; index ETFs largely avoid this through their in-kind creation and redemption process.
Index mutual funds trade once per day at the closing net asset value, so an order placed at 10 a.m. and one placed at 3 p.m. execute at the same end-of-day price. Index ETFs trade throughout the day at live market prices, which matters if you want more control over your exact entry or exit price, but rarely changes the outcome much for long-term, buy-and-hold investors.
Choosing between two similar index funds
When comparing index funds that track the same benchmark, expense ratio is the most reliable differentiator since the underlying holdings are nearly identical. A 0.03% fund versus a 0.20% fund on a $50,000 balance costs $15 versus $100 per year—a gap of roughly $2,550 over 30 years even before accounting for compounding on the difference.
Beyond cost, compare the exact index each fund tracks (some 'total market' funds use slightly different index providers with different rules), the fund's tracking error over the past several years, and whether it's offered inside your existing retirement account, since account availability sometimes matters more than a few basis points of cost.
Common mistakes
- Believing 'index fund' means only US stocks—there are index funds for bonds, international stocks, and specific sectors too.
- Assuming a low expense ratio guarantees good performance relative to the index; tracking error still matters.
- Overlapping multiple index funds that hold nearly identical companies, creating a false sense of diversification.
- Expecting an index fund to protect against market-wide declines—it moves down with its index just as it moves up.
- Switching frequently between index funds tracking nearly the same benchmark, which can generate unnecessary taxable events in a regular brokerage account.
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Open Decision Lab →Frequently asked questions
Is an index fund the same as an ETF?
Not exactly; an index fund is a strategy that can be offered as either a mutual fund or an ETF.
Can index funds lose value?
Yes, an index fund falls when its underlying index falls, since it's designed to mirror that index's performance.
What index do most beginner index funds track?
Many beginners start with funds tracking the S&P 500 or a total US stock market index.
How much money do I need to start?
Some index mutual funds require a minimum of $1,000-$3,000, while index ETFs can often be bought for the price of a single share.
Do index funds pay dividends?
Yes, if the underlying companies in the index pay dividends, the fund distributes them to shareholders.