What an index fund is, plainly
An index fund is a fund that holds everything in a market index, in the same proportions as the index, instead of employing a team to pick which stocks will do best. If a company is in the S&P 500, an S&P 500 index fund owns it, weighted by its size. No forecasts, no trading around news, no star manager. The fund simply mirrors the list.
That sounds like giving up, and it is: you are giving up the attempt to beat the market in exchange for a guarantee of matching it, before a very small fee. The surprise, covered in the evidence section below, is that this "average" result reliably lands you ahead of most professionals who do try.
- Passive management: the fund only trades when the index itself changes (a company is added, removed, or its weight shifts), which keeps trading costs and taxable churn low.
- Near-zero fees: because there is no research staff to pay, broad index funds commonly charge 0.02% to 0.10% a year, versus roughly 0.5% to 1% for active funds.
- Built-in diversification: one purchase can hold 500 companies, or several thousand in a total-market fund, so no single company can sink you.
- Transparency: you always know what you own, because the holdings are the published index.
Index funds come in two wrappers, a traditional mutual fund or an ETF (more on the difference below), and you hold either one inside an ordinary investment account: a 401(k), an IRA, or a taxable brokerage account.
The big indexes and what they track
An index is just a defined, rules-based list of securities with a method for weighting them, usually by company size. Different indexes slice the market differently, and every major slice has cheap index funds tracking it:
| Index | What it tracks | What it represents |
|---|---|---|
| S&P 500 | About 500 of the largest US companies | Roughly 80% of the US stock market by value; the default "the market" benchmark |
| Total US stock market (e.g. CRSP US Total Market, Dow Jones US Total Stock Market) | Essentially every listed US company, thousands of stocks | The whole US market, including small and mid-size companies |
| Nasdaq-100 | The 100 largest non-financial companies on the Nasdaq | A growth- and technology-heavy slice, more concentrated than the S&P 500 |
| Russell 2000 | About 2,000 US small-cap companies | The small-company segment of the US market |
| MSCI EAFE / FTSE Developed ex-US | Large and mid-size companies in developed markets outside the US | International developed-market exposure (Europe, Japan, Australia, and others) |
| MSCI Emerging Markets | Companies in developing economies such as India, Brazil, and Taiwan | The higher-growth, higher-volatility international segment |
| Bloomberg US Aggregate Bond | Investment-grade US bonds: Treasuries, corporate bonds, mortgage-backed securities | The core US bond market; the standard bond-fund benchmark |
A practical reading of the table: an S&P 500 fund and a total US market fund overlap so heavily that they behave almost identically; pick one, not both. International and bond indexes exist to add the pieces a US stock index does not cover, and a three-fund combination (US stocks, international stocks, bonds) is the classic simple portfolio built entirely from index funds.
Active vs passive: what the evidence says
The scoreboard has been kept for 25 years. S&P Dow Jones Indices publishes the SPIVA scorecard (S&P Indices Versus Active), which counts how many actively managed funds beat their benchmark index. Per the Year-End 2025 edition, 79% of active US large-cap funds underperformed the S&P 500 in 2025, the fourth-worst year for active managers in the scorecard's history. Stretch the horizon and the numbers get worse for the pros: over the 10 years ending December 2024, 84.3% underperformed, and over 15 years, 89.5%.
And that is measured against surviving funds; SPIVA also adjusts for the many active funds that closed or merged along the way, which an unadjusted comparison would quietly forget. Nor do the few winners repeat reliably: the companion Persistence Scorecard finds that top-quartile funds rarely stay top-quartile, so past outperformance is a poor way to pick the next winner.
Why is beating the index so hard? It is arithmetic more than skill:
- The market return is the average of everyone in it. Before costs, the average actively managed dollar earns exactly the market return, because active managers largely trade against each other. After costs, the average active dollar must earn less than the index. This is not a market anomaly; it is subtraction.
- Costs recur every year. A 0.7% annual fee gap does not need a bad manager to sink a fund; it just needs time.
- A few stocks drive most returns. A small share of companies produce the bulk of the market's long-run gains. Miss them while holding a concentrated active portfolio and there is no catching up. The index never misses them, because it holds everything.
None of this says no fund ever beats the index. Some do, every year. The problem is identifying them in advance, and the persistence data says you mostly cannot.
Why costs decide it: the fee drag, in dollars
Percentages hide the damage, so here it is in dollars. Suppose you invest $500 a month for 30 years and the market delivers a 7% annual return before fees. The only difference between these three paths is the yearly fee skimmed off that return: a broad index fund at 0.03%, a typical active fund at 0.75%, and an advisor-plus-fund stack totaling 1.00%.
| What you pay | Yearly cost | Balance after 30 years | Lost to fees vs the index fund |
|---|---|---|---|
| Broad index fund | 0.03% | $606,387 | - |
| Typical active fund | 0.75% | $526,960 | $79,427 |
| Advisor + fund stack | 1.00% | $502,258 | $104,129 |
Read the last column again: the 1% stack costs about $104,129, well over half of the $180,000 you contributed in the first place. The fee is charged on your whole balance every year, so it compounds against you exactly the way returns compound for you. And this table generously assumes the pricier funds match the market before fees; the SPIVA numbers above say most do not.
This is why the expense ratio is the one number to check before buying any fund, and why fee differences that sound trivial ("only 0.75%") are the most expensive line in most portfolios. Run your own contributions and fee levels through the investment fee calculator to see the dollar drag on your plan, or compare two specific funds side by side with the expense ratio calculator.
Index fund vs ETF: same idea, different wrapper
People often say "index fund" for the mutual-fund version and "ETF" as if it were something else, but most big ETFs are index funds. The strategy (track an index, charge almost nothing) is identical; what differs is the wrapper you buy it through:
| Index mutual fund | Index ETF | |
|---|---|---|
| How you buy | Directly from the fund company, in dollar amounts | On an exchange like a stock, through any broker |
| When trades happen | Once a day, at that day's closing value (NAV) | All day, at the live market price |
| Minimum investment | Sometimes $1,000 to $3,000, often $0 at the fund's own brokerage | One share, or a few dollars with fractional shares |
| Automatic investing | Excellent: recurring dollar-based purchases are standard | Depends on the broker; increasingly supported |
| Tax efficiency (taxable accounts) | Good, but can distribute capital gains you did not sell for | Usually better: the ETF structure rarely distributes gains |
| Best fit | Hands-off automation inside 401(k)s and IRAs | Taxable accounts, flexibility, and low minimums |
For a long-term buy-and-hold investor the differences are minor. Inside a 401(k) or IRA the tax point is irrelevant, and the intraday trading of an ETF is a feature you should mostly ignore. Pick whichever wrapper your account makes cheap and automatic, and spend your attention on the expense ratio and the index being tracked.
What index funds do not protect you from
Index funds remove manager risk and fee drag. They do not remove market risk, and honest indexing means knowing what you still own:
- Crashes, in full. An index fund is the market, so you get the whole ride down too. The S&P 500 lost roughly half its value peak to trough in 2007-09, about a third in a few weeks in March 2020, and about 25% in 2022. Diversification across 500 companies protects you from any one company failing, not from all of them falling together.
- Concentration at the top. Because the S&P 500 weights companies by size, its ten largest companies currently make up close to 40% of the index. "Owning 500 companies" is true, but a large share of your dollars ride on a handful of mega-cap stocks, and the index gets more concentrated exactly when those few have already run up.
- A single country, if you stop at the S&P 500. A US-only index fund carries US-specific risk. International index funds exist precisely to spread that.
- Your own behavior. The fund will match the index; whether you do depends on holding through the drops. Selling in a crash converts a temporary decline into a permanent loss, and no fund structure prevents that.
The right response to these is not stock picking; it is sizing. Money you need within a few years does not belong in any stock index fund, and bonds or cash exist to dampen the swings you cannot afford.
How to actually start
Index investing is deliberately boring to set up. Three steps:
- 1. Open (or use) an account. If your employer offers a 401(k) with a match, start there; index funds are usually on the menu. Otherwise open an IRA or a taxable brokerage account; our guide to what a brokerage account is walks through the account choice and setup in about 15 minutes.
- 2. Pick one broad, cheap fund. A total US market or S&P 500 index fund with an expense ratio under 0.10% is the standard first holding, in either the mutual fund or ETF wrapper. One fund is genuinely enough to start; international and bond index funds can come later as the balance grows. Choose by index type and cost, not by brand or last year's return.
- 3. Automate it. Set a recurring transfer and a recurring buy for the same day each month, then stop checking daily prices. The habit matters more than the timing: consistent monthly buying purchases more shares when prices are down automatically.
That is the whole system. To see what the habit compounds into, project a monthly amount, an expected return, and your time horizon in the ETF growth calculator; the fee table above shows why the projection barely changes when the fund costs 0.03% and changes a lot when it costs 1%.
Common myths about index funds
- "Indexing is settling for average." The market return is the average of all invested dollars, but earning it puts you ahead of most professionals: 84.3% of active US large-cap funds trailed the S&P 500 over the 10 years ending December 2024, per SPIVA. In investing, the average outcome is an above-average result, because everyone else is paying fees to chase more.
- "You need to pick the right index fund." Among broad, cheap funds tracking the same index, the differences are rounding errors. The decisions that matter are how much you invest, how long you stay invested, and keeping the expense ratio near zero.
- "Index funds are only for beginners." The flow runs the other way: pensions, endowments, and professional advisors have moved trillions into index strategies precisely because the evidence accumulated. It is the default for sophisticated money, not the training wheels.
- "When the market is expensive, active managers protect you." The SPIVA scorecards cover multiple crashes, and in most down years the majority of active funds still trailed their benchmark. Paying more for protection that mostly fails to show up is a bad trade in any market.