Over a 20-year period, roughly 80% of actively managed U.S. stock funds underperform a simple index fund. Not because the managers are bad at their jobs — many are genuinely skilled — but because their fees eat the edge, and consistently predicting which stocks will outperform is harder than the financial industry has ever wanted to admit. You've been paying for expertise that, on average, delivers worse results than doing nothing. Index funds are the answer to that problem, and they've been the answer for 50 years. Here's how they work.
What a Stock Market Index Is
Before you can understand an index fund, you need to understand an index.
A stock market index is just a list of companies, selected according to some criteria, used to track how the market (or a part of it) is performing. The most famous one is the S&P 500 — a list of 500 large American companies, weighted by their size, maintained by S&P Global.
When you hear "the market was up 1.2% today," that's usually referring to the S&P 500. It's a measuring stick. It doesn't exist as a thing you can own — it's just a number that reflects what 500 companies' stocks are collectively doing.
What an Index Fund Is
An index fund is an investment fund that owns the stocks in an index — all of them, in the same proportions. An S&P 500 index fund owns a little piece of all 500 companies on the S&P 500. When Apple goes up, your fund goes up a little. When some company tanks, your fund goes down a tiny amount, cushioned by the other 499 companies.
Nobody is actively choosing which stocks to buy or sell. The fund just mirrors the index mechanically. That's the key word: passive. Index funds are passive investments. No stock-picking, no market timing, no team of analysts. Just: own everything on the list.
Because there's no active management, the fees are extremely low. A typical S&P 500 index fund charges around 0.03–0.05% per year in fees — that's $3–5 per year on a $10,000 investment. Actively managed funds often charge 0.5–1.5% or more, which sounds small but compounds into a massive difference over 20 years.
Why They Work
Two reasons.
Diversification. When you own 500 companies, no single company's failure ruins you. Enron collapses? You owned a tiny sliver of it. The other 499 companies are fine. Concentration is risk; spreading across the whole market reduces it.
The market goes up over time. Not every year, not every decade — but over long stretches, the U.S. stock market has historically returned about 10% per year on average (roughly 7% after inflation). The 2008 crash looked catastrophic. The market fully recovered and kept going. So did 1987, 2000, and 2020. An index fund captures that long-term upward drift without requiring you to be right about anything.
Here's the part that feels wrong until you sit with it: by owning everything and doing nothing, you outperform most people who are actively trying to do better. S&P Dow Jones publishes a SPIVA scorecard every year comparing active funds to their benchmark index. In 2023, over a 15-year period, 92% of U.S. large-cap active funds underperformed the S&P 500. That's not a fluke — it's the consistent result across decades and markets.
Index Fund vs. ETF — What's the Difference?
You'll hear both terms. They're more similar than different.
A traditional index fund (like a Vanguard mutual fund) is priced once per day after the market closes. You buy it directly from the fund company at that day's price. There's often a minimum investment.
An ETF (Exchange-Traded Fund) is an index fund that trades on the stock exchange like a stock — you can buy or sell it any time the market is open, in any amount, even one share. No minimums at most brokerages.
For most people just getting started, an ETF is easier. The most popular ones are VOO (Vanguard S&P 500 ETF) and IVV (iShares S&P 500 ETF). They track the same index. The differences are negligible. Pick one.
How to Actually Buy One
You need a brokerage account. The main ones for beginners: Fidelity, Vanguard, and Schwab. All three are reputable, have no account minimums, and offer fractional shares (meaning you can invest $50 even if one share costs $500).
- Open an account. Takes about 10 minutes. If it's for retirement, open a Roth IRA or Traditional IRA — not just a regular taxable account. The tax advantage matters enormously over time. (See our Roth vs. Traditional breakdown if you're not sure which.)
- Fund it. Link your bank account and transfer money in.
- Buy the fund. Search for VOO, IVV, or FXAIX (Fidelity's equivalent). Click buy. Enter an amount.
- Do nothing. This is the hardest part. When the market drops 20%, do not sell. You haven't lost anything until you sell. Time recovers downturns. Panic-selling locks in losses permanently.
One More Thing
Index funds aren't exotic. They're not a trick or a shortcut. They're the single most recommended investment vehicle by financial economists, Nobel laureates, and Warren Buffett himself — who has publicly instructed that his estate be invested in index funds after his death.
You don't need to understand them more deeply than this to use them effectively. Buy a broad market index fund, inside a tax-advantaged account, consistently over time. That's the whole strategy. It works because you're not trying to be clever — you're just capturing what the market gives everyone who stays in it long enough. If you're building this from scratch in your 40s, see our guide to starting from zero at 40 for the full order of operations.