Index Funds Explained: What They Are and Why Most People Should Own One

An index fund buys a little bit of everything in a particular market rather than trying to pick winners. If the market goes up, the fund goes up. If the market goes down, the fund goes down. Nobody is making decisions about what to buy or sell—the fund just mirrors whatever list it’s tracking.

That sounds passive. It is. And for most people saving for retirement, passive has consistently beaten active over the long run—not because index funds are clever, but because they’re cheap and they don’t make mistakes.

What an Index Actually Is

An index is just a list of companies grouped by some criteria. The S&P 500 is a list of 500 large American companies—Apple, Microsoft, Amazon, and 497 others. The Dow Jones Industrial Average is a list of 30 large companies. The Russell 2000 tracks 2,000 smaller ones.

The companies aren’t chosen by a fund manager. A committee sets the rules—market size, liquidity, profitability—and the companies that qualify get added. Companies that no longer qualify get removed. The index itself doesn’t belong to any fund company; it’s a published list that anyone can track.

An index fund’s job is to hold the same companies in the same proportions as the index it tracks. If Apple makes up 7% of the S&P 500, an S&P 500 index fund holds 7% of its money in Apple. When the index changes, the fund adjusts. No human is deciding what’s a good buy.

Why the Cost Difference Matters So Much

A typical actively managed mutual fund charges 0.50%–1.50% per year in fees. An S&P 500 index fund from Fidelity or Vanguard charges 0.03%–0.04%. That gap looks small. Over 30 years, it isn’t.

$10,000 growing at 7% annually for 30 years with no fees: $76,123. The same $10,000 at 7% with a 1% annual fee: $57,435. The fee difference costs roughly $18,700 on a $10,000 starting investment—not because the market performed differently, but because 1% came out every year before the compounding could do its work.

This is why fee comparisons matter. The fund charging more isn’t necessarily doing better work—it’s just keeping more of your return for itself.

Active vs. Index: The Track Record

The S&P Dow Jones SPIVA report tracks how actively managed funds perform against their benchmark index. The 2023 results: over 15 years, 92% of large-cap active funds underperformed the S&P 500. Over 20 years, the number climbs higher.

This isn’t because professional fund managers are incompetent. It’s because markets are competitive. Every trade has a buyer and a seller—for every manager who outperforms by buying a stock cheap, someone else underperforms by selling it. The fees then take another cut regardless of outcome.

Warren Buffett famously bet $500,000 that a simple S&P 500 index fund would beat a selection of hedge funds over 10 years. The index fund won—returning 7.1% annually vs. the hedge funds’ 2.2% average. The bet ended in 2017.

Past performance doesn’t guarantee future results—that applies to index funds too. But the historical gap between low-cost index funds and higher-cost active funds has been consistent enough that most financial planning institutions now recommend index funds as the default for most investors.

The Main Types

S&P 500 Index Funds

The most common starting point. Tracks 500 large US companies across most industries. Not perfectly representative of the whole US market—smaller companies aren’t included—but covers roughly 80% of the total US stock market value. Vanguard’s VOO, Fidelity’s FXAIX, and Schwab’s SCHB are the most widely held versions.

Total Market Index Funds

Tracks both large and small US companies—closer to the full picture of American business. Slightly more volatile than an S&P 500 fund because smaller companies swing more. The fee difference between S&P 500 and total market funds is usually negligible.

International Index Funds

Tracks companies outside the US—Europe, Asia, emerging markets. The US makes up roughly 60% of global stock market value, so an investor holding only US index funds is missing 40% of the world’s publicly traded companies. International funds add that exposure, though with different risks and currency fluctuation.

Bond Index Funds

Tracks US government or corporate bonds rather than stocks. Bonds are loans—the fund lends money to governments or companies and collects interest. Lower potential return than stocks, lower volatility. A common approach for investors closer to retirement is shifting more of the portfolio toward bond index funds as stability becomes more important than growth.

Quick Comparison

Fee data sourced from Morningstar’s 2026 annual fund fee study.

What Index Funds Don’t Do

They don’t protect you from market drops. When the S&P 500 fell 38% in 2008, every S&P 500 index fund fell 38% too. The diversification reduces the risk of any single company collapsing, but it doesn’t reduce market-wide risk. If the whole market goes down, the index fund goes down with it.

They don’t beat the market. By design, an index fund matches the market minus its small fee. If the goal is to significantly outperform the market, an index fund won’t do that. What it will do is match the market consistently, with low costs, and without the risk of a fund manager making a bad call.

They don’t make decisions for you about how much to invest, when to start, or how to split between stocks and bonds. Those choices still belong to the investor.

How to Actually Buy One

Index funds are available through any brokerage account—Fidelity, Vanguard, Schwab, or through a workplace 401(k). Fidelity and Schwab offer index funds with no minimum investment. Vanguard’s ETF versions (like VOO) can be purchased for the price of one share.

The simplest starting point for most people: open a Roth IRA if income qualifies, contribute up to the annual limit ($7,000 in 2024 for people under 50), and put it in a total market or S&P 500 index fund. Leave it alone. Reinvest dividends. Repeat annually.

The Roth IRA contribution limit in 2024 is $7,000 ($8,000 if over 50). Income limits apply—single filers with income above $161,000 begin to phase out.

Bottom Line

An index fund is a low-cost way to own a slice of many companies at once without needing to pick any of them. Fees are low, decisions are minimal, and the long-term track record against active management is hard to argue with. The case for index funds isn’t that markets always go up—it’s that when they do go up, index fund investors keep more of that gain than active fund investors do. And when markets fall, index funds fall the same amount, without the additional drag of a manager’s fee on top.

Disclaimer

This article is for informational purposes only and does not constitute financial advice. Figures and rates are based on publicly available data and may not reflect your personal situation. Consult a licensed financial advisor before making any financial decisions. All rates and limits referenced are subject to change — verify current figures before acting on them.