How Index Funds Work and Why Investors Use Them

An index fund tracks a list of stocks or bonds instead of trying to beat the market

An index fund is a fund that holds the same stocks or bonds as a published list — called an index — and moves up or down with that list. The S&P 500 index, for example, tracks 500 large U.S. companies. An S&P 500 index fund holds those same 500 companies in the same proportions, so when the index goes up 5%, the fund goes up roughly 5%.

The fund manager does not try to pick winning stocks or time the market. Instead, they simply buy and hold the stocks on the list, rebalancing when the index changes. This approach costs less to run than actively managed funds, which is why index funds charge lower fees.

You buy shares of the index fund itself, not the individual stocks. When you own shares, you own a piece of the entire fund — and therefore a piece of all 500 companies in an S&P 500 fund, for example. If the fund pays dividends, you receive your share of them.

Key Takeaways

• Index funds hold the exact stocks or bonds listed in a published index, so their performance matches that index minus a small fee.
• Because index funds do not require active stock-picking, they charge lower fees than actively managed funds.
• You can buy index funds through a brokerage account, retirement account, or employer plan, and they trade during market hours like individual stocks.
• Different indexes track different markets — large U.S. companies, small companies, international stocks, bonds, or combinations of all three.
• Index funds provide instant diversification because one fund holds dozens or hundreds of securities.

How the fund manager keeps it aligned with the index

The manager's job is straightforward: buy the securities on the index list and hold them. When a company is added to the index, the manager buys it. When a company is removed, the manager sells it. When the index is rebalanced — meaning the weights of different holdings change — the manager adjusts the fund's holdings to match.

This passive approach means less trading, which saves on transaction costs and taxes. It also means less room for the manager to make mistakes. The fund will not outperform the index, but it also will not underperform by much — only by the amount of the fund's fee, which is typically 0.03% to 0.20% per year for index funds.

Some index funds are structured as mutual funds, which you can buy or sell once per day at the closing price. Others are exchange-traded funds (ETFs), which trade throughout the day like stocks. Both types track an index; the difference is mainly in how and when you can trade them.

Which indexes exist and what they track

An index is simply a published list maintained by a company or organization. The S&P 500, maintained by S&P Global, tracks 500 large U.S. companies. The Nasdaq-100 tracks 100 large technology and growth companies. The Russell 2000 tracks 2,000 smaller U.S. companies. Each has its own rules for which companies belong on the list.

Beyond U.S. stocks, indexes exist for international stocks, emerging markets, bonds, real estate investment trusts (REITs), and combinations of these. The Bloomberg U.S. Aggregate Bond Index tracks U.S. bonds across many types and maturities. The MSCI Emerging Markets Index tracks stocks in developing countries. A total stock market index, like the Wilshire 5000, attempts to track every publicly traded U.S. company.

Index funds exist for nearly all of these indexes. You can find funds tracking the S&P 500, the Nasdaq, international stocks, bonds, or a mix of stocks and bonds. Some funds track multiple indexes at once — a "total market" fund might hold U.S. stocks, international stocks, and bonds all in one fund.

Why investors choose index funds over active management

The main reason is cost. An actively managed mutual fund typically charges 0.5% to 1.5% per year in fees, while an index fund charges 0.03% to 0.20%. Over 20 years, that difference compounds significantly. On a $10,000 investment, a 1% annual fee costs far more than a 0.10% fee.

The second reason is consistency. Most actively managed funds do not beat their index over long periods. Studies show that in any given year, some active managers outperform, but the ones that do rarely repeat the next year. An index fund will match its index every year, minus the small fee. You know exactly what you are getting.

The third reason is simplicity. Owning one index fund gives you exposure to dozens or hundreds of companies without having to research individual stocks or monitor them constantly. A single S&P 500 fund gives you a stake in 500 large companies across all sectors of the economy.

How to buy an index fund and where they fit in a portfolio

You buy index funds through a brokerage account — online brokers like Fidelity, Schwab, Vanguard, and others all offer them. You can also buy them through an employer retirement plan like a 401(k), where they are often the default investment option. Some employer plans offer index funds as one choice among several.

To buy, you open an account, deposit money, search for the index fund by name or ticker symbol, and place an order. If it is a mutual fund, your order fills at the end-of-day price. If it is an ETF, your order fills during market hours at the current price, just like a stock.

Many investors use index funds as the core of their portfolio — the largest holding that provides broad market exposure — and then add other investments around it. Others build an entire portfolio from index funds alone, using different funds to cover U.S. stocks, international stocks, and bonds. The approach depends on your goals, time horizon, and how much risk you are comfortable taking.

The difference between index funds and actively managed funds

An actively managed fund employs a manager or team to research companies, decide which ones to buy and sell, and try to outperform the index. They charge higher fees because of this work. An index fund simply holds the index and charges a small fee for administration.

Actively managed funds can outperform in any given year, but most do not beat their index consistently over time. The fees eat into returns, and the manager's picks do not reliably beat the market. Index funds, by design, match the market minus fees — which, over time, often beats most active managers after fees are subtracted.

Some investors prefer active management because they believe a skilled manager can find opportunities the market misses. Others prefer index funds because they cost less and deliver predictable results. Both are legitimate choices; the decision depends on your beliefs about whether active management is worth the cost.

Frequently Asked Questions

Can I lose money in an index fund?

Yes. If the index goes down, the fund goes down with it. Index funds are not may provide investments. However, they spread your money across many companies, so you are not betting on any single stock. Historically, broad market indexes have recovered from downturns over time, but past performance does not may provide future results.

Do index funds pay dividends?

Many do. If the companies in the index pay dividends, the fund collects them and passes them to shareholders. You can usually choose to receive dividends as cash or reinvest them automatically into more shares of the fund. Reinvesting dividends can increase your returns over time through compounding.

What is the difference between an index fund and an ETF?

Both can track an index. The main difference is trading: mutual fund index funds trade once per day at the closing price, while ETF index funds trade throughout the day like stocks. ETFs often have lower fees and are more tax-efficient, but mutual funds may be simpler if you are buying through an employer plan.

How often does an index change?

It depends on the index. The S&P 500 changes when companies are added or removed, which happens several times per year. Some indexes rebalance quarterly or annually. When an index changes, the fund manager adjusts the fund's holdings to match, which may trigger small trading costs and tax consequences.

Can I use index funds in a retirement account?

Yes. Index funds are available in most retirement accounts, including 401(k)s, IRAs, and Roth IRAs. Many employers offer index funds as the default or primary investment option in their 401(k) plans because of the low fees and broad diversification they provide.