Index Funds Explained

An index fund is a type of investment fund, either a mutual fund or an ETF, designed to track a specific market index. Rather than employing analysts to pick individual winning stocks, an index fund simply buys all (or a representative sample of) the securities that make up its target index.

The S&P 500 index, for example, tracks 500 of the largest publicly traded US companies. An S&P 500 index fund holds those same 500 stocks in the same proportions, automatically updating when the index adds or removes companies. When Apple, Microsoft, and Nvidia rise, so does your index fund. When the index falls, the fund falls with it.

Index funds were invented by John Bogle, the founder of Vanguard, who launched the first index mutual fund for retail investors in 1976. Wall Street mocked it at launch, calling it 'Bogle's Folly.' Today, index funds manage tens of trillions of dollars globally and represent the dominant strategy for institutional and individual investors alike.

When you invest in an index fund, your money is pooled with thousands of other investors. The fund manager uses this pool to buy the stocks or bonds in the tracked index, weighted by market capitalization, meaning larger companies receive a proportionally larger share of the fund.

As the prices of those underlying securities fluctuate, so does the value of your fund. If the S&P 500 rises 12% in a year, a well-constructed S&P 500 index fund will rise approximately 12%, minus only the tiny expense ratio. You capture the index's return, no more, no less.

ETF index funds trade on exchanges throughout the day, just like individual stocks. Mutual fund index funds price once per day after the market closes. Functionally, both achieve the same goal: broad market exposure at low cost. The choice between them is largely one of personal preference and broker availability.

Fees are the clearest, most reliable edge index funds have. Because the fee is taken every year as a percentage of your balance, even a small difference compounds into a large gap over a long career. The table below shows the yearly cost on a $10,000 balance at a few common fee levels.

Actively managed funds employ teams of analysts and portfolio managers who research companies and try to build a portfolio that beats the market. They charge higher fees for this effort, often 0.50% to 1.00% or more per year. Index funds charge very little, because they simply follow the index.

The uncomfortable data for the fund-management industry is that most active funds trail their benchmark over long periods. The SPIVA (S&P Indices Versus Active) scorecard from S&P Dow Jones Indices tracks this. Over the 10-year period ending December 2024, at least 80% of active US equity funds underperformed their benchmark, and over the 15-year period, no major category saw a majority of active funds beat it. Underperformance generally rises the longer the time horizon.

Why? Partly fees: an active fund charging 1% per year starts every year already 1% behind the index. Partly market efficiency: millions of sophisticated participants compete continuously to find mispriced securities, so consistently outsmarting the crowd is extraordinarily hard, even for skilled professionals.

In his 2013 letter to Berkshire Hathaway shareholders, Warren Buffett, widely considered one of the greatest investors of all time, revealed that his estate instructions direct 90% of his wife's inheritance into a single low-cost S&P 500 index fund. It is a striking message: the man who has beaten the market for decades steers ordinary investors away from trying to copy him.

His reasoning has been consistent for years. Most investors, including professionals, cannot reliably pick stocks that beat the market after fees over long periods. And even when they pick well, they often hurt their own returns through overtrading, emotional reactions to volatility, and high costs. An index fund quietly keeps you invested, diversified, and low-cost.

In 2007 he backed that view with a $1 million charitable wager: that a simple, low-cost S&P 500 index fund would beat a basket of hedge funds over the next decade, after all fees. Ted Seides of Protege Partners took the other side and chose five funds of funds, each spreading money across many underlying hedge funds. Over the ten years from 2008 through 2017, Buffett's index fund gained about 125.8% in total, roughly 7.1% a year, while the five funds of funds averaged about 36%, roughly 2.1% a year. The proceeds, more than $2.2 million, went to Girls Inc. of Omaha.

The bet mattered because it was a real, public, decade-long test with money on the line, against professionals who were paid well to win. The simple index fund still came out far ahead, mostly because the layers of hedge-fund fees, often around 2% a year plus a share of profits, were a steady drag that skill did not overcome.

Not all index funds are the same. They differ by which index they track, and that choice shapes what you own. The most common building blocks are broad US stock funds, international funds, and bond funds. Many investors combine a broad US fund with an international fund for global coverage.

Getting started is simpler than most people expect. It takes three things: a brokerage account, money to invest, and a single broad index fund to begin with. You do not need a complicated portfolio to start.

Once you have made your first purchase, the most useful habit is to automate regular contributions and check your portfolio infrequently. The investors who look least often tend to do best, because they are least tempted to react to short-term swings. Patience is genuinely the main skill required.

Index funds work because broad markets have risen over long periods, but that long-run return only reaches investors who actually stay invested. The single most common and costly mistake is selling at the wrong time, almost always during a market drop.

Every major downturn comes with convincing reasons to sell and wait for things to calm down. The trouble is that the strongest recovery days often cluster close to the worst ones, so an investor who sells during a decline tends to lock in the loss and miss much of the rebound. The fund did its job; the behavior undid it.

This is why a plain index fund paired with a steady, automatic schedule is so effective. It removes most of the moments where a hasty decision can do damage. Deciding in advance that you will hold through downturns, and not checking the balance too often, protects returns more than almost any clever fund choice.