What Is an Index Fund? The Simplest Way to Start Investing

Index funds are the most beginner-friendly investing tool ever created — low cost, low maintenance, and proven to outperform most professional fund managers. Here's how they work.

The Investing Tool Warren Buffett Recommends to Almost Everyone

Warren Buffett — one of the greatest investors in history — has said repeatedly that most people, including professional investors, should put their money in a simple S&P 500 index fund. He's even specified in his will that the cash left to his wife should be invested 90% in index funds.

That's a remarkable statement. The man who has spent decades analyzing individual companies and making sophisticated investment decisions says the best strategy for most people is an index fund. Why? Because index funds are simple, cheap, and remarkably effective — and they outperform most actively managed funds over the long run.

Here's what you need to know.

What Is an Index Fund?

An index fund is a type of investment fund designed to track the performance of a specific market index — a list of stocks or bonds that represents a segment of the market.

The most famous index is the S&P 500, which tracks 500 of the largest publicly traded companies in the United States — companies like Apple, Microsoft, Amazon, Google, JPMorgan Chase, and Berkshire Hathaway. When you invest in an S&P 500 index fund, you own a tiny piece of all 500 of those companies at once.

Other common indexes include:

Total Stock Market Index — all publicly traded U.S. companies (thousands of them)
Nasdaq-100 — 100 of the largest tech-heavy companies
International Developed Market Index — large companies outside the U.S. (Europe, Japan, Australia)
Bond Index — U.S. government or corporate bonds

Index funds are "passive" — they don't have a manager making decisions about which stocks to buy or sell. They simply replicate the index. That passivity is their greatest strength.

Index Funds vs. Actively Managed Funds

Actively managed funds employ professional portfolio managers who research companies, analyze data, and make buy/sell decisions aimed at beating the market. They charge more for this service — usually 0.5% to 1.5% per year in fees.

Here's the uncomfortable truth about actively managed funds: most of them underperform the market over the long run. S&P research (the SPIVA reports) consistently shows that over 10-15 year periods, roughly 85–90% of actively managed U.S. large-cap funds fail to beat the S&P 500 index.

The managers who beat it one year often don't beat it the next. The managers who charge more don't outperform the ones who charge less. And the fees compound painfully over decades.

Expense ratios matter more than most investors realize. An expense ratio is the annual fee charged as a percentage of your investment. A 1% fee sounds small, but on a $100,000 portfolio over 30 years, it costs you more than $100,000 in lost growth compared to a 0.03% index fund. That's real money taken directly from your retirement.

Vanguard's S&P 500 index fund (VFIAX) charges 0.04% annually. Fidelity's equivalent (FZROX) charges 0.00%. Compare that to actively managed funds charging 20–50x more for worse results.

How Index Funds Work in Practice

When you invest $1,000 in an S&P 500 index fund:

The fund manager buys shares proportionally across all 500 companies.
You own a tiny slice of every company in the index.
When the index goes up 10%, your investment goes up roughly 10%.
When the index drops, your investment drops — but historically, the market recovers and grows over long periods.

There is no active decision-making, no trading in and out of positions, no research team. The fund mechanically mirrors the index and charges you almost nothing to do it.

Dividends paid by the companies in the index are either distributed to you or automatically reinvested — most investors choose reinvestment to compound returns.

How to Buy Your First Index Fund

Buying an index fund is simpler than most people expect:

Open a brokerage or retirement account. For retirement savings: open a Roth IRA or contribute to your 401(k). For taxable investing: open a brokerage account at Fidelity, Vanguard, Schwab, or similar.
Search for an S&P 500 or Total Market index fund. At Fidelity, look for FZROX or FXAIX. At Vanguard, look for VTSAX or VFIAX. At Schwab, look for SWTSX.
Choose how much to invest. Many index funds have no minimum now — you can start with $1. Others require $1,000–$3,000.
Set up automatic contributions. The most powerful move is automating regular investments — $100 or $200 per month automatically invested means you never have to think about it.
Leave it alone. The biggest enemy of index fund returns is the urge to sell when markets drop. Time in the market beats timing the market — consistently and dramatically.

Why Index Funds Win Over Time

The math is simple: markets go up over long periods. The U.S. stock market has averaged roughly 10% annually for nearly 100 years — through recessions, wars, crashes, and crises. An index fund captures that return. An actively managed fund tries to beat that return, fails most of the time, and charges you more for the attempt.

This isn't a secret. It's been documented in academic research for decades. Jack Bogle, who founded Vanguard and created the first index fund in 1976, built his entire career around this insight: low costs and broad diversification beat active management over time.

For a first-time investor, there is no simpler, more proven starting point than a low-cost index fund. Open an account, set up automatic contributions, and let compounding do the work.