What Is an Index Fund?

What Is an Index Fund? (And Why It’s Probably the Only Investment You Need)

A few years ago, someone asked me what I was invested in and I listed off a bunch of individual stocks I’d picked based on articles I’d read and tips I’d overheard. I felt pretty smart about it.

Then they asked me how those picks compared to just buying an S&P 500 index fund.

I checked. My carefully researched, hand-selected portfolio had returned about 6% that year. The S&P 500 index fund had returned 14%. I’d spent hours picking stocks and underperformed the most boring investment on earth by more than double.

That was the last year I picked individual stocks.

Here’s the uncomfortable truth that Wall Street doesn’t want you to know: the simplest, cheapest, most boring investment strategy available consistently beats the vast majority of professional fund managers. It requires no research, no stock-picking skill, and no financial advisor. It’s called an index fund, and once you understand it, you’ll wonder why anyone does anything else.

The Simplest Explanation Possible

An index fund is a collection of stocks bundled into one investment that tracks a specific market index.

The most popular one tracks the S&P 500 — a list of the 500 largest companies in America. When you buy an S&P 500 index fund, you instantly own a tiny slice of Apple, Microsoft, Amazon, Google, JPMorgan, Johnson & Johnson, and 494 other companies. One purchase. Five hundred companies. Instant diversification.

You’re not picking winners. You’re buying the whole market. And the whole market, historically, goes up over time.

That’s the entire concept. Everything else is just details.

Why Index Funds Beat (Almost) Everyone

Here’s the stat that should end every stock-picking debate: over any given 15-year period, roughly 90% of actively managed funds fail to beat the S&P 500 index. These are funds run by teams of MBAs and PhDs with Bloomberg terminals and research budgets in the millions. Nine out of ten of them can’t beat the thing you can buy for $50 on your phone.

Warren Buffett — arguably the greatest investor alive — has said repeatedly that most people should just buy an S&P 500 index fund. He even bet a hedge fund manager $1 million that an index fund would beat a portfolio of hedge funds over 10 years. Buffett won. It wasn’t even close.

Why do index funds win? Three reasons:

Fees are almost nothing. The best S&P 500 index funds charge 0.015% to 0.03% per year. On a $10,000 investment, that’s $1.50 to $3. Actively managed funds charge 0.5% to 1.5% — which sounds small until you realize that over 30 years, a 1% fee difference on the same portfolio costs you roughly $100,000 in lost growth. Fees are the silent wealth killer, and index funds charge almost nothing.

No human error. An index fund doesn’t try to time the market, chase trends, or make emotional decisions. It just buys everything in the index and holds it. That mechanical simplicity removes the human mistakes that plague even professional investors — panic-selling during crashes, FOMO-buying during bubbles, and overconfidence in stock picks.

Diversification is built in. Owning 500 companies means no single company can tank your portfolio. If one company goes bankrupt, you barely feel it because the other 499 cushion the blow. Individual stock-pickers live and die by their picks. Index fund investors ride the entire economy.

The Numbers That Matter

The S&P 500 has returned an average of roughly 10% per year over the long term. Not every year — some years it’s up 25%, some years it’s down 20%. But across 10, 20, and 30-year periods, the average holds remarkably steady.

Here’s what that 10% average does to your money over time:

$200/month invested for 10 years: roughly $38,000. For 20 years: roughly $137,000. For 30 years: roughly $395,000. For 40 years: roughly $1,060,000.

A million dollars. From $200 a month. In the most boring investment on the planet. That’s not a fantasy — that’s what the historical average actually produces.

The key phrase is “over time.” In any single year, the market can drop 30% or more. In 2008 it lost nearly 40%. In 2020 it dropped 34% in one month. Both times, it recovered and went on to reach new highs. The people who panicked and sold locked in their losses. The people who held on — or better yet, kept buying — came out significantly ahead.

Index funds reward patience. They punish panic. The strategy is aggressively boring: buy regularly, hold forever, don’t look at it too often.

Which Index Fund Should You Buy?

There are hundreds of index funds, but for beginners, you really only need to pick from a handful. These are the best S&P 500 index funds available in 2026:

FXAIX (Fidelity 500 Index Fund) — expense ratio 0.015%. No minimum investment. Available at Fidelity. The cheapest major S&P 500 fund.

VOO (Vanguard S&P 500 ETF) — expense ratio 0.03%. No minimum. Available at any brokerage. The most popular S&P 500 ETF in the world.

SWPPX (Schwab S&P 500 Index Fund) — expense ratio 0.02%. No minimum. Available at Schwab.

IVV (iShares Core S&P 500 ETF) — expense ratio 0.03%. No minimum. Available anywhere.

FNILX (Fidelity ZERO Large Cap Index) — expense ratio literally 0%. No minimum. Only available at Fidelity. It technically doesn’t track the S&P 500 by name (to avoid licensing fees), but it holds essentially the same stocks.

Here’s the honest truth: it barely matters which one you pick. They all own the same 500 stocks. They all return virtually the same amount. The performance difference between them over five years is less than 0.1%. Pick whichever one is available at your brokerage and move on.

If you’re at Fidelity, buy FXAIX or FNILX. If you’re at Vanguard, buy VOO. If you’re at Schwab, buy SWPPX. If you’re somewhere else, buy VOO or IVV. Done.

What About Total Stock Market Funds?

You’ll also see index funds that track the “total stock market” instead of just the S&P 500. These include all U.S. publicly traded companies — not just the top 500 but also mid-sized and small companies. About 3,000 to 4,000 stocks total.

The most popular ones are VTI (Vanguard Total Stock Market ETF) and FSKAX (Fidelity Total Market Index Fund).

The difference between a total market fund and an S&P 500 fund is surprisingly small. The S&P 500 companies make up about 80% of the total market’s value, so the returns track very closely. Over the past 20 years, the performance gap between VTI and VOO has been negligible.

Both are excellent. If I had to pick one for a beginner, I’d lean toward the S&P 500 fund simply because it’s easier to understand and explain. But a total market fund is equally valid. You’re not making a bad choice either way.

The One-Fund Portfolio (Yes, Really)

Financial influencers love to complicate investing. They talk about sector allocation, international exposure, bond ratios, rebalancing schedules, and alternative assets until your eyes glaze over and you do nothing.

Here’s what they won’t say out loud: a single S&P 500 index fund, bought regularly and held for decades, is a complete investment strategy. Not a starter strategy. Not a “until you learn more” strategy. A genuine, lifelong wealth-building strategy that outperforms most professional portfolios.

JL Collins wrote an entire book about this called “The Simple Path to Wealth.” His advice to his daughter was essentially: buy VTSAX and don’t touch it. That’s the whole book. (It’s excellent, and it’s on our Resources page.)

As you get more comfortable and your portfolio grows, you might want to add international stocks or bonds for additional diversification. That’s fine. But you don’t need to. One index fund, bought consistently, is enough to build serious wealth over a lifetime.

Don’t let the pursuit of the perfect portfolio stop you from starting an adequate one.

The Mistakes That Cost People Money

Waiting for a “dip” to invest. The market hits new highs constantly. If you wait for it to drop, you miss the growth while you’re waiting. Time in the market beats timing the market. Every study confirms this.

Checking your portfolio too often. When you look daily, you see terrifying drops and exciting spikes. When you look annually, you see steady growth. The less often you check, the better your decision-making. I check mine once a month during my net worth update and that’s it.

Selling during a crash. The S&P 500 has recovered from every crash in its history. Every. Single. One. Selling during a downturn is the one guaranteed way to lose money in an index fund. If the market drops 30%, the correct response is to keep buying. You’re getting a 30% discount on the same companies.

Paying for something you can get for almost free. If a financial advisor puts you in actively managed funds charging 1% when you could own an index fund charging 0.03%, they’re not working for you. Over 30 years on a $200,000 portfolio, that fee difference costs you roughly $200,000. That’s not advice — that’s expensive underperformance.

Start With One Fund and One Transfer

If this article does its job, you should walk away knowing three things:

An index fund owns the whole market so you don’t have to pick winners. It costs almost nothing. And it beats most professional investors over time.

The action is simple: open a brokerage account (or use the one you already have), buy $50 or $100 of an S&P 500 index fund, and set up an automatic monthly purchase. Then go live your life. Check in once a month. Increase your contributions when you can afford to. And let time and compound interest do what they’ve always done.

Investing doesn’t have to be complicated. The finance industry wants you to think it’s complex because complexity justifies their fees. But the most effective strategy in the history of the stock market fits in one sentence:

Buy an index fund every month and don’t sell it.

That’s the whole secret. Now you know it too.