What Is an Index Fund? How They Work, Why Owning the Whole Market Beats Picking, What a 1% Fee Costs, and What to Do When the Market Falls (Money School, Lesson 12)

An index fund is a fund that buys every company in a market index, such as the S&P 500, in proportion to its size, instead of paying a manager to pick winners. You own a tiny slice of hundreds or thousands of companies in one purchase, for a yearly fee that can be as low as a few hundredths of a percent. That is the whole idea, and it wins for a reason that sounds too simple to be true: most professional stock pickers fail to beat the index. In 2025, 79% of actively managed large-company U.S. funds did worse than the S&P 500, and over 15 years roughly nine in ten have. Use the fee comparison below on any fund you are offered before you put a dollar in it. Here is the fact that should change how you feel about the scary years: from 1928 through 2025, the S&P 500 with dividends fell in 26 of 98 years, once by 44% in a single year, and yet across every 20-year stretch in that history, it never lost money. The worst 20 years, starting in 1929, still returned about 2.4% a year. This is Lesson 12 of Money School.

Jake's first investment, years before Money School, was a stock a customer swore was "about to go big." He bought $800 of it on his phone in the shop's back room. It went big for a week, then went to $140, and Jake decided investing was a casino for people with inside information. When Ethan told him, at the start of Stage 3, that the plan was to buy "everything, slowly, forever," Jake laughed. "That's not investing," he said. "That's giving up." Ethan said that was exactly right, and that giving up on picking winners was the most profitable decision most investors ever make.

If you have never invested because it seems like a world of jargon, tips and people who know more than you, you are in the majority, and your caution is not foolish. The world of stock tips is mostly noise, and some of it is designed to separate you from your money. Nobody explained index funds to me either; I spent my first two years buying funds a bank adviser recommended, with fees I never looked at, before someone showed me the fee line on my statement. This lesson is the explanation I wish I had had first: the boring thing that works, and why it works.

What an index fund is, in plain English

Start with the index. A stock market index is a list of companies with a rule for how much of each to count. The S&P 500 lists about 500 of the largest U.S. companies, weighted by their size. A "total market" index lists nearly every public company in the country. A world index lists companies across many countries. The index itself is just a list; you cannot buy it.

An index fund turns the list into something you can own. It pools money from many investors and buys every company on the list, in the list's proportions, then keeps matching the list as companies grow, shrink, join and leave. Nobody at the fund decides which companies are good. That is the point: because there is no team of analysts picking stocks, the fund can charge very little. When you buy one share of an S&P 500 index fund, you own a sliver of all 500 companies at once.

Index funds come in two wrappers, and the difference matters less than people think:

• Index mutual funds are bought directly from a fund company or inside a retirement plan, priced once a day after the market closes, often with no trading fee.
• Index ETFs (exchange-traded funds) trade on the stock market like a share, at prices that move during the day, through any brokerage account.

Both can track the same index at similarly low costs. Choose whichever your account offers most cheaply; the index and the fee matter far more than the wrapper.

Jake: "So instead of guessing which company wins, I own all of them, winners and losers."

Ethan: "And the winners, over time, pay for the losers and then some. You don't have to know which is which. Nobody reliably does."

Why owning the whole market beats picking: the evidence

It seems as if professionals with research teams should beat a fund that does no research. They mostly do not, year after year. In 2025, 79% of actively managed large-company U.S. stock funds returned less than the S&P 500, up from 65% in 2024 and the fourth-worst year for active managers in the 25 years the comparison has been published. Over longer periods it gets worse for the pickers: in the year-end 2024 results, about 89.5% of active large-cap funds had trailed the index over 15 years.

There are three reasons, and none of them is that fund managers are not smart.

1. Fees. An active fund has to beat the market by more than its fee just to match an index fund. If the index fund costs 0.03% and the active fund costs 0.66%, the active manager starts every year more than half a percent behind.
2. The market is the average. Before costs, all investors together own the whole market and earn its return. For every manager who beats it, another must trail it. After costs, the average investor must trail it, and the index fund, with the lowest costs, sits near the front.
3. Winners do not stay winners. The funds that beat the index one year are rarely the same ones that beat it the next. Choosing last year's winner is mostly choosing luck that has already run out.

None of this means no active manager ever wins; some do, for a while. It means that picking which one will win, in advance, is as hard as picking stocks, and the index fund removes the need to do either.

What a 0.03% fee versus a 1% fee does over 30 years

The fee on a fund is called its expense ratio: the share of your balance it takes every year to run the fund. It is small enough to ignore and large enough to matter more than almost anything else you control. In 2025, the average equity mutual fund charged 0.40% a year, down from 1.04% in 1996; index equity ETFs averaged 0.14%, and broad index mutual funds are often cheaper still, around 0.05% or less. Actively managed equity funds have averaged roughly 0.66%, and some funds sold through advisers or insurance products charge 1% or more before any adviser fee.

Here is what those numbers do to $300 a month for 30 years, assuming the market returns 10% a year before fees, close to the S&P 500's long-run average since 1928:

A 1% fee does not sound like it could cost $124,600 out of $673,800, nearly a fifth of the result, but it compounds every year exactly like the returns do, as Lesson 11 showed. The fee is also the only part of investing that is certain: you cannot know the market's return in advance, and you can know the fee to two decimal places. That makes it the one decision where being careful always pays.

When an adviser charges a separate fee on top of the fund's, usually as a percentage of your balance, add the two together before you compare. A 0.5% fund plus a 1% advice fee is a 1.5% drag every year.

"The market went down": what the history says, and what it means for you

Index funds go down when the market goes down; owning everything means owning the bad years too. Here is what the S&P 500 with dividends reinvested did from 1928 through 2025, ninety-eight years of real history:

• It averaged about 10.0% a year compounded, compared with about 3.4% for short-term Treasury bills and 4.5% for ten-year Treasury bonds.
• It fell in 26 of the 98 years, roughly one year in four.
• Its worst years were brutal: -43.8% in 1931, -36.6% in 2008, -35.3% in 1937, -25.9% in 1974. In 2022 it fell 18.0%; in 2025 it rose 17.7%.
• Over every 10-year stretch in that history, it lost money in only 5 of 89.
• Over every 20-year stretch, it never lost money, not once in 79 periods. The worst, starting just before the 1929 crash, still returned about 2.4% a year; the best, starting in 1980, about 17.7%.

And here is what came after the worst years, from the same history, with dividends reinvested:

The 1937 row is there on purpose: recoveries are not always fast, and five years later that investor was only modestly ahead. That is the reason this lesson keeps saying "decades," not "years."

That is the honest picture: in any single year, stocks can fall a lot; over two decades, the history so far has always rewarded people who held on. The second fact is why index investing is for money you will not need for many years, and the first fact is why your emergency fund lives in a savings account, not in stocks. The floor from Stage 2 is what lets you hold through a bad year instead of selling at the bottom to pay a bill.

What does "the market went down" mean for a 25-year-old adding money every month? It means they are buying at a discount. Every monthly contribution during a fall buys more shares than it would have at the peak, and those shares recover with everything else. The person most hurt by a crash is the one who sells during it; the person most helped is the one who keeps buying.

Jake: "So when the news says the market dropped 20%, the plan is to do nothing."

Ethan: "The plan is to keep doing exactly what you were doing. The transfer on the first of the month doesn't read the news."

Which index? S&P 500, total market, world, and bonds

"Buy an index fund" leaves one choice: which index. For most people the answer is one of a few broad ones, and the differences matter less than starting.

• S&P 500: about 500 large U.S. companies. Simple, very widely available, and the index most comparisons use.
• U.S. total market: nearly every public U.S. company, large and small. Very similar to the S&P 500 in practice, because large companies dominate both, with a little more spread.
• World or international: companies outside the U.S., or the whole world in one fund. It spreads your money across countries, which matters if you do not want your entire future to depend on one economy.
• Bond index funds: lend to governments and companies. Lower returns, smaller falls. Useful for money you will need within several years and for steadying a portfolio as you get closer to needing it.
• Target-date funds: a single fund that holds a mix of stock and bond index funds and gradually shifts toward bonds as a chosen year approaches. If it is built from index funds and its fee is low, it is a reasonable one-decision choice inside a retirement plan.

Two things to avoid at the start: narrow or themed funds (one industry, one trend, one country you have read about), which reintroduce the picking problem, and leveraged or "inverse" funds, which are trading tools rather than long-term holdings and can lose value even when the index ends where it started.

How to buy an index fund: five steps

1. Check the floor. Emergency fund in place (Lesson 2), no card balances at 22% (Lesson 3). Index funds are for money you can leave alone for years.
2. Use your workplace plan first, if you have one. Most retirement plans offer at least one broad index fund or an index-based target-date fund. Contribute at least enough to get any employer match; Lesson 13 covers the accounts.
3. Otherwise open a low-cost brokerage or retirement account in your own name. Look for no account fees and no trading commissions on the funds you want.
4. Pick a broad index fund and read two numbers: the index it tracks (it should be a broad one) and the expense ratio (for a broad index fund, a fraction of a tenth of a percent is normal). If the expense ratio is above about 0.5%, keep looking.
5. Automate a monthly purchase on payday, the same habit as Lesson 1. Then stop looking at it every day. Once a year is plenty.

That is all an index investment plan has to be. It is deliberately boring. The excitement in investing is mostly where the fees and the mistakes are.

The mistakes that turn index investing into stock picking

Index funds protect you from picking stocks; they cannot protect you from picking moments. These are the common ways people undo their own good plan.

• Selling in a fall. The single most expensive mistake. Selling after a drop locks in the loss and usually means missing the recovery, which often comes in a few strong days nobody predicts.
• Waiting for the dip. Holding cash until the market "comes down" means missing growth while you wait. Regular monthly buying makes the timing question disappear.
• Chasing last year's winner. Switching to whichever fund or sector did best last year is buying high and is the active-fund problem in a new form.
• Owning ten funds that hold the same thing. Several S&P 500 funds, a total market fund and a target-date fund mostly own the same companies. More funds is not more diversification; it is more paperwork.
• Ignoring the fee because the fund is called "index." Some index funds charge far more than others for the same index. Read the expense ratio every time.
• Checking every day. Daily prices are noise. Watching them makes the falls feel larger and the urge to act stronger.

The United Kingdom and Australia: same idea, local wrappers

Index funds exist everywhere, and the reasoning travels unchanged: own the market, keep fees low, hold for a long time. In the United Kingdom, the common route is a global or U.K. index fund held inside a Stocks and Shares ISA, where growth and dividends are free of U.K. tax, or inside a workplace pension. Watch the platform fee as well as the fund's own charge; together they are what you pay.

In Australia, index funds and ETFs tracking the Australian market or the world are widely available through brokers, and many superannuation funds offer an indexed investment option, often at lower fees than their active options. Checking whether your super is in an indexed option, and what it charges, is the Australian version of this lesson's exercise.

Wherever your paycheck lands: a broad index, a low fee, a monthly automatic purchase, and decades of patience.

The fee check: what a fund's fee will cost you

Put in your monthly amount, the years, an assumed market return, and the fees of two funds you are comparing. The difference at the bottom is what the more expensive fund costs you over the whole period.

The calculator starts at a 7% return, a more cautious figure than the 10% historical average, because it is better to plan with a number you might beat. The fee difference is large at any return.

This week's exercise: find the fee you are already paying, do this with me

Twenty minutes. Most people who have a retirement account have never looked at what it charges; I hadn't, for years.

1. Log in to every retirement or investment account you have: workplace plan, IRA, old employer plans, anything.
2. For each fund you hold, find two numbers: what it tracks (or whether it is actively managed) and its expense ratio. They are on the fund's fact sheet in the account.
3. Write them down with your balance in each. Any fund above about 0.5% goes on a list to look at.
4. In your workplace plan, find the cheapest broad index fund or index-based target-date fund on the menu. Write down its expense ratio.
5. Run the fee check above with your own monthly amount, using your current fund's fee as Fund B and the cheapest broad index option as Fund A.
6. If the difference is large, change where your future contributions go. Moving existing money inside a retirement account is usually free of tax; check with the plan before you move anything held in a taxable account.

That is all. One evening of looking at a number most people never look at, and the payoff can run to six figures.

Honest aside: index investing is not exciting, and that is the point

If you tell people at a party you own a world index fund and add to it every month, the conversation will move on. If you tell them about a stock that tripled, they will lean in. That asymmetry is why so much financial media is about picking, timing and predicting: it is more interesting. The strategy that has worked best for most people over long periods is too boring to make good television.

There is also something freeing about it. You no longer need to know which company will win, which country will grow, or what the market will do next month. You need a broad index, a low fee, an automatic transfer and the patience to leave it alone through the bad years. That is not giving up on investing; it is giving up on the parts of investing that cost the most.

Where this fits in Money School, and what comes next

Lesson 12 is the second lesson of Stage 3, "Build the reservoir." Lesson 11 showed why time and fees matter so much; this lesson showed the simplest way to put both on your side. Lesson 13 covers the accounts where index funds grow most efficiently: the 401(k), the IRA, pensions and super, how to get the full employer match, and why cashing out when you change jobs is one of the costliest mistakes in personal finance.

After that, Lesson 14 works out how much is enough, your own number from your own spending, and Lesson 15 runs the honest comparison of renting and buying. The course page shows every lesson and which ones are live.

Index funds: your questions, answered straight

Jake's workplace plan, it turned out, had his money in a fund charging 0.94% a year, chosen for him by default when he had joined years earlier. The same menu had a broad index fund at 0.04%. He moved his future contributions and his balance in one evening, inside the plan, at no cost, and the fee check put the difference on his monthly contributions alone, over his remaining 27 years, at about $40,000. The $140 stock is still in his phone app, untouched. He keeps it as a reminder. Ethan wrote the line below on the back of the plan's fee sheet.

Revision note. Written October 24, 2026, as Lesson 12 of Money School. The figures carry their dates in the text: the year-end 2025 SPIVA U.S. Scorecard, the Investment Company Institute's fund expense figures for 2025, and annual S&P 500 total returns from 1928 through 2025 from the NYU Stern historical returns dataset; the rolling-period and fee figures are our own arithmetic on that data. "The tortoise beats the hare," says the old fable; in investing, the tortoise also pays lower fees.