Say a manager earns 20% in a year when the market earns 10%. The result makes paying for skill look easy. But that winning manager is obvious only after the year is over.
Index investing means accepting a market's return, less costs, without trying to pick the winning manager. The case rests on arithmetic and the record of active funds as a group.
Choose a market, then follow its rules
An index fund is a mutual fund or ETF that seeks to track a specified index. Either wrapper can follow the same rules; you cannot buy the index itself.
Passive management means following those rules. Active management gives a manager discretion over what to own and how much of each investment to hold, often aiming to beat a market benchmark.
A fund can hold every investment in the index or a representative sample designed to track it. Choosing the market remains your decision, even when following it is the manager's job.
“Everything” means the chosen basket. A broad US stock index leaves out foreign stocks, bonds and other assets. Some indexes cover just one industry. The label tells you how a fund invests; its coverage tells you what you own.
The arithmetic behind the idea
Imagine a whole market owned by three investors, each starting with $1,000. No one adds or withdraws money during the year. The passive investor holds every security in the market's proportions; the other two make active choices.
Say the market gains 10% before costs. The passive holding earns 10%, active A earns 20%, and active B earns 0%. All three together go from $3,000 to $3,300. The active pair's $200 gain on $2,000 also equals 10%.
Subtract hypothetical annual costs of $1 for passive and $10 for each active holding, based on starting balances:
| Holding | Start | Gain | After costs |
|---|---|---|---|
| Passive | $1,000 | $100 | $1,099 |
| Active A | $1,000 | $200 | $1,190 |
| Active B | $1,000 | $0 | $990 |
The active pair finishes with $1,190 + $990 = $2,180. Its net return is ($2,180 − $2,000) / $2,000 = 9%. Passive keeps $99 on $1,000, or 9.9%.
Active A still wins. The active group falls behind because more of its gain pays the bills.
This is William Sharpe's arithmetic of active management. Together, investors own the market. If passive dollars earn its return, all the remaining active dollars must earn it too, before costs. Nothing in that argument requires market prices to be correct.
What the long record shows
The SPIVA US Year-End 2024 scorecard compares active large-cap US stock funds with the S&P 500. Over fifteen years ending December 31, 2024, it counted 89.50% as underperformers—about nine in ten funds available at the start. The shorter periods also show a majority falling short.
Survivorship bias means judging only the funds still standing. SPIVA keeps merged and closed funds in the starting count; a winner must survive the full period and beat the index.
Fund returns are after fees but exclude sales charges called loads. The benchmark uses US-dollar total return, including reinvested distributions, with no costs deducted. An actual index fund has costs.
These bars count funds; Sharpe's arithmetic counts dollars. Results vary by market, category and period, so nine in ten is a historical result, not your future odds. The record shows why relying on a future winner is a demanding plan.
Put the approach into practice
For your $200 decision, the work shifts from picking companies to choosing a market:
- Choose the exposure. “Broad US stocks” names the investments you want to own.
- Read the index's coverage. Check what its rules include and leave out.
- Compare funds following it. Check their costs and how closely their results follow the index.
Suppose our fictional Market Basket Fund follows a broad US stock index. Its 5% Harbor and 4% Tessel weights then come from the index's rules, rather than a manager's forecast. You still choose whether US stocks suit the money's time horizon.
Fees, trading costs and imperfect tracking can leave a fund behind its index. The index label alone does not promise a low price.
The appeal is less reliance on picking winning companies or managers. The trade-off is accepting the chosen market's ups and downs, with costs reducing what you keep.
What indexing leaves unsolved
A fund can follow its index perfectly through a market fall. Broad diversification spreads company risk; it does not make stocks safe for money you need soon. A concentrated index also stays concentrated. Passive is a method, not a safety rating.
Some active funds outperform after costs. Others offer a different mix of investments or more freedom to respond to changing conditions. Those features can have value, but a higher fee still needs a reason beyond last year's ranking. A suitable benchmark makes the performance comparison meaningful.
For your $200, the case can be simple: “I want broad US stocks at a low cost, and I can accept their market risk. This fund leaves out foreign stocks and bonds.” The reason is the exposure and the cost, without a forecast of next year's winning manager.
Living with bad periods remains part of the job. Index investing can be your whole approach; company analysis is optional. Once you know the market you want, expense ratios put a price on the fund's ongoing work.
In short
- An index fund follows a specified index; it does not own every investment.
- Across a defined market, active dollars collectively earn the market before costs. Higher costs lower what remains.
- SPIVA counted about nine in ten active US large-cap funds as underperformers over 15 years ending in 2024.
- Broad indexing reduces the need to pick winners. You still choose the market, bear its risks and decide whether you can stay invested.