Index funds

A fund is a shared pot of money that buys many investments at once, so everyone who chips in owns a small slice of all of them. An index fund is a fund with an unusual rule: instead of paying someone to pick winners, it buys everything on a published list and holds it. That sounds too passive to work. The passivity is the design, not a shortcut.

An index is a list with rules

An index is a defined list of investments used to measure a market. The S&P 500, for example, is a list of about 500 of the largest U.S. companies. Every index has rules for what belongs on it — how big a company must be, where its shares trade — and when a company stops fitting the rules, it comes off the list and something else goes on.

You can't buy an index directly. It's a measuring stick, not a product. When the news says "the market was up today," it's usually quoting an index.

An index fund buys the list

An index fund holds the investments on one index's list, in roughly the same proportions as the list. That's the whole job. Nobody at the fund is studying companies or forming opinions — the list decides, and the fund copies it. Compare that with an actively managed fund, where a manager picks investments and aims to beat an index. An index fund isn't trying to beat anything. It tries to match its index, and because one purchase buys you every company on the list, it also hands you diversification — spreading your money across many investments so no single one can sink you.

Nobody picks the winners — on purpose

Skipping the picking sounds like a weakness. Here's why it's the feature.

A stock's price already reflects what a huge crowd of buyers and sellers, many of them full-time professionals, collectively thinks the company is worth. Beating that crowd once happens. Beating it for decades, after paying yourself for the effort, is very rare — and nobody has found a reliable way to spot, in advance, which managers will pull it off. An active fund also has a higher bar than "beat the market": it has to beat the market by more than it charges you, every year, or you'd have ended up with more by simply matching the market cheaply.

An index fund gives up the chance of beating the market. In exchange, it never trails its index by more than its small fee. You're trading a slim chance at extraordinary for a near-certainty of average — and average, delivered at low cost, has historically been a hard result for professionals to beat over long stretches.

The fee is where the money is

Every fund charges an expense ratio — its annual fee, taken as a percent of your money, charged whether the fund gains or loses. Because an index fund pays no one to pick, that fee can be tiny. The difference looks boring. It compounds into real money.

Imagine $10,000 left alone for 40 years, earning 7% a year — a made-up smooth number, not a forecast; real markets are never that polite. In a nearly-free fund, you'd end with roughly $150,000. In a fund charging 1% a year, you'd keep about 6% and end with roughly $100,000. Same investments, same 40 years. The fee quietly took about a third of the ending balance.

Now the honest other half. Markets have losing years, and an index fund follows its list down as faithfully as it follows it up: if the index falls 20%, that $10,000 is $8,000, and the fee gets charged anyway. The 7% was an assumption. The fee is in writing. Returns are hopes; the expense ratio is the one number in that whole example that's certain.

Read the list anyway

One caution: index fund does not automatically mean diversified. An index can be a narrow list — a single industry, a single corner of the market — and buying the list is only as good as the list. Checking what's actually on it is how you know what you own.