This is general information, not investment advice.

The most popular investment of the past half-century works by not trying to be clever. Here's why that matters.

What a market index is

A market index is a measurement — a snapshot of how a slice of the market is doing. The S&P 500, run by S&P Dow Jones Indices, tracks about 500 large US companies; the Dow tracks 30; the Nasdaq Composite skews to tech. An index isn't something you can buy — it's a yardstick. What you can buy is a fund that copies it.

How the S&P 500 works

The S&P 500 is market-cap weighted: each company's slice is proportional to its total market value (share price × shares outstanding). So the biggest companies dominate — the ten largest constituents account for roughly 38% of the index's value, per S&P data. Membership isn't automatic: an S&P committee reviews candidates against rules on size, US domicile, liquidity and recent profitability.

What an index fund is

An index fund is a mutual fund or ETF that holds the same securities as an index, in the same proportions, aiming to match its return rather than beat it — "passive" investing. The opposite is active management, where a manager researches and picks stocks trying to outperform a benchmark.

The record: passive vs. active

The case for index funds rests on a blunt fact: most active managers fail to beat their benchmark, and the longer the horizon, the worse it looks. S&P's SPIVA scorecard found 84.3% of active large-cap US funds underperformed the S&P 500 over the 10 years ending 2024 — and over 15 years, zero of 22 US equity fund categories had a majority of active managers beating their benchmark, per S&P Dow Jones Indices.

Why index funds won: fees

The engine of the argument is cost — every dollar in fees is a dollar that doesn't compound. The average active equity mutual fund charged 0.64% in 2024, versus 0.05% for the average index fund, per the Investment Company Institute. Compounded over decades, that gap erodes a meaningful chunk of returns. It was the insight behind Jack Bogle's first index fund for ordinary investors, launched at Vanguard in 1976 — dismissed at the time as "Bogle's folly," it raised just $11 million and went on to become one of the largest funds in the world.

ETFs vs. index mutual funds

Both can track the same index. Index mutual funds price once daily at net asset value; ETFs trade all day on an exchange like a stock and are often more tax-efficient (see our ETF explainer). The practical differences for a long-term holder are usually modest.

The risks

Index funds aren't magic. They give you the market's losses too — track the S&P 500 down 30% and your fund falls about 30%, with no manager playing defense. Market-cap weighting concentrates risk in the biggest names (today, a handful of megacap tech stocks). And critics raise governance questions as passive funds own ever-larger stakes across corporate America.

What it means

For most investors without a genuine edge, a low-cost index fund offers broad market exposure cheaply — a benchmark the data show is hard to beat after fees. Many advisers treat one as the core of a portfolio, often paired with a bond index fund, and built up over time through dollar-cost averaging (see our explainer). The proposition is simple: capture the market's long-run return at minimal cost, and don't bet on outguessing it.