Stocks

What Is an Index Fund?

Name: CFGI Fear and Greed Index
Creator: CFGI

By Lucas, CFGI ResearchUpdated June 28, 2026Reviewed by Rob

Diagram of an index fund: a fund holding the same companies as a market index, in the same weights, to match its return. — Own the whole slice of the market, cheaply, and match it. Source: CFGI.

Quick answer

An index fund is a fund that simply tracks a market index, such as the S&P 500, by holding the same companies in the same proportions. It does not try to pick winners or beat the market; it aims to match it, at very low cost. That passive approach, plus broad diversification in a single holding, is why index funds have grown from a ridiculed experiment in the 1970s into the default choice for millions of long-term investors.

CFGI data

An index fund removes the hard part of picking stocks, but not the hardest part of all: sitting still when the crowd panics. The biggest threat to an index investor’s returns is selling in fear and buying in greed, the exact mood CFGI scores on a 0 to 100 scale, daily for stocks since 2021, which makes the gauge a useful gut-check against undoing a sound strategy at the worst moment.

Source: CFGI dataset, 2021 to June 2026.

An index fund tracks an index instead of trying to beat it.
It holds the index’s companies in the same proportions, at very low cost.
John Bogle launched the first retail index fund in 1976, to ridicule.
Over 15 years, around 90% of active US large-cap funds trail the S&P 500.
The hard part is not the strategy but holding it through fear and greed.

Matching the Market, Not Beating It

A market index like the S&P 500 measures a slice of the market. An index fund buys that slice: the same companies, weighted the same way, so its return tracks the index. There is no manager trying to outguess the market, which keeps costs low and removes the risk of a manager underperforming. Because one fund holds hundreds of companies, it delivers diversification in a single purchase. An index fund can be packaged as a traditional mutual fund or as an ETF that trades on an exchange.

Where It Came From: "Bogle’s Folly"

The index fund was not always obvious. John Bogle founded Vanguard in 1974 and, in 1976, launched the first index fund for ordinary investors, tracking the S&P 500, giving individuals a strategy that had been reserved for big institutions. The reception was brutal. Critics mocked it as "un-American" and a guaranteed path to mediocrity, and the launch raised only around 11 million dollars, which Bogle himself later called an abject failure. "Bogle’s Folly", they called it. That folly grew into one of the largest funds on earth, worth well over a trillion dollars, and reshaped how the world invests.

Why It Works: Fees and Averages

The logic is almost arithmetic. As a group, all investors collectively own the market, so as a group they earn the market return, minus their costs. That means the average actively managed dollar, weighed down by higher fees, must underperform a cheap index dollar over time. The evidence is overwhelming: the long-running SPIVA studies find that over a 15-year horizon, roughly 90% of actively managed US large-cap funds underperform the S&P 500 after fees. Paying more to try to beat the market, on average, buys you lower returns, which is the entire case for owning it cheaply instead.

The Core Insight

You cannot control the market’s return, but you can control your costs. An index fund maximises the thing you control and stops fighting the thing you cannot.

Index Fund, ETF Or Mutual Fund?

These names overlap and confuse people, so it helps to separate them. "Index fund" describes a strategy: tracking an index rather than trying to beat it. "Mutual fund" and "ETF" describe structures: how the fund is bought and priced. An index fund can be either, an index mutual fund priced once a day, or an index ETF that trades all day on an exchange. What they share, and what matters most, is a tiny fee and the goal of matching the market. When people praise "low-cost index investing", they mean exactly this, whichever wrapper it comes in.

What Index Funds Do and Don’t Protect Against

It is important to be clear-eyed about the risk. An index fund spreads single-company risk: if one firm in the index collapses, it is a tiny fraction of your holding. But it does nothing to protect against market-wide risk. In a crash, an index fund falls right along with the index it tracks, because it is the index. Index funds reduce one important kind of risk, the danger of betting everything on a single company, but they leave you fully exposed to the ups and downs of the whole market, which is the price of capturing its long-run returns.

The Hard Part Is Doing Nothing

The strategy is simple to describe and surprisingly hard to follow. The biggest threat to an index investor is not the market; it is their own behaviour. Study after study finds a "behaviour gap": investors earn less than the very funds they own, because they sell in panic near the bottom and pile back in near the top. An index fund only delivers its famous long-run returns to the person who actually holds it through the scary parts. Doing nothing, when every instinct screams to act, turns out to be the hardest and most valuable skill in passive investing.

Index Funds and Market Sentiment

This is where sentiment quietly matters even to the most hands-off investor. The emotion that wrecks an index strategy, panic in a crash, euphoria in a boom, is exactly what a Stock Fear and Greed Index measures on a 0 to 100 scale. Used well, the gauge is not a trading signal but a mirror: when it screams Extreme Fear and you feel the urge to sell your index fund, that is precisely the moment the data, and history, suggest holding on. Knowing the crowd is at an extreme can be the nudge that keeps you from undoing a sound plan at the worst possible time.

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