Sometime in late 2024, a line was crossed that almost no one noticed. For the first time, more investor money sat in index funds than in funds run by professional stock pickers. By the end of 2025, index funds held roughly $19.3 trillion against about $17.4 trillion in active strategies, according to the Investment Company Institute. Understanding how index funds work explains that shift, and it explains something more useful for your own money: why the cheapest, laziest-looking investment product has quietly become the default for everyone from first-time savers to pension managers.
The idea is older than it looks, and once the mechanics click into place, the appeal is hard to unsee.
How index funds work, starting with the basket you can’t buy
Start with what an index actually is. The Securities and Exchange Commission, through its Investor.gov education site, defines a market index as a measurement of the performance of a “basket” of securities meant to represent a slice of the market or the broader economy. The S&P 500 tracks 500 large U.S. companies. The Russell 2000 tracks smaller ones. The Wilshire 5000 reaches for nearly the entire U.S. stock market. These indexes are scorekeepers, not products. You cannot send money to the S&P 500.
An index fund solves that. As the SEC puts it, an index fund is a mutual fund or exchange-traded fund that seeks to track the returns of a market index. Buy a single share of an S&P 500 index fund and you own a sliver of all 500 companies at once, in roughly the proportions the index uses. Some index funds hold every security in their target index, while others hold a representative sample that behaves the same way. Either approach gives you the index’s return, minus a small fee, in one transaction.
The contrast that matters is the one with active funds. An active fund pays a manager and a team of analysts to pick which stocks to buy and when to sell, trying to beat the market. An index fund does not try to beat anything. It just owns the market and follows along. That sounds like settling for average. The data says otherwise.
Why the cheap option usually wins
This is where the story turns counterintuitive. The fund that tries hardest tends to lose, and the gap widens with time.
Every year, S&P Dow Jones Indices publishes its SPIVA scorecard, which measures actively managed funds against the indexes they aim to beat. The findings are remarkably consistent. In 2024, about 65% of actively managed large-cap U.S. equity funds underperformed the S&P 500, slightly worse than the 60% that lagged in 2023. Stretch the window and the picture gets bleaker for the stock pickers. Over the 20 years ending in 2024, roughly 92% of active domestic funds underperformed their benchmarks. SPIVA also found that across the 15-year period ending that December, there was not a single category where a majority of active managers beat the index.
The reasons are structural rather than a knock on anyone’s intelligence. Active managers trade more, which generates costs and taxes. They charge more to fund their research staff. And as a group, professional investors essentially are the market, so before fees their collective return must roughly equal the market’s return, leaving fees to drag them below it. A handful beat the index in any given year, but identifying those winners in advance, and watching them stay ahead for decades, has proven close to impossible.
The expense ratio is the whole game
If active management usually loses, the next question is by how much, and the answer lives in a single number: the expense ratio. That is the annual percentage a fund charges to run itself, skimmed quietly from your balance whether the fund rises or falls.
The spread here is enormous. The Investment Company Institute reports that at the end of 2025, passive index mutual funds carried an average expense ratio of just 0.058%, and passive ETFs averaged 0.135%. Active mutual funds, by contrast, averaged 0.57%. On a $100,000 balance, that is the difference between paying about $58 a year and paying $570. Compound that gap over thirty years and it can quietly cost a saver tens of thousands of dollars, money handed to a fund company in exchange for performance that, per SPIVA, usually trails a far cheaper index fund anyway.
This is the mechanism behind the takeover. Investors did not suddenly decide active managers were incompetent. They noticed that low costs are one of the only reliable predictors of future returns, and they followed the math. Morningstar’s research has long pointed to fees as among the most dependable signals of how a fund will treat you over time, precisely because a fee is the one variable you know with certainty before you invest.
What index funds will not do for you
None of this makes index funds magic, and an honest explainer has to name the tradeoffs. Owning the index means owning the whole ride down as well as up. When the S&P 500 falls 20%, your index fund falls about 20% too, because tracking the market faithfully cuts both directions. There is no manager to move you to cash before a crash, which is the feature, not a flaw, since most managers who try get the timing wrong.
Index funds also concentrate where the index concentrates. A market-cap-weighted fund like an S&P 500 fund puts more of your money into the biggest companies, so when a few giant technology stocks dominate the index, your “diversified” fund leans heavily on them. And an index fund only diversifies within its index. An S&P 500 fund holds no small companies and no international stocks, which is why many investors pair a few index funds to cover the whole market.
The practical takeaway is steadiness rather than cleverness. Index funds work because they keep costs near zero, capture the market’s long-run return, and remove the temptation to outguess it. That is also why they pair so naturally with dollar-cost averaging, which automates buying through every market mood, and why the long arc of compounding they rely on is captured by the Rule of 72. Understanding how index funds work does not just explain the $19 trillion that moved their way. It explains why, for most people saving for a goal years away, the boring basket tends to win.