Index investing: passive vs. active funds and what the data shows

Index investing means owning every security in a market index, in the index's own proportions, instead of trying to pick the ones that will do best. An S&P 500 index fund doesn't ask whether Apple or Exxon is the better buy. It holds all 500 companies, each in proportion to its market value, and accepts the index's return minus a small fee.

That sounds like settling for average. The surprise, documented every six months for more than two decades by S&P Dow Jones Indices, is that "average minus a tiny fee" beats most professional stock pickers over long periods. This guide explains how index funds work, what the evidence on active managers actually says, the arithmetic behind it, how to judge how well an index fund does its one job, and where indexing has real limits.

What an index is, and what an index fund does

An index is a rule-based list of securities with a weighting method: the S&P 500 (500 large US companies chosen by a committee and weighted by float-adjusted market capitalization), the Russell 2000 (small US companies), the Bloomberg US Aggregate (the investment-grade US bond market), and thousands of others. The index itself is just a calculation. You can't buy it.

An index fund is a mutual fund or ETF built to reproduce an index's return. The SEC's bulletin on index funds describes the approach as passive: the fund holds the index's securities, or a representative sample of them, and trades only when the index changes or money flows in or out. Investor.gov notes that passive management usually means less trading, lower fees and lower realized capital gains than active management.

There are two common ways to track:

• Full replication. Hold every security at its index weight. Typical for liquid indexes like the S&P 500.
• Sampling. Hold a representative subset that matches the index's key traits. Typical for bond indexes with thousands of issues, many of which rarely trade.

Most broad indexes are market-cap weighted: a company worth twice as much gets twice the weight. That means the fund automatically owns more of what has grown, never needs to rebalance for price moves, and trades very little. Other methods exist, including equal-weight, dividend-weighted and factor ("smart beta") indexes, but they trade more and usually cost more.

Passive versus active

What the SPIVA scorecards show

Since 2002, S&P Dow Jones Indices has published the SPIVA (S&P Indices Versus Active) scorecards, which compare actively managed US mutual funds with the index that matches each fund's category. Two design choices make them more honest than most performance claims: they count funds that closed or merged during the period as part of the starting group, so failed funds can't quietly vanish, and they compare each fund with its own style benchmark.

The year-end 2025 report, covering periods ending December 31, 2025, found that the share of all US large-cap funds that underperformed the S&P 500 was:

• 78.78% over 1 year;
• 66.84% over 3 years;
• 88.96% over 5 years;
• 85.59% over 10 years;
• 89.93% over 15 years; and
• 92.89% over 20 years.

Across all domestic stock funds measured against the S&P Composite 1500, the 20-year figure was 95.01%. Results vary by category and by year. Mid-cap and small-cap managers have done relatively better in some short periods (only 40.65% of small-cap funds lagged the S&P SmallCap 600 in 2025), but their long-run numbers were similar: over 20 years, 89.74% of mid-cap and 90.28% of small-cap funds trailed their benchmarks. The mid-year 2026 scorecard, published in September 2026, continues the series.

The same report tracked survivorship: of the 2,818 domestic stock funds that existed at the start of the 15-year period, only 50.92% were still around at the end. Funds that lag tend to be merged away or closed, which flatters any track record that only looks at survivors.

The arithmetic behind it

The result follows from a simple accounting identity. Every share of every stock in the market is owned by someone. Split all owners into two groups: those who hold the market in index proportions, and everyone else. The index holders, by definition, earn the market return before costs. Since together the two groups own the whole market, the active group, in aggregate and before costs, must also earn the market return. Active investors can only beat the market at the expense of other active investors.

Costs then break the tie. Active funds charge more and trade more, so after costs the average actively managed dollar must trail the average passively managed dollar.

Judging an index fund: tracking difference and tracking error

Because an index fund's job is narrow, it's easy to grade.

• Tracking difference is the fund's return minus the index's return over a period. It is normally slightly negative, by about the expense ratio, plus or minus small effects from cash drag, sampling, trading costs and securities-lending income.
• Tracking error is the standard deviation of that difference over time. It measures how consistently the fund follows the index. A fund with a steady −0.05% gap has a near-zero tracking error even though it never matches the index exactly.

A real example: three S&P 500 funds

Three of the largest S&P 500 index funds come from three different sponsors: State Street's SPY, BlackRock's IVV and Vanguard's VFIAX. Using Finabase's weekly closes with dividends reinvested, $10,000 put into each in the week of November 8, 2021 was worth $17,674, $17,733 and $17,703 in the week of October 26, 2026, a spread of $59 on a gain of more than $7,600. IVV's lead over SPY is in line with its lower expense ratio, 0.03% against SPY's 0.0945% as of October 2026. Different sponsors, different wrappers, the same index: almost exactly the same result. The S&P 500 price index, which leaves out dividends, would have grown to the equivalent of $16,425, which is why index funds are compared with an index's total return, not its price.

The limits of indexing

You get the whole market, including its excesses. A cap-weighted index owns the most of whatever has risen most. When a few giant companies dominate the index, an S&P 500 fund is less diversified than "500 stocks" suggests.

You get every decline in full. An index fund never moves to cash. In 2008 the S&P 500's total return was about −37%, and an S&P 500 fund delivered it. Indexing manages costs, not risk; diversification across asset classes is what manages risk (see Diversification: why spreading your investments reduces risk and Asset allocation: how stock, bond and cash mixes have behaved).

Not all index funds are cheap or broad. "Index" only means rule-based. A narrow thematic index fund can be expensive, concentrated and high-turnover.

The index itself matters. Two "US stock" funds can track very different indexes, such as the S&P 500 versus a total-market index that adds thousands of mid and small companies.

How Finabase helps

The indices page shows major world indexes live, and the heatmap sizes the market by market capitalization so you can see how much a handful of the largest companies weigh in a cap-weighted index. The comparison tool charts index funds rebased to the same start, so you can check their tracking against each other, and each fund's symbol page lists its dividend history. Finabase does not recommend or sell funds.

Frequently asked questions

Do index funds always underperform the index?

Almost always, slightly, because the index has no costs and the fund does. A well-run index fund trails by roughly its expense ratio; securities-lending income can occasionally close part of the gap.

Is index investing the same as buying ETFs?

No. Most ETFs track indexes, but index funds also come as mutual funds, and some ETFs are actively managed. Index investing describes the strategy; ETF and mutual fund describe the wrapper.

What does SPIVA stand for?

S&P Indices Versus Active. It is S&P Dow Jones Indices' twice-yearly comparison of actively managed mutual funds with their category benchmarks, published since 2002.

Is the S&P 500 enough diversification?

It covers large US companies across all 11 sectors, but no small companies, no international stocks and no bonds. Whether that is enough depends on the goals and risk an investor has, which is the subject of Asset allocation: how stock, bond and cash mixes have behaved.

Summary

Index investing replaces the question "which stocks will win?" with "how cheaply can I own them all?" An index fund holds a benchmark's securities by rule, trades little, and returns the index minus a small fee. S&P's SPIVA scorecards show that most active US stock funds trail their benchmarks, about 90% of large-cap funds over 15 years to 2025, and half of all funds don't survive that long. The underlying reason is arithmetic: before costs active investors earn the market's return as a group, and after costs they must earn less. When choosing an index fund, look at the index it follows, its expense ratio, and its tracking difference and tracking error; when relying on indexing, remember that it controls costs, not market risk.