How Index Funds Give Ordinary Investors Market Returns

The Bet That Changed Investing

In 2007, Warren Buffett made a public $1 million bet that a simple S&P 500 index fund would outperform a basket of five hedge funds over ten years. By 2017, the index fund had gained 125.8%. The hedge funds averaged 36.3%. Buffett's counterpart conceded before the decade was out. That contest encapsulated a transformation in investment philosophy that had been decades in the making — and by 2024, U.S. passive funds held more assets than active funds for the first time in history, with index fund assets exceeding $13 trillion according to Investment Company Institute data.

Index funds are investment vehicles that track a specific market index — a pre-defined basket of securities — by holding the same securities in the same proportions as the index. The most commonly referenced indexes include the S&P 500 (500 large U.S. companies), the Russell 2000 (2,000 smaller U.S. companies), the MSCI World Index (1,500+ companies across 23 developed countries), and the Bloomberg Aggregate Bond Index (thousands of investment-grade U.S. bonds).

How Index Construction and Tracking Work

An index is not itself an investable product — it is a mathematical measurement. The S&P 500, maintained by S&P Dow Jones Indices, selects its 500 constituents based on criteria including market capitalization above $18 billion, U.S. domicile, positive earnings over the most recent four quarters, and public float of at least 50%. The index is weighted by market capitalization, meaning larger companies represent a larger share. As of early 2025, the five largest S&P 500 components — Microsoft, Apple, Nvidia, Amazon, and Meta — collectively represented approximately 27% of the index.

An index fund tracks this composition by purchasing the same securities. When a company is added to or removed from the index, the fund buys or sells accordingly. This passive replication strategy produces several structural advantages over active management:

• Near-zero research costs: no team of analysts picking stocks
• Low portfolio turnover: minimal buying and selling reduces transaction costs and taxable events
• Predictable performance: the fund returns approximately what the index returns, minus expenses
• Transparency: fund holdings are fully disclosed and updated regularly

The Cost Advantage: Where Passive Wins

The expense ratio — the annual percentage fee deducted from fund assets — is the starkest point of comparison between index funds and actively managed funds. Vanguard, the firm founded by John Bogle who launched the first retail index fund in 1976, offers the Vanguard 500 Index Fund (VFIAX) at an expense ratio of 0.04%. The average actively managed U.S. equity fund carries an expense ratio of approximately 0.68%, according to Morningstar's 2024 U.S. Fund Fee Study.

The gap is $80,000 to $145,000 on a $100,000 investment. Fees compound in reverse — every dollar paid in expenses is a dollar that does not participate in future compounding. The math is not subtle.

Active Management's Performance Record

S&P Global's SPIVA (S&P Indices Versus Active) report is the most comprehensive ongoing study of active fund performance relative to benchmarks. The 2023 year-end report found that over a 15-year period, 87% of large-cap U.S. active equity funds underperformed the S&P 500. Over 20 years, the underperformance rate approached 94%. The pattern holds across categories: 80–90% of active mid-cap, small-cap, and international funds also underperformed their respective benchmarks over 15+ year periods.

The core finding is not that active managers lack skill. Many do outperform for several years. The challenge is identifying in advance which managers will outperform, and whether their outperformance will persist after accounting for fees. Research from Vanguard and others consistently finds that past outperformance is a poor predictor of future outperformance — better attribution often goes to factor exposures (value tilt, small-cap tilt) that can be captured through low-cost factor index funds.

Choosing an Index Fund: Key Variables

• Index tracked: Total market funds hold more diversification than S&P 500 funds; international funds provide geographic diversification
• Expense ratio: Lower is almost universally better; any ratio above 0.20% for a broad-market index fund deserves scrutiny
• Tracking error: How closely the fund mirrors its index; smaller tracking errors indicate better execution
• Fund structure: Mutual fund vs. ETF (exchange-traded fund); ETFs trade intraday and may offer tax advantages; mutual funds settle at end-of-day NAV
• Minimum investment: Vanguard mutual funds typically require $1,000–$3,000; Fidelity index funds have no minimums; ETFs require purchasing at least one share (many brokers offer fractional shares)

For most individual investors building long-term wealth through tax-advantaged accounts like 401(k)s or IRAs, a single broad-market index fund covering U.S. or global equities provides sufficient diversification, near-zero cost, and performance that beats the majority of professional active managers over any meaningful time horizon. Simplicity is a feature.

This article is for informational purposes only and does not constitute financial advice.