Direct answer: An index fund is a portfolio that tracks a market index (such as the S&P 500 or the MSCI ACWI) by holding the same securities in the same proportions as the index. Index funds have lower costs than actively managed funds because they do not employ analysts or make active trading decisions. Decades of evidence show that most actively managed equity funds underperform their benchmark index after fees over periods of 10 to 20 years.
Fact Sheet: Index Funds
Key Takeaways
- S&P 500 index funds (expense ratio: 0.03% to 0.04%) track the 500 largest U.S. companies by float-adjusted market capitalization; as of 2024, the S&P 500 represents approximately 80% of U.S. equity market capitalization.
- SPIVA (S&P Indices Versus Active) reports consistently show that over 15-year periods, 85% to 92% of actively managed U.S. large-cap equity funds underperform the S&P 500 after fees.
- Total market index funds (tracking CRSP US Total Market, Russell 3000, or Dow Jones U.S. Total Stock Market) provide exposure to approximately 4,000 U.S. stocks including small and mid-cap companies, not just the 500 largest.
- Index fund risks: concentration risk (a market-cap-weighted index holds more of the largest companies; in the S&P 500 as of 2024, the top 10 stocks represent approximately 35% of the index), no protection against index-level declines (you get the full market decline), and no opportunity to avoid sectors or companies in the index.
- Index funds come in two legal structures: index mutual funds (priced once daily at NAV, purchased directly from fund company) and exchange-traded funds (ETFs, traded throughout the day on exchanges); for long-term investors the differences are minor.
How Index Funds Work
A market-cap-weighted index fund holds each security in the index in proportion to its total market capitalization. When Apple's market cap is 7% of the S&P 500's total market cap, the fund holds 7% of its assets in Apple. When index composition changes (companies are added or removed at quarterly rebalances), the fund trades to match. Because these trades are rules-based and predictable, transaction costs and tax drag are minimal compared to active funds. The fund manager's job is minimizing tracking error (the difference between the fund's return and the index's return), not selecting securities.
Cost Comparison
The cost differential between index funds and active funds is large and persistent. Vanguard's S&P 500 ETF (VOO) charges 0.03% per year; a representative actively managed U.S. large-cap fund charges 0.75% to 1.0% per year. Over 30 years at 8% pre-fee returns on a $100,000 investment: the index fund at 7.97% net grows to $1,050,000; the active fund at 7.25% net (1% fee) grows to $806,000. The 0.97% annual cost difference compounds to a 23% reduction in ending wealth. This is before considering that most active funds also trail the index on a gross (pre-fee) basis, making the net comparison even more favorable to index funds.
Types of Index Funds
By market coverage: S&P 500 (U.S. large cap only); Total U.S. market (large, mid, small cap); International developed markets (Europe, Asia-Pacific, Japan); Emerging markets; Global all-cap (U.S. plus international). By weighting method: market-cap weighted (most common: bigger companies get more weight); equal-weighted (every company in the index gets the same weight, typically rebalanced quarterly); factor-weighted (tilted toward specific factors like value, momentum, or low volatility). By asset class: equity index funds (most common), bond index funds (total bond market, government, corporate, municipal, international), commodity index funds (typically futures-based), real estate index funds (REITs).
Key Limitations
Index funds are not risk-free: a total stock market index fund declined 50% in the 2008 to 2009 financial crisis. They provide no opportunity to avoid clearly overvalued assets (you hold the index regardless of valuation). They have concentration risk from market-cap weighting (momentum produces larger weights in recent outperformers). They do not offer downside protection. And for some markets (emerging markets, small-cap) the index is less liquid and tracking costs are higher. These limitations are worth understanding but do not change the evidence-based conclusion that for most investors, broad market index funds outperform actively managed alternatives after fees over long time horizons.
Frequently Asked Questions
Are index funds and ETFs the same thing?
No. An index fund is a type of investment strategy (passive, tracks an index). An ETF is a legal structure (traded on exchange). Many ETFs are index funds, but not all: there are actively managed ETFs and factor ETFs that are not passive index trackers. Similarly, many index funds are structured as traditional open-end mutual funds, not ETFs. The S&P 500 is tracked by both the Vanguard S&P 500 ETF (VOO, an ETF structure) and the Vanguard 500 Index Fund Admiral Shares (VFIAX, a mutual fund structure). Both track the same index at essentially the same cost; the structure affects how you buy them and when they price.
Can I invest in an index fund in my 401(k)?
Yes, most 401(k) plans offer at least one index fund option, and many plans have expanded their index offerings. The largest 401(k) plans typically offer S&P 500 index funds, total bond market index funds, and international index funds at institutional expense ratios (0.01% to 0.05%). Smaller plans may offer only one or two index funds alongside a larger menu of actively managed funds. Check your plan's fund lineup and expense ratios; if the only index fund charges 0.5% or more, it is worth requesting that the plan administrator add a lower-cost option.
Does index fund investing mean I will never beat the market?
By definition, a market-cap-weighted total market index fund returns the market return minus its cost (currently 0.03% to 0.05% per year). This is better than most investors get: after accounting for the costs and performance shortfall of actively managed funds, the average actively-managed-fund investor earns less than the market return. Beating the market consistently requires a genuine information or execution edge that the evidence suggests most individual and institutional investors do not have. Accepting the market return at minimal cost is often the highest reliably achievable return, not a concession.