An index fund is a type of mutual fund or ETF designed to track a specific market index (like the S&P 500). It provides broad market exposure with minimal fees.
Vanguard 500 Index Fund (VFIAX) holds all 500 S&P 500 stocks in proportion to their market cap. If Apple is 7% of the S&P 500, Apple is ~7% of the fund.
Index funds consistently beat 80-90% of actively managed funds over 15+ years, primarily due to low fees (0.03% vs 1%+ for active funds). Warren Buffett's #1 recommendation for most investors. The evidence is overwhelming: passive investing wins over time.
The SPIVA (S&P Indices Versus Active) Year-End 2024 report provides the most comprehensive annual comparison:
| Time Period | % of Active Funds That Underperformed | Key Insight |
|---|---|---|
| 1 Year (2024) | 65% (large-cap) | Even in a strong year, most active funds lagged |
| 10 Years | ~65% | Consistent underperformance |
| 15+ Years | ~90% | Zero equity categories with majority outperformance |
Asset-weighted 10-year returns: 6.38% for passive index funds vs. 5.41% for active funds (Morningstar). That ~1% annual gap compounds dramatically — on a $100,000 investment over 20 years, it's roughly a $50,000+ difference.
| Fund | Index | Expense Ratio | Min. Investment |
|---|---|---|---|
| VFIAX (Vanguard 500) | S&P 500 | 0.04% | $3,000 |
| FXAIX (Fidelity 500) | S&P 500 | 0.015% | $0 |
| VTI (Vanguard Total Market) | CRSP US Total | 0.03% | Price of 1 share |
| VOO (Vanguard S&P 500 ETF) | S&P 500 | 0.03% | Price of 1 share |
💡 Pro Tip: Buffett's $1M Bet
In 2008, Warren Buffett bet $1 million that a simple Vanguard S&P 500 index fund would outperform a portfolio of hedge funds over 10 years. He won handily: the index fund returned 7.1% annually vs. the hedge funds' 2.2%. Buffett donated the winnings to charity. His advice for most investors: "Consistently buy an S&P 500 low-cost index fund."
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Try DCA Calculator →What's the difference between an index fund and an ETF?
Both track an index, but ETFs trade throughout the day like stocks, while index funds are priced once at market close. ETFs are often more tax-efficient and may have lower expense ratios. However, index funds allow fractional shares and automatic investing without trading fees. For most investors, the difference is minimal.
Can an index fund ever beat the index?
No — by definition, an index fund tracks its benchmark before fees, so it will always slightly underperform the index by its expense ratio. However, because most actively managed funds fail to beat their index after fees (85-95% over 15 years), index funds effectively beat the vast majority of active managers.
How much should I invest in index funds?
Most financial advisors recommend 70-100% of a long-term portfolio in index funds. Warren Buffett has repeatedly said a low-cost S&P 500 index fund is the best investment for most people. A simple three-fund portfolio (US stocks, international stocks, bonds) covers virtually all diversification needs.