Financial Basics · Investing Fundamentals

Index Funds

Definition

An index fund is a mutual fund or ETF that tries to match — not beat — a market index like the S&P 500 or Total US Stock Market. No active manager picks stocks; the fund simply owns everything in the index.

Why it matters in retirement

Over 15+ year periods, roughly 85–90% of actively managed US equity funds underperform their index benchmark after fees. For a retiree, this is the highest-probability investing decision you can make: stop paying for active management and buy the index.

Key Numbers — 2026

15-yr active fund underperformance
~88%
Lowest-cost index ER
0.015%
S&P 500 annual return (since 1926)
~10%
Cost of 1% fee over 30 years
~28% of wealth

Pros

  • Beats most active funds over 10+ years
  • Ultra-low fees
  • Tax-efficient
  • Transparent holdings

Cons

  • Can't outperform the index
  • Fully exposed to market downturns
  • Concentration in mega-cap tech (cap-weighted)

Common mistakes

  • Buying "enhanced" index funds with 0.50%+ fees
  • Owning overlapping funds (S&P 500 + Total Market = 80% overlap)
  • Market-timing index funds instead of holding through downturns
  • Ignoring international — US-only is a 50% concentration bet

Related

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