Index Funds & Market Efficiency

A market is efficient to the extent that current prices already reflect all the publicly available information about an asset — new information gets absorbed into prices quickly as investors trade on it, rather than sitting around unexploited.

An index fund is a fund that simply buys a broad, representative slice of an entire market (or a large segment of it) — hundreds or thousands of companies at once — instead of trying to pick which individual companies will do best.

If a stock were obviously about to rise in price — a genuine, well-known bargain — investors would rush to buy it right away, which per Supply & Demand would push its price up immediately, until it was no longer such an obvious bargain. Any easy, low-risk way to beat the market gets bought away almost as soon as enough people notice it — which is exactly why finding one consistently, year after year, is so difficult even for full-time professional investors with enormous resources.

This connects directly to Risk & Return: if there were a reliable way to earn a higher return without taking on any extra risk, it wouldn't stay available for long — money would flood toward it until the opportunity was gone. So on average, most professional stock-pickers don't reliably beat a simple, low-cost index fund that just owns the whole market, after their fees are accounted for — not because they aren't skilled, but because so many skilled people are all competing for the same mispricings that few are left uncorrected for long.

Index funds also deliver Diversification automatically and cheaply: owning a slice of hundreds or thousands of companies at once means one company's bad news barely moves the total, exactly the idiosyncratic-risk-canceling effect that page describes — without having to research and buy each company individually.

Two investors each put $10,000 into the stock market for 20 years. One spends significant time and money trying to pick individual winning stocks, paying a manager 1.5% a year in fees to do so. The other simply buys a low-cost index fund charging 0.1% a year and holds it the whole time.

If both portfolios earn the same underlying 8% return before fees:
  Stock-picker (8% − 1.5% fees = 6.5% net):  $10,000 → ≈ $35,236 after 20 years
  Index fund (8% − 0.1% fees = 7.9% net):    $10,000 → ≈ $45,755 after 20 years

Even with identical underlying performance, the 1.4 percentage point fee gap alone costs the stock-picker over $10,500 here — and that's before accounting for the very real chance the stock-picker's choices underperform the market average rather than just matching it.