Index investing means buying funds that simply replicate the composition of a stock market index — the S&P 500, the FTSE All-World, or any other benchmark — rather than trying to pick winning stocks or time the market. It sounds unremarkable. In practice, it has outperformed the vast majority of professional fund managers over most long-term periods.
The case for index investing rests on the efficient market hypothesis: the idea that stock prices already reflect all publicly available information. If this is true — or even approximately true — it is very hard for active managers to consistently find mispriced stocks. Any advantage they find is quickly competed away as others spot the same opportunity.
The evidence largely supports this view. Study after study — and the annual S&P SPIVA reports are the most comprehensive — shows that the majority of actively managed equity funds underperform their benchmark index over 10, 15, and 20-year periods. The proportion of underperformers grows the longer the period examined.
Why does this happen? It is partly about costs. Active funds charge 0.5-1.5% per year. An index fund charges 0.05-0.10%. Over 20 years, a 1% annual cost difference compounds to a huge performance gap — roughly 20% of total returns. Active managers must not just pick better stocks than the market; they must pick enough better stocks to overcome this cost disadvantage every year.
It is also about skill distribution. The stock market is increasingly dominated by professional managers. When professionals are competing against other professionals, average active performance converges toward the index return — minus costs.
Index investing does not promise high returns. It promises you will not underperform the market by more than the (very low) fund costs. For long-term investors, this is a compelling promise. Warren Buffett has repeatedly recommended index funds for investors who do not have the time, skill, or inclination to analyse individual companies.