Establishing the Long-Run Record
The case for holding equities rests on a long history of outperformance over safer assets. Roger Ibbotson and Rex Sinquefield's 1976 study, "Stocks, Bonds, Bills and Inflation," established the definitive long-run record for US asset class returns. They documented that from 1926 onwards, US equities returned approximately 10% per year in nominal terms, compared to around 5% for long-term government bonds and 3% for Treasury bills. The gap — roughly 5% per year over bonds — is what finance calls the equity risk premium.
That 5% compounded over decades produces a dramatic difference in accumulated wealth. A £10,000 investment growing at 5% for 40 years becomes approximately £70,400. At 10%, it becomes approximately £452,600. The equity premium is not a small refinement — it is the dominant factor in long-run wealth accumulation.
Global Evidence
Ibbotson and Sinquefield's work focused on the US, which was the world's most successful economy over the 20th century. A potential objection: perhaps the US is an outlier. Elroy Dimson, Paul Marsh, and Mike Staunton addressed this in their landmark 2002 book, "Triumph of the Optimists," which extended the analysis to 16 countries over the full 20th century.
Their finding was robust: equities outperformed bonds in every developed country studied, with a global equity premium of roughly 5% per year over bonds on average. Even countries that suffered major disruptions — Germany through two world wars, Japan through postwar reconstruction — eventually delivered positive equity premiums over the very long run. The evidence that equities outperform over long horizons is not a US-specific result.
What the Premium Compensates
The equity premium is not free money. Equities carry substantially higher volatility than bonds — annual standard deviations of 15–20% are typical, versus 5–8% for government bonds. Equities also carry the possibility of catastrophic and permanent loss: a company can go bankrupt, an index can fall 50% and take decades to recover. The premium compensates investors for accepting this uncertainty.
The Japanese stock market provides a salutary caution: the Nikkei 225 peaked at nearly 39,000 in December 1989 and did not surpass that level for over 35 years. An investor who retired at the peak of the Japanese bubble and held only domestic equities experienced no premium at all over their retirement period. Diversification across countries is one response to this concentration risk.
The Forward-Looking Debate
Whether the historical equity premium is a reliable guide to future returns is genuinely contested. Some argue that current valuations — elevated price-earnings ratios in many markets — imply lower future returns. Others argue that the premium will persist because it compensates for irreducible risk. Most institutional investors plan using forward equity premiums of 3–5% over bonds, modestly lower than the historical average, to reflect current valuation levels.