Episode 408: Elroy Dimson – Investing & Optimism

Episode Summary

Executive Summary: A wide-ranging conversation with Elroy Dimson centered on the value of studying long-run financial history to avoid common investing biases. Dimson explained how global return data were built, why survivorship and easy-data biases distort conventional histories, and how his work reshaped views on equity premia, diversification, growth, and factor investing. The key message: invest broadly, keep costs low, and remain humble about forecasts.

Main Topics: Why financial history matters (Priority: 5/5): Dimson argues that understanding where markets came from is essential for thinking sensibly about where they may go, because future expectations are impossible to ground without historical context. Building the Global Investment Returns Yearbook and DMS data (Priority: 5/5): He describes how the data set evolved from a private millennium project into a global, century-long compilation of stock, bond, and bill returns across many countries, assembled from academic and commercial sources. Biases in historical market data (Priority: 5/5): The discussion covers survivorship bias, easy-data bias, and index construction errors that can materially overstate returns if markets, periods, or failed securities are omitted. Equity risk premium and expected returns (Priority: 5/5): Dimson explains that long-run data imply a lower equity premium than many institutions assume, and that realistic return expectations should be more modest than historical U.S.-centric narratives suggest. Diversification across countries and industries (Priority: 4/5): He emphasizes that global and industry diversification mainly reduce risk, not create return, while home-country bias persists for behavioral, practical, and geopolitical reasons. Factor effects: value, size, momentum (Priority: 4/5): The conversation reviews long-term evidence on factor premiums, noting that value and size have been weaker recently while momentum remains distinctive but costly to implement. Optimism, pessimism, and portfolio behavior (Priority: 4/5): Dimson frames investing as a long-term, modest-lifestyle choice: stay invested, avoid market timing, and prioritize education and human capital as the safest asset.

Key Arguments: Financial history is necessary because investors cannot reason well about the future without understanding the path that led to the present. Standard historical market series often overstate returns because they exclude wars, failed markets, dead firms, and other inconvenient data. The globalization of the data set showed that U.S. exceptionalism was not a universal baseline; international evidence changes expected return assumptions. Economic growth does not automatically translate into higher stock returns because expected growth is generally priced in advance and benefits many stakeholders besides shareholders. Emerging markets may grow faster economically, but stock returns can disappoint because investors often enter after optimism is already embedded in prices. International diversification is primarily a risk-reduction tool; it cannot improve returns for everyone at once, only redistribute relative outcomes. Home-country bias is partly emotional and partly practical, but overly concentrated domestic portfolios are hard to justify on diversification grounds alone. The equity risk premium is probably lower than many endowments assume, making spending policies based on 5%+ expected equity premia difficult to defend. Value, size, and momentum effects exist, but implementation costs and changing market structures can weaken or reverse their apparent attractiveness. A disciplined, low-cost, broadly diversified, long-term strategy is more defensible than trying to forecast near-term market direction.

Data Points: Episode number: 408 - Rational Reminder episode featuring Elroy Dimson. Years of podcast history: 8 years - Hosts note they had wanted to interview Dimson for a long time. Countries in early millennium project: 10 countries - Private publication reached 10 countries with 100 years of data each. Common start date for main dataset: 1900 - Dimson says the primary start date for the global series is New Year 1900. Coverage of global equity market cap in 1900: over 98% - He says their histories now cover significantly over 98% of global equity market capitalization in 1900. Majority of UK and US common stocks by value in 1900: railroads - Railroad stocks dominated market capitalization in both markets. Largest market in 1900: Britain - Dimson says Britain was the biggest market by market cap in 1900. Current share of global equity market among top 10 companies: about 25% - He notes roughly a quarter of global equity market value is represented by 10 companies. Top 10 companies by country: 9 in the United States, 1 in Taiwan - Used to illustrate current concentration and diversification challenges. Suggested long-run real bond return: about 2% - Dimson uses this as a plausible long-run real bond return assumption. Plausible equity risk premium: about 3% - He suggests 3% is a reasonable current estimate for the equity premium over safe assets. Potential real stock return implied by bond return plus premium: about 5% - He combines 2% real bonds with a 3% equity premium. Typical endowment spending assumption criticized: 5% - He says many institutions still assume spending or premium levels around 5%, which he views as high. Duration of Norway strategy council role: about a decade - Dimson chaired the Strategy Council for Norway for roughly ten years. Early year of current-century bond shock: 2022 - He cites 2022 as the year when bonds stopped behaving like the safe diversifier many had assumed.

Pivotal Quotes: "It's extremely difficult to think about the future without knowing where you've come from." — Elroy Dimson: Opening answer on why financial market history matters. "The role of international diversification is risk reduction, and that you can promise." — Elroy Dimson: Explaining why global diversification helps even though it cannot raise returns for everyone. "The very best is investing in yourself, that's getting an education." — Elroy Dimson: Final discussion on personal finance priorities and what counts as the safest asset.

Implications: Listeners should expect lower future returns than optimistic institutional models suggest, favor broad and low-cost diversification, and treat market history as a guide to humility rather than prediction. The industry should place less weight on U.S.-centric backtests and more on robust global evidence.