Why International Diversification Delivers Less Than It Used To
A generation ago, adding foreign stocks to a U.S. portfolio meaningfully cut risk, because national markets moved somewhat independently of one another. Capital, information, and multinational companies now cross borders so freely that markets have become far more correlated, and that shift changes what an investor can honestly expect diversification to deliver.
What globalization actually changed
Three forces drove the integration of the world's stock markets over the past half century. Capital controls that once restricted how freely money could cross borders were dismantled across most developed and many emerging economies from the 1970s through the 1990s, allowing institutional and retail capital to flow toward whichever market offered the best relative opportunity. Multinational corporations grew to earn a substantial share of revenue outside their home country, so a company legally headquartered and listed in the United States might derive a third or more of its profit from Asian and European operations, meaning its stock price already reflects global, not purely domestic, economic conditions. And instant, globally shared information, news, earnings data, and macroeconomic releases arriving simultaneously everywhere, erased much of the lag that once let one country's market react to news before another's.
The combined result shows up in a single statistic that matters more to a diversified investor than almost any other: correlation, a measure between negative one and positive one of how closely two assets' returns move together. In the 1970s, researchers estimating the correlation between U.S. and major European equity market returns typically found figures in the range of 0.4 to 0.5, meaningfully less than perfectly aligned. By the 2010s and 2020s, that same correlation frequently measured 0.8 or higher during ordinary periods and climbed toward 0.9 during global stress events such as 2008 and 2020, when investors around the world sold risk assets simultaneously regardless of a specific company's or country's underlying fundamentals.
Globalization also changed how foreign companies actually reach individual investors. Before the mid-twentieth century, buying a foreign stock typically meant opening an account with a broker in that country, dealing in an unfamiliar currency, and navigating local settlement rules. The modern solution is the American Depositary Receipt (ADR), a certificate issued by a U.S. bank representing a specific number of shares of a foreign company held in custody overseas, traded on U.S. exchanges in dollars during U.S. market hours exactly like a domestic stock. Thousands of foreign companies now trade this way, and dozens of the largest multinational companies maintain simultaneous listings on two or more exchanges around the world, so the same underlying business can be bought in London, Hong Kong, and New York, with arbitrage keeping the dollar-equivalent prices closely aligned across venues. This infrastructure, more than any single policy change, is what made global diversification operationally easy for an ordinary investor, even as the diversification benefit itself has shrunk on a correlation basis.
The math: correlation, diversification, and currency
Worked example 1: how rising correlation shrinks the diversification benefit. The variance of a two-asset portfolio is portfolio variance = w1^2 x variance1 + w2^2 x variance2 + 2 x w1 x w2 x correlation x volatility1 x volatility2, where w1 and w2 are the portfolio weights. Assume a U.S. stock index with 15 percent annual volatility and a foreign stock index with 18 percent annual volatility, held in equal 50/50 weights.
At a 1970s-style correlation of 0.4: portfolio variance = 0.25 x 0.0225 + 0.25 x 0.0324 + 2 x 0.5 x 0.5 x 0.4 x 0.15 x 0.18 = 0.005625 + 0.0081 + 0.0054 = 0.019125. Taking the square root gives a portfolio volatility of about 13.8 percent, notably lower than either the 15 percent or 18 percent volatility of the individual markets alone, and well below the simple weighted average volatility of 16.5 percent. That gap between 13.8 percent and 16.5 percent is the diversification benefit, and it is substantial.
At a modern correlation of 0.9: portfolio variance = 0.005625 + 0.0081 + 2 x 0.5 x 0.5 x 0.9 x 0.15 x 0.18 = 0.005625 + 0.0081 + 0.01215 = 0.025875. The square root gives a portfolio volatility of about 16.1 percent, barely below the weighted average of 16.5 percent, and actually higher than the 15 percent volatility of the U.S. market held alone. In other words, at today's typical correlation levels, blending in the more volatile foreign market provides almost no risk reduction and can even raise portfolio volatility above what a purely domestic holding would have delivered. The mathematics of diversification did not change; the input that changed is correlation itself.
Worked example 2: currency risk in an unhedged international holding. Suppose a Japanese company's stock trades at 5,000 yen per share, and the yen-to-dollar exchange rate is 150 yen per dollar. An American holding this stock through an unhedged fund effectively owns a claim worth 5,000 / 150 = $33.33 per share. If the stock price in yen is completely unchanged a year later but the yen strengthens to 130 per dollar, the same 5,000 yen is now worth 5,000 / 130 = $38.46, a currency-driven gain of ($38.46 - $33.33) / $33.33 = 15.4 percent, entirely independent of how the underlying business performed. Currency movements of this size are common over a single year between major currency pairs, which means an unhedged international holding carries a return driver layered on top of, and sometimes larger than, the stock market return itself.
What the evidence shows
The empirical finance literature on international correlation is remarkably consistent on the direction of the trend, even if exact correlation figures vary by study period and methodology: cross-country equity correlations have risen substantially since the 1970s, with the sharpest increases occurring during periods of financial crisis, when a phenomenon researchers call correlation breakdown or contagion causes markets worldwide to sell off together regardless of local fundamentals, precisely when diversification would be most valuable if it still worked as it once did. Studies of the 2008 financial crisis and the 2020 pandemic selloff both found correlations among major developed markets pushing toward 0.9 or higher during the acute selling phase, higher than their already elevated longer-run averages.
At the same time, the evidence does not support abandoning international diversification altogether. Emerging markets, and to a lesser degree small developed markets with less multinational corporate overlap with the United States, have continued to show somewhat lower correlation to U.S. large-cap stocks than developed markets show to each other, and multi-decade return data shows meaningful periods, including much of the 2000s, when non-U.S. markets outperformed U.S. markets by a wide margin. A portfolio concentrated entirely in one country also concentrates exposure to that country's specific regulatory, currency, and valuation cycle, a risk that shows up clearly in the historical record of countries whose markets stagnated or declined for a decade or more even while global equities as a whole advanced.
Building a global portfolio today
Given a smaller but still real diversification benefit, most evidence-based portfolio construction today treats international allocation as a moderate, not dominant, position, commonly in the range of 20 to 40 percent of total equity exposure, rather than the 50/50 split that pure market-capitalization weighting of the global economy would suggest and rather than the near-zero international exposure many U.S. investors held historically due to home bias, the well documented tendency to overweight one's own country regardless of the diversification math. Broad, low-cost international index funds are the most efficient way to implement this, since they eliminate the cost and effort of researching individual foreign companies while still capturing the currency and economic-cycle diversification that remains available even at 0.8 to 0.9 correlation.
The currency question deserves a deliberate decision rather than a default. Currency-hedged international funds strip out the exchange-rate swings shown in the worked example above, leaving pure equity-market exposure, while unhedged funds add currency as an additional, somewhat independent source of return and risk. Over long horizons, currency effects tend to average out closer to zero, since exchange rates do not have a structural long-run drift the way equity markets do, which is why many long-term investors choose to leave international holdings unhedged and accept the added short-term volatility in exchange for simplicity and modestly lower fund expenses.
It is also worth distinguishing where in a portfolio international exposure does its most useful work. For equities, the correlation math above shows the benefit has narrowed considerably. For bonds and cash-like instruments, however, foreign interest rate cycles often remain considerably less synchronized with the United States than foreign equity markets are with U.S. equity markets, since monetary policy still responds primarily to domestic inflation and employment conditions in each country. An investor looking for the diversification international assets used to provide in equities may find more of that benefit today in a modest allocation to foreign or global bonds, where correlation to U.S. fixed income has risen far less dramatically than the equity correlation trend described above.
Actionable breakdown
- Hold meaningful international exposure, but expect a modest benefit.
- Target roughly 20 to 40 percent of equities in non-U.S. markets.
- Use broad low-cost index funds rather than individual foreign stocks.
- Include some emerging market exposure for lower correlation.
- Decide deliberately on currency hedging rather than by default.
- Do not expect international stocks to hedge a U.S. crisis.
Common pitfalls
The most common pitfall is holding no international exposure at all because domestic markets have outperformed in recent memory, a pattern that has reversed for extended multi-decade stretches historically and concentrates risk in a single country's valuation and policy cycle. A second pitfall is expecting international stocks to reliably cushion a U.S. downturn, when the evidence shows correlations tend to rise precisely during global crises, the moments diversification is needed most. A third is ignoring currency exposure when comparing historical fund returns, since an unhedged fund's past performance already embeds currency swings that may not repeat in the same direction going forward. A fourth is overpaying for actively managed international funds when broad index alternatives capture most of the available diversification benefit at a fraction of the cost. A fifth, easy to miss pitfall is treating an ADR-based fund and a currency-hedged fund as interchangeable; an ADR simply changes where and how a foreign stock trades, it does not remove currency exposure, while a hedged fund specifically strips that exposure out through offsetting currency contracts, at a small additional cost.
None of this argues for timing international exposure, buying more when foreign markets have recently outperformed and less when they have lagged. The correlation and currency dynamics described here are structural features of a globalized economy, not signals about which region will do better next year, and there is no reliable evidence that investors can successfully time the rotation between domestic and international leadership. The more defensible approach is to set a target international weight consistent with your own risk tolerance and time horizon, and hold it through both the periods when it helps and the periods when it does not.
The bottom line
Global markets are more connected than they have ever been, so international diversification still helps meaningfully at the margins and against single-country risk, but it should no longer be relied on as a major buffer during the kind of global downturn where an investor might want it most.
All articles · International investing · Asset allocation · U.S. markets