In the 1830s, French historian Alexis de Tocqueville toured the United States and chronicled his observations in a book titled Democracy in America. What mainly impressed him was Americans’ focus on trade and commerce. They have a “purely practical” mindset, he wrote, and concluded that, “the position of the American is quite exceptional.” In the years since, others have picked up on this concept of “American exceptionalism.”

Despite recent political and economic crosscurrents, the gap between the U.S. economy and its peers has only widened, especially over the past dozen years. Between 1990 and 2012, according to an analysis by author Larry Swedroe, corporate earnings in the United States grew no faster than in other countries. But in the years since 2012, American companies’ profits have multiplied while—in aggregate—international companies’ earnings have stagnated. As a result, markets in the U.S. have far outpaced their international peers. This has made investing outside the U.S. feel like a losing proposition for quite some time.

On the surface, this seems easy to explain. In the U.S., entrepreneurship is fundamental to our DNA, and our regulatory regime makes it easy to start a business. Hewlett and Packard got their start in Packard’s garage. Gates and Zuckerberg founded trillion-dollar companies in their dorm rooms. Jensen Huang launched Nvidia from a booth at Denny’s.

In contrast, on the other side of the Atlantic, regulations make it harder to build a business. And thus, there aren’t any companies in Europe comparable to the “Magnificent Seven” technology firms in the U.S., and there are just a handful elsewhere in the world. In the European Union, working hours are strictly limited. In 2023, when the French government tried to raise the official retirement age from 62 to 64, more than a million people took to the streets to protest.

Through this lens, the outperformance of the U.S. economy seems to make sense. But this might also be an oversimplification. Indeed, since the beginning of this year, markets in the U.S. have begun to falter. Domestic stocks are mostly in negative territory, while stocks outside the U.S. have delivered solid positive performance. This has people taking a second look at the question of American exceptionalism. Specifically, the question investors are asking is: To what degree should a portfolio be diversified internationally?

This isn’t such an easy question. Ask the Vanguard Group to construct a portfolio, and it’ll be split roughly 60/40 between domestic and international stocks. Vanguard’s view is that there’s no reason to favor any one country or region of the world over another, so investors’ portfolios should simply reflect the relative weightings of world markets. But Vanguard’s founder, the late Jack Bogle, took an entirely different view. He didn’t hesitate to tell people that his personal portfolio was 100% domestic. Domestic stocks, he felt, were entirely sufficient.

There is, in other words, no consensus on this question. To gain clarity, though, we can consult the data.

In a 2023 paper titled “Still Not Crazy After All These Years,” hedge fund manager Cliff Asness examined the outperformance of domestic stocks, performing what’s known as attribution analysis to uncover the sources of that performance. His conclusion: The lion’s share of domestic stocks’ impressive gains over the prior 15 years isn’t attributable to earnings growth. It wasn’t, in other words, driven by the exceptionalism of American companies. Instead, those companies’ stocks had, for the most part, just become more expensive.

Even though domestic stocks have given up some of their lead this year, that valuation gap is still very significant. Using the price-to-earnings (P/E) ratio as a measure, domestic stocks today are still 40% more expensive than their peers in developed markets outside the U.S.

Boosters of a domestic-only approach are quick to reply that American stocks deserve higher valuations. Since there is no Magnificent Seven anywhere outside the U.S., they argue, these companies’ scale and growth justify domestic markets’ higher valuations. But that argument quickly falls apart. As Asness points out, domestic and international stocks traded at comparable valuations as recently as 2007. This valuation gap, in other words, is a new phenomenon.

According to the Swedroe analysis referenced above, another factor has contributed to domestic stocks’ outperformance: Between 2008 and 2024, the U.S. dollar appreciated nearly 20% against international currencies. This depressed the value of international stocks for U.S. investors, thus further boosting the relative performance of domestic stocks. There is, however, no guarantee that this trend will continue. And indeed it could reverse.

To be sure, there are unique aspects to the U.S. economy, and Tocqueville’s observations have validity. But a quantitative analysis suggests that the extreme outperformance of U.S. markets we’ve seen over the past dozen years may not continue indefinitely. Despite some erosion this year, domestic stocks still carry elevated valuations, and the U.S. dollar is still expensive. This is worth paying attention to, because ultimately valuations do matter. Investments that are expensive usually don’t offer the same prospective returns. And that’s certainly what history suggests.

While it may be hard to remember, there have been multi-year periods when international stocks have outperformed domestic markets. In fact, a chart of domestic vs. international stocks looks a little like a sine wave, with performance alternating over time. For that reason, I continue to recommend an allocation to international stocks. How much? I suggest something in the neighborhood of 20%. According to the data, that’s enough to deliver a diversification benefit, but not so much that it introduces significant currency risk.

A final note: You might notice that I haven’t mentioned the proximate cause of the valuation shifts we’ve seen this year—the new administration’s tariff policies. I’m not focusing on this specifically because I see it as just one example of how markets can shift unexpectedly. In choosing an international allocation—or any other aspect of your portfolio—I recommend taking the long view. Choose a structure that you think will make sense regardless of who is in the White House or where the economy happens to stand at any given time.​