If you are tired of waiting for the international stocks in your portfolio to pull their weight, your frustration is more than justified. Compared to U.S. stocks, international equities have never lagged this much, this consistently, for this long.

An investor who put money into the MSCI All-Country World ex-US index 15 years ago has gained roughly 60% total, or 3.1% per year. Emerging market equities have performed especially poorly in that time: +33% total (1.9% per year). The S&P 500, by comparison, is up more than 360% over the same period (10.7% per year). That’s six times more return for U.S. stocks than International stocks since September 2008.

There are several reasons for this. The first is currency fluctuation. The U.S. dollar has strengthened significantly (more than 30%) in the last 15 years. For the same reason a strong dollar “buys more” when Americans travel abroad, corporate profits paid in foreign currencies are “worth less” when converted back into dollar-denominated investment accounts.

Monetary policy has also affected relative returns. The U.S. Federal Reserve was among the first central banks to cut interest rates to zero during the Great Recession. Others, including the European Central Bank, were far behind the curve or far less aggressive in implementing stimulus. This laid the foundation for a slower economic recovery and less favorable equity conditions abroad.

The composition of the benchmarks are also vastly different. In terms of market capitalization, 28% of the S&P 500 is technology companies. By comparison, less than 12% of the MSCI All-Country World ex-U.S. index is allocated to technology.

Vanguard founder John Bogle saw little value in owning international stocks. He felt whatever benefits might be gained from geographic diversification were offset (negatively) by the risks associated with foreign currencies, less transparent governments, and less stable economies.

It’s certainly true that most of the largest American companies carry indirect international exposure by way of selling products and services all over the world, even if they are based in the U.S. and have stocks traded on a U.S. exchange.

We still recommend clients allocate around 20% of their equity dollars to international strategies. If that sounds like a lot, it’s less than half the international exposure you would have if your stock portfolio mirrored the market cap of the global economy (60% U.S., 40% international).

It’s impossible to know whether U.S. equities will continue to outperform their international counterparts over the next 3-5 years, though it’s fair to expect a smaller degree of outperformance if the current trend persists.

Valuation is probably the most obvious argument in favor of maintaining or increasing International exposure. The average stock in the MSCI All-Country World ex-U.S. index trades at a trailing price-to-earnings (P/E) ratio of 13.5. That’s a steep discount to the 20.2 P/E you get with the S&P 500. More reasonable valuations and less tech exposure make International benchmarks more value-oriented. But if you want international exposure without sacrificing growth, there’s no shortage of active managers who offer such strategies.

Looking forward, we remain neutral on international equities, which are on pace to underperform U.S. stocks (again) for the 12th time in 16 calendar years. There’s no denying that international stocks are still lagging after all these years.

The opinions voiced in this material are for general information only and are not intended to provide specific advice or recommendations for any individual. All performance referenced is historical and is no guarantee of future results. All indices are unmanaged and may not be invested into directly.