My lunch bill got stranger by the day. Every year, I come to the same Malaysian paradise. Time magazine once rated it one of the 10 most beautiful islands in the world. In 2015, it was better than ever because it just kept getting cheaper. The fresh curried fish, pizza, fruit smoothie and coconut that I had for lunch cost just $7.

That’s when I pulled out my laptop. I checked the U.S. dollar against global currencies. The greenback was a rocket beside a bunch of balloons. Last year, it gained more than 21 percent over Malaysia’s ringgit. Over the past 36 months, it gained 43 percent.

U.S. Dollar Rises Against Malaysian Ringgit 2013-2016

U.S. Dollar Rises Against Malaysian Ringgit
Source: OANDA Historical exchange rates

I pushed my coconut aside and started to write, Think Like Warren Buffett While Vacationing Abroad. For American travelers, the world was on sale. But there was a benefit for those whose currencies took a whooping. Perhaps they couldn’t enjoy a cheap trip to Disneyland. But if they bought U.S. stocks they hitched a ride on the dollar. With U.S. exposure, they fared quite well.

American investors can’t say the same. Vanguard’s S&P 500 Index Fund mostly climbed from January to July. But it fell hard in August. By year-end it had gained a paltry 1.24 percent.

Vanguard’s S&P 500 Index Fund 2015: Measured In U.S. Dollars

Vanguard’s S&P 500 Index Fund
Source: Morningstar

Wild currency swings, however, gave non-American investors a far better ride. When we invest in foreign stocks, some years we’re rewarded. Other years we aren’t. But adding foreign stocks adds diversification. It can also juice returns. Many non-American investors found this out last year.

The S&P 500 is a lot like Starbucks. You can find it almost anywhere. On foreign exchanges, it usually trades in other currencies. For example, Vanguard Canada’s S&P 500 Index (VFV) trades on Toronto’s stock exchange. It’s priced in Canadian dollars, where it proved to be hotter than a steaming Cappuccino. It gained 22.22 percent last year.

Australians also saw U.S. stocks soar. Vanguard U.S. Total Market Shares Index (VTS) trades in Aussie dollars on the Australian exchange. By November 30, 2015 it had gained 20.71 percent (as I write, Vanguard Australia hasn’t updated year-end results).

The U.S. dollar also beat the euro and the British pound. That means Vanguard’s S&P 500 UCITS ETF (VUSA) gained slightly more than 10 percent in euros. Priced in UK pounds, it gained 3.9 percent.

Global stocks also disappointed Americans in 2015. Measured in U.S. dollars, Vanguard’s Total World Stock Market Index ETF dropped 1.88 percent. The rising U.S. dollar had created an upstream swim for Americans, while investors around the globe mounted a small motorboat.

British investors in Vanguard FTSE All World UCITS ETF (VWRL) gained 4.28 percent. Canadian investors in Vanguard’s FTSE Global All Cap ex Canada ETF (VXC) gained 17.04 percent. Australians in Vanguard’s All World ex U.S. Shares Index ETF (VEU) – which doesn’t include U.S. stocks– gained 10.42 percent by November 30, 2015.

Americans with diversified portfolios may have performed poorly in 2015. International investors with equal diversification did much better. But such roles will change. Currencies are like seasons. There will always be a summer. There will always be a fall. But unlike seasons, we can’t predict currencies.

For this reason, many investors believe that investing in foreign markets equates to certain risks. This may be true for a short-term trader. But it’s different for a long-term investor. Global portfolios increase diversification. Long term, this reduces volatility. If the portfolio is rebalanced once a year, it might even boost returns.

You’ll never know which countries’ stocks or currencies are going to rise or fall. It’s best not to guess.

That doesn’t mean you shouldn’t look at global currencies. I do it all the time­–but not to invest. I scope out bargain countries for my next vacation. It really is better to think like Warren Buffett when vacationing abroad.

2015 U.S. Stock Market Fund Returns Measured In Different Currencies

Index Currency Exchange ETF 2015 Return
S&P 500 USD New York Vanguard S&P 500 ETF (VOO) +1.35%
S&P 500 British pounds London Vanguard S&P 500 UCITS ETF (VUSA) +3.9%
S&P 500 Canadian dollars Toronto TSX Vanguard Canada’s S&P 500 Index (VFV) +20.22%
Total U.S. Market Australian dollars Australian ASX Vanguard U.S. Total Market Shares Index (VTS) +20.72%*

2015 International/Global Stock Market Fund Returns Measured In Different Currencies

Index Holdings Currency Exchange ETF 2015 Return
Global Stock Market Index USD New York Vanguard Total World Stock Market Index ETF (VT) -1.88%
Global Stock Index British pounds London Vanguard FTSE All World UCITS ETF (VWRL) +4.28%
Global Stocks (excluding Canada) Canadian dollars Toronto TSX FTSE Global All Cap ex Canada ETF (VXC) +17.04%
Global Stock Market Index (not including the USA) Australian dollars Australian ASX Vanguard All World ex U.S. Shares Index ETF (VEU) +10.42%*

2015 U.S. Dollar Gain vs. Canadian Dollar

2015 U.S. Dollar Gain vs. Canadian Dollar

Source: OANDA Historical exchange rates

2015 U.S. Dollar Gain vs. Australian Dollar

2015 U.S. Dollar Gain vs. Australian Dollar

Source: OANDA Historical exchange rates

2015 U.S. Dollar Gain vs. Euro

2015 U.S. Dollar Gain vs. Euro

Source: OANDA Historical exchange rates

2015 U.S. Dollar Gain vs. UK Pound

2015 U.S. Dollar Gain vs. UK Pound

Source: OANDA Historical exchange rates

Andrew Hallam is a Digital Nomad. He’s the author of the bestseller, Millionaire Teacher and The Global Expatriate's Guide to Investing: From Millionaire Teacher to Millionaire Expat.