We didn't need for scientists to tell us that a lot of us suffer from illusions when it comes to our money, but it turns out there's data to support it—and the problem gets worse when we travel overseas.

A recent study published in the Journal of Retailing—titled "Europoly Money: How Do Tourists Convert Foreign Currencies to Make Spending Decisions?"—showed that we do plenty of fuzzy math when we figure out exchange rates abroad. One culprit for the so-called "money illusion:" we tend to round down when doing calculations in our head. When in India, for instance, where $1 equals 54 rupees these days, a tourist will typically just use 50 for easier math, and as a result estimate that items will cost less than they really do. Meanwhile, blithely thinking that one euro is about the same as a dollar—they're always kinda close, right?—can make what seems to be a $400 bill top $500.

One way to offset that effect: travel to countries where a different phenomenonactually helps you spend less. In Japan, 3600 yen may seem like a lot for a tea set at first glance, even though a simple calculation shows that it's just $40. But according to the study, that first glance still reins you in.

Of course, an idiot-proof method is not to use your head, in one sense: do the calculations on your phone's calculator, or use a money-exchanging app. What's your favorite tip or app for doing exchanges quickly?