Tunisia, Argentina, Brazil and Thailand are home to some of the world’s most math-phobic 15-year-olds.

Levels of “mathematics anxiety” in those countries were the highest among the nations tracked by the OECD as part of the Program for International Student Assessment (PISA) tests that the organization coordinates.

The chart below shows Tunisian teenagers are most anxious about math among those tested, and the Netherlands as the least, with the Czechs and Slovaks closest to the OECD average.

The OECD concocted this index based on responses to a series of questions students around the world were asked as part of the PISA exams. The questions were intended to gauge the level of stress students felt about math. Here’s an example.

Why bother measuring math anxiety? Because it tends to be associated with poor performance in mathematics. Some believe this is because the mind is so occupied with worrying about math that it has less bandwidth to actually work on the math problems at hand. The result? Students choke.

It’s worth noting that while high levels of math anxiety are associated with low performance, the converse isn’t true. In other words, low levels of anxiety are not linked with high performance in mathematics. In fact, the students from top-performing countries in mathematics—such as China, Singapore and Korea—are slightly more anxious about math than the average OECD student.

The key to math success seems to be combining a manageable amount of worry with a tendency toward perseverance and a belief in hard work. The PISA exam also presented a series of questions to students that attempted to gauge their level of perseverance. The OECD laid out their findings on those topics here.

Students who reported that they continue to work on tasks until everything is perfect, remain interested in the tasks they start, do not give up easily when confronted with a problem, and, when confronted with a problem, do more than is expected of them, have higher scores in mathematics than students who reported lower levels of perseverance.