People perceived as the most likely to succeed might also be the most likely to crumble under pressure.

A new study finds that individuals with high working-memory capacity, which normally allows them to excel, crack under pressure and do worse on simple exams than when allowed to work with no constraints. Those with less capacity score low, too, but they tend not to be affected by pressure.

"The pressure causes verbal worries, like 'Oh no, I can't screw up,'" said Sian Beilock, assistant professor of psychology at Miami University of Ohio. "These thoughts reside in the working memory." And that takes up space that would otherwise be pondering the task at hand.

"When they begin to worry, then they're in trouble," Beilock told LiveScience. "People with lower working-memory capacities are not using that capacity to begin with, so they're not affected by pressure."

The findings are detailed this week's issue of Psychological Science.

Working memory, also known as short-term memory, holds information that is relevant to performance and ensures task focus. It's what allows us to remember and retrieve information from an early step of a long task, such as long-division math.

"In these math problems students have to perform subtraction and division, and if you're trying to hold information in your memory and you start worrying about performance, then you can't use your entire mental capacity to do the math," Beilock explained.

The study analyzed 93 undergraduate students from Michigan State University to determine their working-memory capacities. The students were divided into two groups, a high working-memory group (HWM) and a low working-memory group (LWM). Each person was given a 24-problem math test in a low-pressure environment. The HWM group did substantially better.

Then the two groups were given the same test, but were told that they were part of a "team effort" and an improved score would earn the team a cash reward. They were also told their performance was being evaluated by math professors.

Under this higher, real world pressure situation, the HWM group's score dropped to that of the LWM group, which was not affected by the increased pressure.

Since working memory is known to predict many higher-level brain functions, the research calls into question the ability of high-pressure tests such as the SAT, GRE, LSAT, and MCAT to accurately gauge who will succeed in future academic endeavors.