A new study out of the University of Chicago tracks “math anxiety” in early elementary students. Interestingly, they found that because anxiety most notably affects working memory, it also tends to affect higher-performing students more profoundly than lower-performing students. The idea is that because so often lower-performing students have less robust working memory to begin with, and so use externalization strategies, their achievement on math tests is less affected by anxiety.
(Link to an article about the study here.)