I just came across this quote:
If you are in a field and are being chased by a bull you will probably perform above your usual level at jumping the gate, but below your usual level at undoing the combination lock. Anxiety may enhance motor skills, but it depresses complex intellectual performances.
The authors of the book where I found the quote (Bell et al. 1983) attribute this to the Yerkes-Dodson Law. After a bit of Googling, I’m not sure that’s right, but it is a lovely quote. I came across it whilst checking through old notes on papers for something I’m writing on students’ mathematical misunderstandings. However, it is deeply relevant in the context of students’ exam anxiety.
Bell, A.W., Costello, J. and Kuckermann, D. (1983) A review of research in mathematical education. Part 1: Research on learning and teaching. NFER-Nelson.