A new study by L. Stankov and J. Lee found that "the best predictors of cognitive performance are a cluster of self-beliefs consisting of confidence, self-efficacy, anxiety, and self-concept."
Furthermore, self-belief (aka confidence) is a key component of Mental Toughness! Studies show that overall mental toughness can be developed.
On a practical note, teachers, tutors and parents can help children build their self-belief or confidence in math by:
having lots of problem solving practice,
learning from the best teachers or tutors,
using positive affirmations (for self) and receiving positive encouragement (from others),
learning composure or relaxation skills (especially for exams stress)