This paper investigates the extent to which test performance is affected by shocks to non cognitive skills. In a field experiment, students took a mathematics test. Students were clustered into several student groups that were randomly assigned to treatment and control group. The treatment consisted of positive affirmation before students began taking the test. This affirmation significantly raised students' test scores. In particular, students with low maths grades and with self-assessed difficulties in maths achieved higher test scores. Results suggest that teachers may raise their students' performance by interventions to their non-cognitive skills.*