Summary. Relationships between personality and mechanical arithmetic test measures in 118 eleven to twelve‐year‐old normal school pupils were studied. Speed was found to be related to attitudes, mainly Jesness Inventory subscores, and accuracy to cognitive styles. Specifically, errors involving carelessness, including those due to ‘set’, were related to Impulsivity on Kagan's test. Then CHE boys were found to differ from the normal pupils in terms of Jesness scores and Impulsivity. They attempted fewer arithmetic items and, on a more difficult test, were markedly less accurate than normal pupils. They made proportionately more ‘set‐errors’.Lower arithmetic attainment scores of CHE boys are not wholly attributable to lack of knowledge but are related partly to attitude and partly to Impulsivity.