The paradoxical findings about students' mathematics self-concept and academic achievement shown in international and comparative studies prompt this exploration of the function and development of mathematics self-concept. That is, when examining data within individual countries, a positive relationship exists between students' self-concept and achievement in mathematics while a negative relationship emerges in cross-country comparisons. This challenges the popular and commonly held assumption among North American teachers who generally believe that self-concept predicts student achievement and thus, the improvement of students' self-concept in mathematics leads to higher mathematics achievement. Using comparative studies of Chinese and US student mathematics learning, this study further analyses the inadequacy of existing theories and then seeks to explain the relationship between self-concept and achievement in mathematics using an alternative interpretation.