Academic self-concept and achievement are positively related among students from the same school and country. Yet negative associations between these variables may be found at the level of schools and countries. In the present article, we propose how this apparent paradox can be explained in terms of reference group effects, in which high standards, norms, or benchmarks act to decrease academic self-concept, whereas low ones have the contrary effect. Multilevel regression analyses of the PISA 2006 data consisting of 353,403 students, 13,886 schools from 53 countries revealed an interesting pattern of relationships. Within schools, students’ science achievement and science self-concept were positively related although the size of this relationship varied considerably between countries, whereas between schools and between countries, this association was negative. Consistent with our hypothesis, the size of the between-school relationship was larger in countries with a higher percentage of selective schools. At the country level, the negative relationship between country mean achievement and self-concept was explained by country differences in educational benchmarks, standards, and norms next to country differences in response styles. In this article, we also discuss the implications for the validity of cross-cultural comparisons of self-concept.