Dimensional comparisons between math and verbal achievements affect students' academic self-concepts by increasing (decreasing) students' self-concept in the subject with their intraindividual higher (lower) achievement. Dimensional comparison theory assumes that dimensional comparisons are also triggered by external influences. The present research aimed to test this assumption by examining whether a minimal intervention, asking students to think more or less intensively about their subject-specific abilities, would affect the strength of dimensional comparison effects. In two experimental studies, N = 415 (Study 1) and N = 204 (Study 2) students from Germany rated their self-concepts in the subjects math and German after they had written down three thoughts about their math abilities and one or five thoughts about their German abilities (Study 1) or after they had written down three thoughts about their German abilities and one or five thoughts about their math abilities (Study 2). As expected, multiple group comparisons showed in particular a stronger negative dimensional comparison effect of students' German (math) achievement on their math (German) self-concept after students had written down five thoughts instead of one thought about their German (math) abilities. This finding suggests that, as predicted in dimensional comparison theory, external influences can trigger dimensional comparisons.