Even though marks in different subjects are substantially correlated, the corresponding self-concepts often display a very weak association. The ''Internal-External Frame of Reference Model'' (I/E-model; Marsh, H. W. (1986). Verbal and math self-concepts: an internal/external frame of reference model. "American Educational Research Journal, 23," 129-149) explains this finding: social comparisons (resulting in positive correlations of the self-concepts) and dimensional comparisons (resulting in negative correlations of the self-concepts) are confounded. A rarely tested hypothesis derived from the I/E-model is that the dimensional comparison process only affects the self-concepts, if the students achieve differently in the corresponding subjects. In a sample of N=1508 students (grades 7 and 8), low correlations of the self-concepts for four subjects could be observed only in students displaying different school-marks in the corresponding subjects. In students who have the same marks in different subjects, the self-concepts showed a substantial positive correlation.