This study compares the effects on Mental Rotation Test (MRT) performance of instructions that stress the importance of (a) personal effort, and (b) genetically driven ability. A total of 120 high-school students were assigned to three groups, and administered two sub-tests of the MRT. Between the first and second sub-tests, the groups received one of the following instructions: effort ("anyone can succeed in this task by putting in effort"), ability ("performance on the test depends on genetic determinants"), and neutral for control ("this is an important test used in many countries"). We predicted that effort but not ability instructions would affect performance. Results confirmed the prediction and showed that, after controlling for baseline performance, the effort group outperformed the ability and control groups, which did not differ from each other. Discussion focuses on the mechanisms implied in believing ability to be genetically determined or experientially driven. (Contains 1 table and 2 figures.)