Psychological research typically shows large gender differences in mental rotation abilities, with men performing faster and more accurately than women. This study examines the effect of instructions on performance on the Mental Rotations Test (MRT), specifically whether or not a penalty for guessing is mentioned in the instructions. The relationship between MRT performance and participant characteristics is also examined through a self-assessment questionnaire. Results showed that omitting the description of a guessing penalty from the instructions led to an attenuated gender difference in the number of attempts made on the MRT. However, the overall MRT score remained significantly different for men and women. Further analyses revealed that the number of attempted items was significantly related to the number of items answered correctly on the MRT. Overall, the results suggest that performance factors contribute to the differential spatial performance of men and women.