Shifting the image of male superiority in spatial ability: An examination of sex differences in mental rotation

Spatial ability is a crucial mental capacity that enables individuals to react to and understand objects in physical space. Among polygynous mammals, including humans, males consistently are found to have better spatial ability than females. Numerous theories have proposed the evolutionary pressures that may account for this sex difference. In humans, the largest male advantage is associated with tests of mental rotation, in which subjects imagine the appearance of small objects from different perspectives. Two experiments were undertaken to investigate the source of the sex difference in mental rotation test performance. These experiments tested whether male testosterone and sex, respectively, were related to rotation or non-rotation aspects of performance on the Shepard and Metzler computerized mental rotation test. In the second experiment, we related rotation and non-rotation components of performance on the Shepard and Metzler test (represented by the slope and intercept of the error rate and response time rotation functions) to the same subjects' scores on the Vandenberg and Kuse mental rotation test. We found that while high testosterone and male sex predicted low error rates, these variables were not related to mental rotation processes per se on either test. Both sex and testosterone level were instead related to non-rotation processes that contributed to performance---visually encoding or making decisions about stimuli. These results specify how the large sex difference in test scores is amenable to evolutionary explication. It is hypothesized that the sex difference in mental rotation tests may be related to a female adaptation for more cautious decisions, particularly in the spatial domain. Taken together, these findings suggest that the sexes are similar in the ability to mentally rotate, and thus, the male advantage in spatial ability is substantially smaller than was previously thought.