Males have consistently been noted to perform better than females on Piagetian horizontality and verticality tasks. To examine whether females are less competent than males or whether mediating variables influence females' performance, 42 male and 42 female college students performed horizontality and verticality tasks. Subjects also rated their expectancies for both success and failure on the tasks and their perceptions about the scientific/mathematical nature of the tasks. An analysis of the results showed that females performed more poorly than male subjects on both tasks and had lower expectancies for successful task performance. Individuals with lower expectancies for success tended to have lower task scores. When expectancies about performance were partialled out, the amount of horizontality variance explained by gender was reduced by 60% and the sex difference in performance was not significant. For verticality, while the sex difference remained significant with expectancies partialled out, there was a 40% reduction in the proportion of variance explained by gender. These findings support the view that expectancies mediate performance on these tasks. The sex difference in expectancies could not be accounted for by perceptions of the tasks as scientific/mathematical. Nor were ratings of the tasks as scientific related to actual task achievement. Performance on the horizontality and verticality tasks was moderately correlated for both sexes, indicating that the two concepts do tap the same underlying Euclidean structure. (Author/BL)