Males consistently outperform females on mental rotation tasks, such as the Vandenberg and Kuse (1978) Perceptual and Motor Skills, 47(2), 599–604, mental rotation test (MRT; e.g. Voyer et al. 1995) in Psychological Bulletin, 117, 250–265. The present study investigates whether these sex differences in MRT scores can be explained in part by early spatial activity experience, particularly those spatial activities that have been sex-typed as masculine/male-oriented. Utilizing an online survey, 571 ethnically diverse adult university students completed a brief demographic survey, an 81-item spatial activity survey, and the MRT. Results suggest that the significant relation between sex of the participant and MRT score is partially mediated by the number of masculine spatial activities participants had engaged in as youth. Closing the gap between males and females in spatial ability, a skill linked to science, technology, engineering, and mathematics success, may be accomplished in part by encouraging female youth to engage in more particular kinds of spatial activities.