Some predictions on handedness and spatial task performance were tested in the present study. Recent theorizing on this posit the role of genetic factors and also the intellectual disadvantages of dextrality. Right-handed and left-handed males with or without familial sinistrality (FS+ / - ) were assessed on a spatial task. The main effects of handedness and familial sinistrality were found to be significant. FS+ right handers and FS - left handers outperformed FS - right handers and FS+ left handers. Results are discussed in terms of the right shift model of Annett and also that of Geschwind and Galaburda.