Sex and familial sinistrality differences in cognitive abilities.

Cognitive functioning was assessed in 69 left-handed males and females with a positive family history of left-handedness and in 77 left-handed and 55 right-handed males and females without familial left-handedness. Compared to females, males performed better on numerical reasoning and on two visuospatial tasks involving spatial manipulations (Figure Rotation and Surface Development). Within the group of left-handers, the multivariate effect for Familial Sinistrality was significant. Left-handers with familial left-handedness exhibited better scores on numerical reasoning, on verbal reasoning, and on two visuospatial tasks involving visual closure (Hidden Figures, Picture Completion) than did left-handers without left-handed relatives. The nonfamilial left-handers also exhibited lower scores on both inductive reasoning tasks when they were compared to their right-handed counterparts. The outcome runs contrary to the prevalent conclusion that left-handers with left-handed relatives are more likely to exhibit lower performances on visuospatial tasks than left-handers without such relatives.