Relation of spatial reasoning ability to hand performance in male and female left-handers to familial sinistrality and writing hand.

The relation of mental ability for spatial reasoning to hand performance was studied in male and female left-handers considering familial sinistrality and writing hand. Hand performance was assessed by a dot-filling test; hand preference was assessed by the Edinburgh Handedness Inventory (Geschwind scores). Nonverbal intelligence (spatial reasoning) was measured by the Cattell's Culture Fair Intelligence Test. The relationship between IQ and hand performance was found to be more complicated than expected. This was associated with sex, familial sinistrality, and writing hand, which created different patterns in interactions between motor and cognitive systems. It was concluded that the brain benefits from different strategies by using both hemispheres in a competitive and complementary manner where necessary to achieve a high visual-spatial performance depending upon genetic preprograms.