Right and left hand skill in left-handers: distribution, learning, and relation to nonverbal intelligence

The relation of hand skill to nonverbal intelligence (spatial reasoning) tested by Cattle's Culture Fair Intelligence Test was studied in left-handed subjects. Hand skill was assessed by the peg moving test, which revealed that some of the left-handers (Geschwind scores and self-reports) were ambidexters. The distribution for right hand skill fitted to Gaussian data better than that for left hand skill. The R-L difference between the hands in peg moving times could also be described by normal distributions in the total sample, ambidexters, and left-handers. The right and left hands of ambidexters were as fast as the left-handers. In left-handers, the right hand was significantly slower than the left hand. In males, right-hand skill fitted to Gaussian data better than left-hand skill. In males and females, right and left-hand skill fitted to Gaussian data. Left-hand skill fitted to Gaussian data better in females than males. The difference between right- and left-hand peg-moving times was found to be larger in females than males. There was no significant difference between the right- and left-hand peg moving times of the male and female left-handers. The mean peg moving times for the right and left hands linearly decreased at each successive trial (N = 10). This motor learning was found to be better in left-handers with high IQs than those with low IQs. The right hand of left-handers with high IQs was faster than that with low IQs. The motor learning of the left hand was not different in left-handers with high and low IQs. The regression line for ambidexters exhibited less learning than that for left-handers. It was suggested that right hand skill is associated with nonverbal intelligence in left-handers, indicating the importance of left hemispheric processing in this kind of cognitive performance.