The left brain determines the degree of left-handedness.

The contribution of right- and left-hand skills to left-handedness was studied in 42 left-handed male subjects. Hand preference was assessed by the Edinburgh Handedness Inventory. Hand skill was assessed by a peg-moving task; 10 trials were given to each hand. Peg-moving times decreased linearly with each trial (visuomotor learning). Both hands exhibited equal learning capacities. The learning curves were the same for the left-hands of left-handers with and without familial sinistrality (FS). The right-hand of left-handers with FS was found to be slower than that without FS. The right- and left-hand skills and their learning curves were about the same in left-handers with right-hand writing, exhibiting no difference from the left-hand skill and learning curve of left-handers with left-hand writing. Right-hand skill decreased linearly as left-hand preference increased from -40 to -100; left-hand skill was not related to hand preference. Right-minus left (R-L) time for peg moving increased linearly with hand preference from -40 to -100. R-L time for peg moving linearly decreased as the right-hand skill increased; the left-hand skill was not associated with R-L time for peg moving. It was concluded that the right hand (left brain) determines left-handedness; the neural structures only on the left side exhibit pronounced plastic changes to genetic and environmental influences in left-handers.