Abstract
Volumetric measurements show that right-handed individuals have larger right hands than left hands. In contrast, the hands of left-handers are much more nearly symmetrical. Based on what is known about trophic interactions between neurons and targets, these findings predict a corresponding asymmetry of the relevant parts of the sensorimotor system in right-handers. The lack of an opposite-hand asymmetry among left-handers further implies that right- and left-handed phenotypes do not arise according to the same developmental rules.