Fallacious yet widespread and documented beliefs courtesy of Wikipedia.
Mental abilities are not absolutely separated into the left and right cerebral hemispheres of the brain. Some mental functions such as speech and language (cf. Broca's area, Wernicke's area) tend to activate one hemisphere of the brain more than the other, in some kinds of tasks. If one hemisphere is damaged at a very early age, however, these functions can often be recovered in part or even in full by the other hemisphere (see Neuroplasticity). Other abilities such as motor control, memory, and general reasoning are served equally by the two hemispheres.