This book is about differences between humans for handedness and for brain asymmetry. It has been known at least since the time that Galen was a surgeon to the gladiators in Rome that each side of the body is controlled by the opposite side of the brain. It was not until the mid 19th century that Paul Broca proposed his famous rule, “On parle avec l’hémisphère gauche” (Broca, 1865). Broca had performed post mortem examinations on a number of patients who had lost their ability to speak but were able to understand speech. He found lesions in the left frontal lobe in all cases. A few years later, the Austrian neurologist Carl Wernicke described another type of speech disorder in which patients produced a stream of fluent speech that was incomprehensible, and the patients themselves could not understand what was said to them. The cerebral lesion in these cases was in the region of the superior gyrus of the temporal lobe, again on the left side. Figure 1.1 illustrates some key features of the human cerebral cortex and the regions now known as Broca’s and Wernicke’s areas. Disorders of language may be associated with lesions over a wide area of the left cerebral hemisphere, with varying patterns of loss of function. (a) Human cerebral cortex, lateral view; (b) language associated cortex. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203759646/2b9e7cbc-ac77-4d09-a72b-49cc9e71f90a/content/fig01_01_B.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>