Paul Broca is unanimously recognized as the founder of neuropsychology. Helis development of the scientific method to map mental functions onto brain topographpy has been enormously influential. Nevertheless, Dax's paper on the left hemisphere dominance for speech was written and published before Broca explicitely proposed the same theory. Probably, Broca was aware of the paper prior to 1865, but he never acknowledged Dax's original theoretical contribution. On the contrary, he always claimed to be the first to espouse the theory of left hemisphere dominance for language and never quoted Marc Dax (Broca, 1877 p 536), 'I do not like dealing with the questions of priority concerning myself. That is the reason why I did not mention the name of Dax in my paper'. In our opinion, the weight of evidence reported here suggests that the theory of the left hemisphere dominance for speech must be attributed equally to Dax and Broca, and henceforth should be called 'the theory of Dax-Broca'.