If men and women are equal, why have males been the dominant sex virtually throughout history? Here, geneticist Moir and BBC- TV writer-producer Jessel argue convincingly that the answer lies in the difference between the male and female brain. Writing with clarity and style, and documenting their data every step of the way, Moir and Jessel explain how the embryonic brain is shaped as either male or female at about six weeks, when the male fetus begins producing hormones that organize its brain's neural networks into a male pattern; in their absence, the brain will be female. Not surprisingly, there are endless variations in degree of maleness, and mishaps can lead to a male brain in a female body and vice versa. Moir and Jessel include a brain sex test that lets the reader discover just how masculine or feminine his (or her) brain is. For the nonscientist, they translate considerable research into the structural and organizational differences between male and female brains, demonstrating how these differences make men more aggressive and competitive and better at skills that require spatial ability and mathematical reasoning, and women more sensitive to nuances of expression and gesture, more adept at judging character. Women, it seems, are more people-oriented than men, who are more interested in things. Moir and Jessel assert that it is necessary to ``accept who we are before arguing about what we should be,'' and that denying gender differences means ignoring their value. A literate, entertaining, and, for some, surely wrath- provoking presentation of scientific data about the differences between the sexes.