Ask people to explain why women remain so dramatically underrepresented in the senior ranks of most companies, and you will hear from the vast majority a lament that goes something like this: High-level jobs require extremely long hours, women’s devotion to family makes it impossible to put in those hours, and so their careers inevitably suffer. Not so, say the authors, who spent 18 months working with a global consulting firm that wanted to know why it had so few women in positions of…