For most of history, being female defined the limits of a woman's achievements. But now, women are successful careerists equal to men. In Norway, women legally must constitute a third of all boards; in America, women have gone from 3% of practising lawyers in 1970 to 40% today, and over half of all law students are female. These changes are revolutionary - but not universal: the 'sisterhood' of working women is deeply divided. Making enormous strides in the workplace are young, educated, full-time professionals who have put children on hold. But for a second group of women this is unattainable: instead, they work part-time, earn less, are concentrated in heavily feminised occupations like cleaning and gain income and self-worth from having children young. As these two groups move ever further apart, shared gender no longer automatically creates interests in common with other women. The XX Factor lifts the curtain on the social, cultural and economic schisms.