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. 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 these social, cultural and economic schisms.