Alison Wolf's "XX Factor" - NYTimes.com Earlier this year, Sheryl Sandberg asked the members of a mainly female audience of undergraduates at Harvard University to stand up if they thought one day they could be C.E.O. of their company or even president of the United States. Many stayed seated. For the chief operating officer of Facebook and the fledgling elite she was addressing, one crucial barrier on the road to gender equality may well be what Sandberg calls the leadership ambition gap - the lack of women with the self-confidence to "lean in" like men. But running for president is not a concern most women lie awake nights thinking about. And it is this quiet majority that Alison Wolf, a professor of public sector management at King's College London, draws attention to in "The XX Factor," her brisk, rigorously researched volume on the economic rise of (some) women. I say "some" because Wolf, like the French writer Elisabeth Badinter, argues that universal sisterhood is dead, with an elite of well-educated women moving away from the rest. "Whereas through most of human history it made sense to talk about "women" en masse," Wolf writes, "today it very rarely does." The women of recent feminist manifestoes, she contends, have become a class apart, with their own ambitions and concerns - concerns shared not by most women, but by elite men. These women, like their male counterparts, now marry later and have fewer children than their less-educated sisters. They take shorter breaks from paid full-time employment (a reverse from past trends) and claim an ever greater share of overall female income while relying on nannies and other household help. And although the professional world of the elites is increasingly integrated when it comes to gender, most women, including those domestic helpers, tend to work in professions that are still very much segregated along gender lines. One result: Men and women at the top marry each other more, exacerbating income inequality and reducing social mobility in Western societies. Wolf's book is a sobering reminder that as it is currently framed, the work-family debate smacks of elitism. The dominant narrative of middle-class women seeking to combine fulfilling jobs with more quality time for family clashes with that of poorer women, who have long worked for pay because they had to. Stereotype-busting innovations like shared parental leave and shared part-time working arrangements between parents are not just cultural propositions. They are luxuries that only relatively well-off couples can afford. Here is where Wolf makes her most provocative but also her most problematic assertion: "Adult female employment today isn't a common shared experience in the way that tending home and family used to be," she writes. Gender equality for professional women comes at a price, Wolf implies. She does not explicitly attribute blame, though she seems to come close: "Without the new servant classes, elite women's employment would splutter and stall," she writes. True. But their elite husbands rely on those nannies and housekeepers just as much. Class has always been a major divider. Surely, it would have been difficult to speak of shared experiences between the working-class women of the Industrial Revolution and the "daughters of educated men," as Virginia Woolf described them. Yet for all the important differences, women in the early 21st century still share distinct hurdles based on their sex, independent of class - from everyday sexism and sexual violence to the mother of all challenges, motherhood itself. If there is one thing the would-be Sandbergs of this world and their nannies have in common, it is that maternal stereotypes still govern how women are expected to behave. It explains why female leaders who display traditionally "male" characteristics are penalized - "aggressiveness" and "brusqueness" clashing with the compassion and softness associated with women, with mothers. It explains why the caring professions are so overwhelmingly dominated by women (and why they are so badly paid). And it may explain why, even though Wolf's book successfully reminds readers of the many revolutionary changes to women's lives over the past century, it is also surprisingly quick to regard certain remaining gender differences as "normal." Katrin Bennhold is a correspondent for The International Herald Tribune and a columnist for The Female Factor.