As it happens, U.S. Fortune 500 companies as a group are slightly ahead of major European firms in women directors (16 percent to 13 percent female representation overall), but Europe is now poised to move well ahead of the US. The governments of France, Italy, Spain, Holland, Belgium, and Iceland have adopted quota systems that require boards of 30 to 40 percent women. Viviane Reding, the European Commissioner for Justice, Fundamental Rights and Citizenship, touts these initiatives as business-based policies: "Companies are finally starting to understand that if they want to remain competitive in an aging society they cannot afford to ignore female talent."

The economic case for gender quotas is, however, shakier than its advocates seem to realize. Morgan Stanley and the CED cite studies by both Catalyst, a women's advocacy group, and McKinsey & Company, a consulting firm, showing large economic gains accruing to companies with female directors. Catalyst's 2007 study found that, "On average, companies with the highest percentages of women board directors outperformed those with the least by 53 percent." But neither study has established causation: More female directors could be a consequence, rather than a cause, of business success. Aspiring upstart firms often need to fill their boards with financial backers and business partners (who may be disproportionately male), while rich, established firms may have more leeway and more interest in broader considerations like gender equality.

Some studies, moreover, find a negative relationship between women directors and firm performance. In a 2008 paper in the Journal of Financial Economics, economists Renee Adams and Daniel Ferreira found that female directors had better meeting attendance and were more active in "monitoring" their firms—but that "the average effect of gender diversity on firm performance is negative... Our results suggest that mandating gender quotas for directors can reduce firm value for well-governed firms." Deborah Rhode and Amanda Packel, both directors at Stanford University's Center on the Legal Profession, are committed to getting women on boards but object to hyperbole and overstatement in the research. In their exhaustive 2010 review of the literature, they conclude: "The relationship between diversity and financial performance has not been convincingly established." And here is an intriguing 2012 finding from business professors Charles O'Reilly (Stanford) and Brian Main (University of Edinburgh): "We find no evidence that adding women outsiders to the board enhances corporate performance. We do find some evidence that male CEOs with higher levels of compensation are more likely to appoint women outsiders and that boards with more women outside members are more generous in paying the CEO."