WHEN Rwandans went to the polls on September 15th, one part of the result was not in doubt: nearly one-third of their elected parliamentarians would be women. And probably more: in the outgoing parliament, nearly half the members were female. That level of representation—once seldom seen outside Scandinavia—has less to do with an upsurge in feminist thinking than with a law passed in 2003 that guaranteed women 30% of the seats. The aim was to break up “old boy” networks and help the country make a new start in its first elections since the 1994 genocide.

Such legal privileges, known as “gender-based quotas” by supporters, are catching on. Around 110 countries have rules helping women to get elected, joined in recent years by such feminist-friendly places as Afghanistan, Iraq and Sudan. On September 5th Angola had its first election with a new quota in place that says 30% of candidates must be women. Yemen is discussing a similar measure.

Quota laws may reserve a certain number of seats in the legislature for women, or instruct political parties to present a minimum proportion of female candidates (something parties in countries such as Britain and Germany may do voluntarily anyway).

The trend to quotas is most visible in countries where a legal leg-up can overcome prejudice, in the form of violence, shortage of cash, or lack of media attention. But even in the European Union—probably the best place on the planet from the point of view of women's rights—the issue is a hot one. In the European Parliament, Lissy Gröner, a German Social Democrat, is campaigning for half of all the top jobs in EU institutions to be reserved for women.

Supporters argue that having more women in politics is not only fair, but also beneficial. Research suggests that, at least in poorer countries, quota laws change the subjects that lawmakers discuss. A law in Rwanda that defines rape and protects victims of sexual abuse got through thanks to women legislators; their male counterparts saw the subject as taboo. A study co-authored by Esther Duflo, an academic at the Massachusetts Institute of Technology, says that women lawmakers focus more on public goods and take fewer bribes. Another study carried out in India showed that female politicians promoted public-works projects that mattered particularly to women, such as well-construction.

Yet quotas also raise hackles. Why should voters' choices be limited by artificial categories? Quotas also risk diluting the quality of decision-making: if women candidates are good, voters will choose them anyway; if they are not good, why do they deserve to be elected? Another objection is that plenty of women have done well without them. Angela Merkel in Germany and Tzipi Livni in Israel, as well as Hillary Clinton and Sarah Palin in America, have won elections without help from quotas. Legal privileges may help get women into politics. But something else makes them reach the top.