Rwanda has the highest proportion of women in parliament in the world - even above the proportion of women in the general population at 64%.
Why is this the case?
Geobits hit the first piece of the puzzle: Gender quotas. The Rwandan constitution requires at least 30% of positions to be filled be women. See Article 9 of their constitution.
30% is a long way from 64%. Where do the rest come from?
Political scientists focus on group mobilization as one key to electoral success. There may be many parties competing for power, but ultimately it is the group that can mobilize it's members that is going to win. Groups that are well organized are easier to mobilize.
Rwanda has a unique system to make sure that women are mobilized to participate in government: The Rwandan Ministry of Gender and Women in Development. They organized a series of "women's councils" to mobilize women in politics. It exists at the most local levels, every leader is a woman, elected by the women in her area. Women's councils act as a kind of shadow government, providing input to local and regional governments. Because of women's councils, Rwandan women can mobilize and and work in politics much easier than women in other countries.