The adoption of gender quotas inpart y lists has beena voluntary decision by many parties in many countries, and is now a subject of discussioninman y others. The Parity Low passed inF rance in 2001 is particularly interesting because for the first time the quota was set at 50 percent, and the deputies passing the reform are elected in single member districts. In this paper we rationalize parity onthe basis of the self interest of male incumbent deputies: The existence of a voters' bias in favor of male candidates is sufficient to convince the incumbents to advocate for equal gender representationinpart y lists, because it raises the incumbents chances of being re-elected. We confirm empirically the existence of male bias in the French electorate and we show that parity law may have Assembly compositioneffects and policy effects that vary with the electoral system.