This study aims to advance our understanding of why women are underrepresented in legislatures around the world, and what accounts for the wide variation over time and across countries. Scholars generally agree on many of the favorable conditions for women to enter parliament, including, inter alia, proportional representation, leftism in government, and female employment. However, the mechanisms that link women's seat shares to the supposed explanatory factors are still poorly understood. In this study, we argue that the key link resides in welfare state policies that 1) free women to enter the paid workforce, 2) provide public sector jobs that disproportionately employ women, and 3) change the political interests of working women enough to create an ideological gender gap. The emergence of this gender gap, in turn, creates incentives for parties to compete for the female vote, and one way that they do so is to include more and more women in their parliamentary delegations. We are grateful to Natsu Matsuda for capable research assistance, to Torben Iversen for sharing data, and to Ethan Scheiner and Bing Powell for helpful comments as discussant for this paper at a panel at the 2004 American Political Science Association meetings in Chicago. Thies also acknowledges the support of a grant from the UCLA Academic Senate's Committee on Research.