Parties’ parliamentary delegations contain a multitude of interests. While scholars suspect that this variation affects party behavior, most work on parties’ policy statements treats parties as unitary actors. This reflects the absence of strong expectations concerning when (and how) the parliamentary caucus matters for platform construction, as well as the difficulties inherent in testing such claims. Drawing on the literature on women’s descriptive representation, we argue that the makeup of the parliamentary party likely has important consequences for issue entrepreneurship, the scope of issues represented on the manifesto, and even the left-right position of election platforms. With the most comprehensive party-level study of women’s representation ever conducted, we test our three diversity hypotheses using data on the gender makeup of parties’ parliamentary delegations and the content of their manifestos for 110 parties in 20 democracies between 1952 and 2011. We show that as the percentage of women in the parliamentary party increases, parties address a greater diversity of issues in their election campaigns. Women’s presence is also associated with more left-leaning manifestos, even when controlling for parties’ prior ideological positions. Together, these findings illustrate a previously overlooked consequence of descriptive representation and provide a framework for understanding when and why the parliamentary party influences manifesto formation. They show that diversity—or lack thereof—has important consequences for parties’ policy statements, and thus the overall quality of representation.