We investigated the tendency of women to undervalue their contributions in collaborative contexts. Participants, who believed they were working with another study participant on a male sex-typed task, received positive feedback about the team’s performance. Results indicated that women and men allocated credit for the joint success very differently. Women gave more credit to their male teammates and took less credit themselves unless their role in bringing about the performance outcome was irrefutably clear (Studies 1 and 2) or they were given explicit information about their likely task competence (Study 4). However, women did not credit themselves less when their teammate was female (Study 3). Together these studies demonstrate that women devalue their contributions to collaborative work, and that they do so by engaging in attributional rationalization, a process sparked by women’s negative performance expectations and facilitated by source ambiguity and a satisfactory "other" to whom to allocate credit.