Meta-analytic results show that board gender diversity is modestly associated with firm performance, but there is notable heterogeneity among findings. Board gender diversity allows access to women’s perspectives, potentially helping boards, but diversity can also trigger biases that exclude women directors, such that boards do not integrate meaningful perspectives. Addressing this problem, we leverage the categorization-elaboration model, contact theory, and critical mass theory to build new theory as to how men directors can serve as allies to women directors to better (...) leverage diverse perspectives. We empirically test how considerations that reduce out-group categorization and bias against women moderate the board gender diversity-firm performance relationship. Our results show that gender diverse boards perform better with more formal contact among men and women director colleagues, and that gender diverse boards with more men directors who only have prior experience working with token-women, perform worse. Our work helps explain how and why board gender diversity can improve or detract from firm performance. This extends the literature by illustrating the important consequences that occur when firms do (or do not) have men directors who are likely to be allies of gender diversity. (shrink)