This article develops an integrative framework for understanding genderbased group dynamics based on sex composition. The authors study decisions made by male and female members of a group to eliminate or promote other male and female group members. Study 1 uses game simulations modeled on the television show The Weakest Link to show how male and female players trade off individual versus group utility in a group setting and how such trade-offs affect group dynamics. They identify four patterns of sex-based group dynamics: old boys' club, queen bee, bounded rationality, and females as finalists but not winners. These patterns are part of a mosaic based on differences in sex composition and sex differences between male and female group members discriminating positively, negatively, or not at all against other male and female group members. The authors propose that differences in the degree of competition versus cooperation in the group, the initial sex composition, and the mean and variance of players' abilities by sex interactively determine which pattern will be noted. Study 2 uses observational data from The Weakest Link to test these predictions in a field setting. Implications for the motivational, cognitive, and strategic antecedents of gender effects based on sex composition of the group are discussed.