Understanding how a task group’s demographic composition influences its effectiveness requires considering situational demands. We explore this insight in a high-pressure situation, Himalayan mountain climbing. We hypothesize that the distribution of members’ nationality within climbing expeditions is a meaningful basis for ingroup categorization, and that national heterogeneity within expeditions is associated with intragroup competition manifested through climbers’ propensity to take more risk to reach the summit. We test this hypothesis using an archival dataset comprised of 2,756 non-commercial Himalayan expeditions undertaken from1950 to 2010. Our results show that nationally diverse expeditions are more likely to experience a climber injury or death but also that a greater proportion of their group will reach the summit of their target mountain. We also conduct individual level analyses to better understand howrelative demography—being part of a more or less heavily represented nation in the expedition—influences climbers’ likelihood of being injured or killed and summiting. We discuss the implications of our findings for group demography research and consider how they might extend to work groups that operate in other types of highpressure environments.