Our project aims to address the impact of gender composition on group interactions and decision making in the context of two typical behavioural biases that are of particular interest to project management: (a) planning fallacy and (b) escalation of commitment. Both biases have been widely shown to have a pervasive influence on project management. This research makes an important contribution to the literature on project decision making, which has mostly focused on studying individual decision makers. Previous research have not considered the impact that group-specific factors—such as gender composition and the resulting quality of within-group interactions—might have. Our work will provide managers with guidelines to manage the increasing gender diversity in organisations and better take advantage of the potential benefits of this diversity while avoiding possible downsides.