How do people judge responsibility in collaborative tasks? Past work has proposed a number of metrics that people may use to attribute blame and credit to others, such as effort, competence, and force. Some theories consider only the actual effort or force (individuals are more responsible if they put forth more effort or force), whereas others consider counterfactuals (individuals are more responsible if some alternative behavior on their or their collaborator’s part could have altered the outcome). Across four experiments (), we found that participants’ judgments are best described by a model that considers both actual and counterfactual effort. This finding generalized to an independent validation data set (). Our results thus support a dual-factor theory of responsibility attribution in collaborative tasks.