International humanitarian law remains under-theorised. Eric Posner pioneered the use of law and economics methodology to provide an alternative explanation of international humanitarian law. The present article examines how the use of the cognitive framework underpinning the law and economics (L&E) lens in international humanitarian law (IHL) transforms this legal regime. First of all, the article argues that, although the law and economics methodology accounts for the fact that self-interest is one of the motivating factors behind state action, it does not accommodate the constructivist dimension of international humanitarian law. Furthermore, while the Chicago School has descriptive capacity for the principle of military necessity, it offers a limited analytic framework for understanding the principle of humanity, both of which are equally important when understanding the foundational basis of IHL. Secondly, the article argues that L&E changes how states interpret the purpose of international humanitarian law, the structure of this legal regime and how individuals apply the legal norms on the battlefield. In other words, it alters the balance between military and humanitarian considerations within IHL norms. The rules of targeting will serve as a case study to illustrate some of the problems associated with the application of the L&E lens to IHL, especially how such rules in fact place limitations on the planning and conduct of military operations.