Proportionality is one of the core principles of international humanitarian law (IHL). Described in Additional Protocol I to the Geneva Conventions (AP I), the proportionality principle directs commanders to ― refrain from deciding to launch any attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated. While a number of nations are not a party to AP I, the proportionality principle is almost certainly a part of customary international law. However, the question is not whether the proportionality principle exists, but how to apply it in practice. The principle is susceptible to broad ranges of judgment and is therefore left to the good faith and common sense of military commanders. Much of the struggle with proportionality comes from weighing the fundamentally dissimilar values of military advantage gained to the harm to civilians and civilian objects. Additionally, the commander has to make proportionality decisions under significant uncertainty, with a lack of complete information, and within narrow timelines. All this points to a decision making environment that is much less than optimal. Some international tribunals and commentators have suggested that the proportionality decision be judged through the eyes of a ― reasonable military commander. However, what does this mean? More has been written about proportionality than perhaps any other IHL principle, but few writers have sought to explain and predict how those commanders actually make decisions as human beings limited by their cognitive capacities in a suboptimal decision making environment, thus a descriptive decision theory analysis of the proportionality principle. Scholarship often focuses on a normative decision theory analysis, looking at what decision makers should do and how they should apply the principle. Normative analysis is essential in a field like IHL, but equally important is understanding how and why human cognitive processes may result in decisions that appear to deviate from what is expected by rational choice theory. The field of descriptive decision theory and law has dramatically expanded in the last few decades, particularly in the field of behavioral economics and the law. However, there are almost no publicly available studies of heuristics, cognitive biases, and IHL principles in targeting decision making. This Article explores how heuristics and cognitive biases might affect the IHL proportionality decision. The Article also aims to be interdisciplinary, providing an overview of both IHL targeting principles and heuristics programs. Additionally, by placing the proportionality decision in the actual context of a detailed deliberate targeting cycle, this Article intends to better inform future theoretical and empirical research in this area. Finally, the Article suggests some ways that cognitive processes might influence the proportionality decision. This is a critical area for future interdisciplinary research and this Article hopes to inform and encourage that research
