Many international law decisions are made by individuals, often possessed with expertise, legal or otherwise. We examine individual international humanitarian law (IHL) decision-making on two levels: military decisions made ex ante regarding real-time operational questions under conditions of uncertainty and imperfect information, and subsequent ex post evaluations of the propriety of military decisions in the context of military investigations regarding legal responsibility with respect to proportionality and reasonableness. IHL requires ex post investigators to consider only information available at the time decisions were made. Through an experimental vignette study conducted with laypersons, legal experts and people with field experience, we test whether they are susceptible to cognitive ‘outcome bias’, specifically the extent to which the knowledge of operational outcomes, especially regarding incidental civilian harm, influences ex post normative evaluations. Our results demonstrate a general tendency towards outcome bias, which is somewhat tempered by expertise. Individuals with operational decision-making experience may be less prone to outcome bias than legal experts. We discuss possible implications for the design of military investigations relating to IHL.