In 2005, the International Committee of the Red Cross (ICRC) published its two-volume Study of customary international humanitarian law (IHL). While upon its publication it was greeted both with acclaim and with criticism, today the Study has become a standard reference work for practitioners and academics alike – indeed, as far as the academia is concerned, it is probably the single most cited work on IHL. But how authoritative has the Study really been in practice? This is the question that we hope to answer in this article.

That question can be framed and approached from many different angles. We have chosen an empirical one, by collecting and analysing citations to the Study in documents containing expressions of State positions, in the judgments of international and domestic courts and tribunals and in the outputs of other influential actors. Our analysis establishes that the Study is increasingly seen as a highly authoritative instrument, such that a particular proposition will be found to reflect customary international law simply on the basis that the Study says so. In the absence of any concerted pushback, particularly by States – and no such pushback appears to be evident today, even if initially that was not the case and there remains some discontent – the Study’s authority will only increase over time, if nothing else then through repetition and force of habit.

The Study’s authority is particularly evident from our survey of the judgments of international courts and tribunals, but the accretion of authority is widespread and not confined to them only. The common tendency to cite the Study as a primary or sole authority for the existence of a customary rule, without any independent analysis and often as if it was a quasi-legislative text, is remarkable. And even relatively trivial but approving citations reinforce the feedback loop of authority. The Study’s authority rests not only on its rigour and the ICRC’s special mandate and expertise, but also on purely pragmatic grounds. The Study fulfils a variety of otherwise unmet needs. Since its publication no rival project was even conceived of, let alone implemented, that could meet those needs. The Study is simply useful, either for genuinely fundamental purposes (such as regulating non-international conflicts) or for purely pedestrian ones (finding cites for non-controversial propositions). And because it will remain useful, and because so many international legal institutions have already treated it as authoritative, the process of accretion is highly likely to continue.