Customary International Law and Public Support for the Use of Force

• In Event: International Collaboration Virtual Poster Session

Abstract

Existing observational studies have been unable to adequately explain why states comply with international legal obligations, especially those deriving from nonbinding international legal principles. Customary international law is defined by custom—evidence of a general practice accepted as law. Some studies suggest that customary international law should have no different effect than treaty law on support for the use of force. This article sheds light on several fundamental questions related to the effect of international law. First, to what extent does customary international law change public preferences about the use of force? Second, does the type of commitment to international legal principles matter? Third, are certain groups of people more likely to respond to international legal commitments? Fourth, through what mechanisms does international law exert an effect?

This article employs a preliminary survey experiment of 1,800 Amazon Mechanical Turk workers fielded in November 2017. This experiment presented respondents with a hypothetical situation in which the United States was considering bombing a building where terrorist leaders would be meeting, but would be in close proximity to a mosque that regularly holds services. The experiment randomized information about the international legal principle of proportionality, which does not permit attacks when the expected harm incidental to such attacks would be excessive in relation to the military advantage anticipated to be gained. It also randomized whether the United States publicly committed to following this principle, and whether the UN found a similar previous bombing to violate those legal principles. After this vignette, the experiment asked whether the respondent would support a U.S. bombing of the targeted building.

This article finds that there is a weak effect of customary international law restraining potentially violative uses of force abroad. Specifically, when information on customary international law is paired with statements that international organizations may deem the use of force as a violation, respondents significantly decreased their support for the use of force (for α = 0.1). Using multiplicative interaction models, this article also shows that the international law treatments had heterogeneous effects, particularly based on military assertiveness and internationalism attitudes of respondents. Lastly, there is weak support for mediation by reputation and morality concerns.

These findings suggest several avenues for fruitful research. The distinction between the effects of customary international law and treaty obligations on compliance is unexplored in the larger literature, and this paper shows that international legal principles may have less effect on their own without an international monitor. Exploring multiple forms that legal commitments can take—beyond public commitments or international pronouncements—may also shed light on what makes international legal principles create binding obligations. This article contributes to studies on the effect of international law on the use of force by showing that it is not necessary to codify legal obligations in treaty law for them to meaningfully restrain public attitudes toward acting in violation of those legal commitments.

Author

• Melinda Haas, University of Pittsburgh