The Emergence of a New Rule of Customary Law: An Experimental Contribution

For most legal rules, an evolutionary perspective may be elucidating. Yet it would be hard to claim that otherwise one misses the essence of the rule-making procedure. This paper uses experimental data to show that customary law is different. While some doctrinal concepts of this source of law try to freeze the resulting rule, and to isolate it from the process of rule generation, Maurice Mendelson (in a 1998 paper in ‘Recueil des Cours’) forcefully argues that this conceptualization misses the very difference between statutory (treaty-based) and customary international law. Consequently, he also challenges the conventional claim that for a new rule of customary law to originate, two conditions must be met: there must be consistent practice, and it must be shown that this practice is motivated by the belief that such behavior is required in law. Mendelson claims that this doctrine misses the fundamentally incomplete nature of public international law. He claims that a new rule emerges when mere practice leads to convergent expectations. This paper uses data from public good experiments to back Mendelson’s theory. This evidence demonstrates that behavior converges even absent verbal communication; that convergence is guided by mean contributions in the previous round, which serve as an implicit norm; that free-riding on this implicit norm is regarded as illegitimate; that cooperation can be stabilized at a high level if the functional equivalent of what public international lawyers call “reprisals” are permitted. Hence the mechanism of norm formation proposed by Maurice Mendelson is fully borne out by the experimental data.