Customary International Law: Can UN Guidelines Affect Domestic Laws?

Below is a guest post by Martin Willner, research associate in the International Institutions and Global Governance program.

More often than not, when we think about customs and laws, we think of them as two distinct categories. Customs are encouraged, but optional: replying to an RSVP request, writing a thank-you note, shaking hands. Laws are mandatory: stopping at red lights, paying taxes, going to jury duty. As Kipling might put it, custom is custom and law is law and never the twain shall meet.

...except when we are talking about international law.

Contrary to popular assumption, international law is more than just treaties. In addition to the catalogues of written agreements, the customs and practices of states play an equally important role in determining international legal issues. But when and how do customs actually become law, and when do they simply remain customs? These questions have been the focus of discussion within the International Law Commission (ILC) for the past year. This summer the ILC is presenting its preliminary findings in a report to the UN General Assembly.

Historically, the customary practices of states have been transformed into international law. For example, the laws of war that are now codified in the Geneva conventions were developed over centuries, not through treaties but through the consistent practice of states regarding issues such as the treatment of civilians. Even before these laws were written down and clarified in Geneva, they were already considered binding under customary international law. While it is relatively easy to trace this type of transformation in retrospect, it is extremely difficult to pinpoint if, when, and how a specific custom reaches the status of binding international law.

The difficulty in identifying customary international laws has real consequences. As courts around the world attempt to ensure their respective countries’ compliance with international law, they must try to determine for themselves if any customary laws apply to domestic cases. Without consensus on how to identify customary international law, what is considered international law by a country such as South Africa may be different from what is considered international law by the United States.

Much of the current problem lies in the fact that customary law is, by definition, unwritten. It is derived, according to the Statute of the International Court of Justice (ICJ), from “international custom, as evidence of a general practice accepted as law.” However, this definition has often been said to be a result of “poor drafting.” Despite the nearly seventy years that have passed since the ICJ articulated this definition, fundamental terms have yet to be clarified. What does general practice mean? Does it mean that a practice is universal? Or, is it simply practiced by a majority of states?

While it is tempting to blame the inefficiency of the ILC, the UN, or any other international body for failing to provide a clearer definition for an important term, this critique fails to address the inherent complications of customary law. It is perhaps the greatest weakness of the ICJ’s definition—its lack of clarity—that allows for its greatest strength—its flexibility. With its currently fuzzy definition, customary law provides domestic courts with the flexibility to (mostly) interpret the application of these laws for their own countries. The flexibility to interpret at a domestic level is particularly important for individual states regarding customary law. Unlike treaty law, once a custom is determined to be part of international law it becomes binding on all states—regardless of whether or not a country has consented to that specific law.

The ILC is therefore currently undertaking a virtually impossible balancing act between providing clarity and preserving flexibility. When ultimately deciding which side to sacrifice for the other, it is likely that the ILC will prioritize flexibility—as it has in the past. The ILC has tried to address the same issues surrounding customary international law twice before, and each time it has failed. In 1971, the second time the task was attempted, the ILC came to the conclusion that “[i]t is counterproductive, and may be impossible, to codify the relatively flexible process by which rules of customary law are formed.”

Furthermore, if the ILC decides to sacrifice flexibility for clarity, the resulting document would have potentially troubling consequences for courts around the world. In effect, any definitive guidance could alter the current state of compliance with international law around the world. Taking Israel’s not-so-secret nuclear arsenal as an example, in the event that Israeli courts were to use new ILC guidelines and hold that non-proliferation was part of customary international law, the country would immediately be in violation of international law despite the fact that it is not party to the NPT. In other words, future guidance provided by the ILC—a closed commission of thirty-seven members—could affect the domestic law-making process around the world.

The United States would certainly not be immune from these ramifications. Currently, the United States is an outlier regarding its practice of capital punishment. Were the ILC’s guidance influential on the Supreme Court, it would be possible that the judicial branch might deem capital punishment illegal under customary law. Already without guidance from the ILC, the Supreme Court has abolished the juvenile death penalty noting that “only seven countries other than the United States ha[d] executed juvenile offenders.” The frequent and significant use of customary international law to supersede the legislative branch’s ability to make law is certainly unlikely, but poses a truly worrisome threat to the existing American balance-of-powers.

Over the next four years it will be interesting to see where the ILC decides to make sacrifices as it looks to provide “guidance not prescription” on the fundamental questions of customary international law that have thus far remained unanswered. Fortunately, there is still time for the commission to successfully provide guidance—this summer’s report is only the first of four. This success, however, will depend on its ability to balance the fundamental issues of flexibility and clarity in customary international law.