Comity as Cover? How U.S. Courts Balance International Law Away

[Serafeim Liakopoulos is a graduate of the University of Pennsylvania Carey Law School (LL.M.) and is currently a study visitor at the European Court of Human Rights]

International comity, as understood by U.S. private international law, is not a binding rule of international law but a principle of discretionary deference towards foreign sovereigns and their legal acts. It encompasses a range of judicial practices, including declining jurisdiction, limiting the extraterritorial application of domestic law, enforcing foreign judgments, and applying foreign law. Comity, it is said, is a flexible principle that lets courts show “respect” for foreign sovereigns when no hard rule of international law applies. Indeed, comity is said to function as a “layer on top of” international law.

Understood broadly, comity appears nearly universal. If any act of deference to foreign authority—such as applying foreign law or recognizing foreign judgments—can be labeled as comity, then all legal systems could be said to engage in ‘comity-like’ behavior. However, the United States presents a distinctive approach. Indeed, U.S. courts do not merely perform comity-like functions; they explicitly invoke comity as a consideration in adjudicating cross-border cases. Thus, although not always treated as a formal legal doctrine, comity in the U.S. plays a quasi-doctrinal role in determining when and how domestic and foreign laws should apply. It operates as a flexible, discretionary justification for judicial restraint, particularly in areas such as prescriptive jurisdiction, recognition of foreign judgments, and forum non conveniens.

The risk with such a flexible principle, however, is that when it is “layered on top of rules of international law” it risks blurring the distinction between comity and binding legal norms, creating the impression that if a result is acceptable under comity, it is consistent with international law as well. Focusing on the presumption against extraterritoriality and the Supreme Court’s approach in Societe Nationale Industrielle Aerospatiale v. United States the post suggests that comity in U.S. practice sometimes functions less as deference and more as a justification for exceptionalism.

The Presumption Against Extraterritoriality: Restraint or License?

The presumption against extraterritoriality functions as a principle of statutory interpretation in U.S. law. Essentially, this principle instructs courts to presume that no legislation is intended to apply extraterritorially unless Congress clearly indicates otherwise. Thus, on its face, the presumption appears to limit the reach of U.S. law abroad by setting a high threshold for applying statutes extraterritorially.

Yet, the presumption does not categorically prohibit extraterritorial application. Rather, it acts as a procedural gatekeeping mechanism. Courts first determine whether the statutory text contains a “clear, affirmative indication” of extraterritorial intent. If no such language exists, the inquiry ends with the statute being confined to domestic application. However, if courts find that there is sufficient clarity for an extraterritorial reach, they may permit the statute to have an extraterritorial effect. In EEOC v. Arabian American Oil Co, the U.S. Supreme Court has admitted that the “presumption ‘serves to protect against unintended clashes between our laws and those of other nations which could result in international discord.’” Importantly, it is considered that the presumption against extraterritoriality is a matter of comity: U.S. courts choose not to interpret statutes as having an extraterritorial effect because they respect the sovereignty of other states.

By contrast, the customary international law principle of non-interference is both broader and stricter. Unlike the presumption against extraterritoriality, non-interference is a substantive norm grounded in the way international law understands sovereignty itself. Fundamentally, it prohibits states from engaging in actions that intrude into the internal affairs of others through almost any means, direct and indirect alike.

As a customary norm, non-interference is not codified in a single treaty, but it is widely recognized and supported by consistent state practice and opinio juris. The International Court of Justice has also confirmed that the principle has the status of customary international law, and subsequently stated that the principle prohibits intervention which “bear[s] on matters in which each State is permitted, by the principle of State sovereignty, to decide freely.” 

When these two principles are examined side by side, a tension emerges: Is the presumption against extraterritoriality genuinely reinforcing the principle of non-interference, or does it, in practice, weaken it? On the surface, comity appears to serve as a self-imposed restraint aligned with international norms, but the foregoing suggests that this framing masks a deeper misalignment: by converting a substantive prohibition under international law into a question of domestic statutory interpretation, comity permits interference under the right conditions. 

This is especially visible in areas like transnational discovery, where U.S. courts order the production of documents abroad despite conflicts with foreign privacy or blocking statutes or even against the will of other sovereigns. Those orders are often treated as legitimate because they are seen as purely procedural and outside the scope of the presumption, even though from a non-interference perspective they may come very close to coercive intervention.

Aerospatiale and the Problem of ‘Balancing’ Sovereignty

The Supreme Court’s decision in Aerospatiale is often taken as the leading example of comity‑based “balancing” in transnational discovery. Rather than requiring U.S. courts to rely exclusively on the Hague Evidence Convention, the Court invited judges to weigh U.S. interests against those of the foreign state when deciding whether to order discovery directly under domestic procedural rules. 

First, if we consider the balancing of the interests of the two states as something granted from one state to another as a matter of deference or comity, then, because the usage of the FRCP to compel discovery abroad has been inherently criticized as interfering with the sovereign rights of other states, a fundamental contradiction is created in that the function of comity is defeated: if comity allows U.S. courts to authorize actions that interfere with another state’s sovereignty—such as compelling discovery in violation of foreign law—then it offers less protection than what customary international law requires. In that case, comity cannot credibly be described as a layer of deference “on top of” international law because it permits conduct that international law prohibits.

The second, perhaps most coherent interpretation that on its face would neither violate customary international law nor the definition of comity, is one that returns to Hilton v. Guyot, a case where the U.S. Supreme Court declared comity as “neither a matter of absolute obligation … nor of mere courtesy and good will.” Although Hilton’s framework is generally regarded as obsolete or unworkable, its core formulation of comity may simply be misunderstood rather than irrelevant. On this reading, the kind of balancing endorsed in Aerospatiale can be seen not as unilateral deference, but as part of a mutual understanding between states: that sovereign interests can legitimately be weighed against one another in certain contexts.

Indeed, if one accepts that states have implicitly consented to some reciprocal limitations on sovereignty, then such balancing could be seen as a pragmatic accommodation within the international legal system without violating norms of customary international law. Comity, in that scenario, would be said to represent a voluntary conceptual framework—an “idea” through which sovereigns have consented to mutual accommodations. Rather than imposing absolute prohibitions, comity would reflect an understanding among states that some degree of friction is inevitable, if not acceptable, so long as it is managed respectfully. This is why comity is not a matter of obligation, nor a matter of courtesy: to be a matter of obligation signals more than mutual understanding, but to be courtesy or deference requires the capacity to act unilaterally.

This defense, however, remains open to criticism precisely because, in order to comply with international law, it requires that other states have consented to such balancing and cooperation through comity. Here, it might be important to note that, as academics acknowledge, many other countries, both in Europe and beyond, maintain a dismissive stance toward international comity in their legal doctrine. Most of these nations do not portray any of their grants to foreign nations (such as foreign judgment recognition) as a matter of comity. Instead, they rely on codified statutes and other procedures that leave little room available to judicial discretion. Although these jurisdictions give effect to foreign law or judgments in a way that may, in practice, resemble outcomes traditionally associated with comity under U.S. law, that does not mean these mechanisms are understood by the states deploying them as resting upon reciprocal consent. Indeed, the fact that a nation has a specific statute for judgment recognition or other specific means of dealing with certain instances of foreign law does not mean it has consented to the broad, open-ended balancing of interests that comity under the previous reading requires. To the contrary, the use of blocking statutes and (anti-)anti-suit injunctions, not only in continental civil law systems but also in countries like China and Canada, demonstrates that when foreign interests conflict with their sovereignty, states explicitly reject such a notion of “consented” cooperation. Besides, even states that do utilize comity, including the U.S., characterize comity as a matter of unilateral discretion with the expectation of reciprocity rather than an already existing tacit agreement. Accordingly, it is difficult to accept that comity is a norm of mutually inferred consent.  

If the foregoing is correct, then the majority of comity definitions that permit U.S. courts to engage in a balancing of sovereign interests are fundamentally problematic under customary international law: the very act of balancing presupposes that a U.S. court is entitled to measure another state’s sovereign interests against its own and decide how much weight they deserve. Without genuine consent to such a framework, that looks less like deference and more like a unilateral assertion of authority over matters that international law treats as falling within another state’s domestic jurisdiction.

What Kind of Comity Do We Want?

None of this means that comity is useless or always abused: there are cases where U.S. courts have relied on it to go beyond what international law strictly requires and to exercise real restraint. Perhaps, then, the problem lies in the selective application of comity as a norm of law with an international relations dimension: used strategically, selectively, and pretextually. If comity is to remain a credible language of respect for foreign sovereignty, it cannot simply be a domestic technique for managing international law away. At the minimum, it should be anchored more explicitly to customary norms and to genuine practices of consent and cooperation, rather than to judicial intuitions about when foreign interests “deserve” to prevail.