In the period 1861 to 1900, the U.S. Supreme Court under the Chief Justiceships of Roger Taney, Salmon Chase, Morrison Waite, and Melville Fuller presided over a jurisprudential transformation of American law. But its impact on the development of U.S. foreign relations law has not been as well studied. Bracketed between the extraordinary contributions of the Marshall Court and the emergence of the United States as a world superpower in the aftermath of World War I, the period from the Civil War to the end of the nineteenth century has been an underappreciated epoch in studies of the legal dimensions of America's place in a global order. Separated in time from both the originalist moment of the Founding and the first years of the New Republic, as well as from the twentieth century's Pax Americana, the late 1800s seem like a backwater of jurisprudential insight for contemporary understandings of the relationship between international law and U.S. law.

This seems particularly so with the role of customary international law. Partly, of course, this may be a function of customary international law's amorphous character and function in the matrix of legitimate sources of law under the U.S. constitutional order, which I discuss in more detail later. But there is also an empirical challenge to any study of U.S. Supreme Court cases involving customary international law: in some periods of the Court's operation, there are not many of those cases, and those that are reported are of an ambiguous and problematic nature.

Customary international law presents the greatest concentration of theoretical and doctrinal problems reviewed in this book. This is, in and of itself, a bit of a puzzle. Why has the role of custom in public international law been so hotly disputed for so long? Why, especially in comparison with discussions about the place of custom in domestic legal doctrines (see Chapters 5–9), has customary international law (CIL) been overemphasized and overtheorized? Why are the doctrinal features and contours of CIL so distinct, even in relation to other, purely public law subjects (such as constitutional law, discussed in Chapter 9)? And, why, after nearly half a millennia of debate, are we no closer to conclusive answers as to what makes a binding custom among nations, the relation between CIL and other international law sources of obligation (particularly treaties), and the process by which customary international law changes or dies?

There are, perhaps, a number of reasons for what may be called exceptionalism for CIL. The first of these is that, alone among legal fields, public international law acknowledges custom as being a leading source of obligation. There is currently enshrined a definitive statement of the sources of international law. It can be found in Article 38 of the Statute of the International Court of Justice (ICJ), or World Court, which indicates that, in disputes submitted to it, the Court shall apply:

1. international conventions, whether general or particular, establishing rules expressly recognized by contesting states;

2. international custom, as evidence of a general practice accepted as law;

3. the general principles of law recognized by civilized nations;

4. … judicial decisions and the teachings of the most highly qualified publicists of the various nations, as a subsidiary means for the determination of rules of law.

As already suggested in the last chapter, the canonical divide between private and public law subjects has always bedeviled much thinking about customary law. As a set of doctrines, family law combines elements of both, and, as just discussed, customary regimes can continue to exert a powerful pull in those jurisdictions where indigenous marital and succession practices were recognized and accepted. So, too, with property law. Very sophisticated contemporary legal systems – most notably those of Australia and the United States – have embraced group title to property, premised on ostensibly indigenous practices, institutions, and procedures. In this respect, common-law systems have sometimes been more receptive of property rights based on indigenous custom, than they have with family statuses and relationships grounded on the same source of law.

Unlike in family law, common law has, since medieval times (see Chapter 3), incorporated local customs as an element affecting property use, particularly of commonages. Unwritten and unrecorded customary servitudes continue to exist and be judicially enforced, even in U.S. jurisdictions. Perhaps as an unintended effect of jurisprudence construing the United States Constitution’s property rights and due process guarantees, customary land rights have emerged as a divisive issue. At stake is nothing less than the essential character of customary norms: though inchoate, are they binding from the moment of their adoption by the relevant community concerned, or must they be statutorily codified or judicially recognized before being constitutionally enforced?

If custom was largely perceived as a cultural phenomenon for medieval Continental jurists, it took on a decidedly historical cast in England. This would have a profound effect on attitudes towards custom in the Anglo-American common-law tradition, particularly with the normative hierarchy of custom in relation to other law sources, the requisite proofs of custom, and other restraints on evolving customary norms. In England, these transformations in the roles of custom took place over three broad periods. In what I refer to here as the “pre-Blackstonian epoch,” two distinctive ideas of customary law emerged from the ius commune (considered in the last chapter). One of these was the idea that the judge-made common law was itself a customary regime. The other was the continuation of a more traditional notion of local or special systems of legal practices being recognized in derogation of the common law.

These two strands of English legal thought were woven together in the writings of Coke, Selden, and Hale as part of the English constitutional struggles of the sixteenth and seventeenth centuries, but were only finally fully synthesized and digested in William Blackstone’s Commentaries on the Laws of England (1765). Blackstone’s work was immensely influential for what would follow in British jurisprudence, but also for developments in the United States and other common-law jurisdictions, and the full scope of his doctrine is recounted at length here. Post-Blackstonian intellectual currents – particularly Jeremy Bentham’s writing, but also late-nineteenth- and early-twentieth-century debates in U.S. legal circles – will also be considered in order to give a full tour d’horizon of the common-law tradition’s historic engagement with the role of custom in law.

Torts are definitionally nonconsensual civil wrongs. Because there is no consent, the actors in a tort or delict – victim and perpetrator – usually have no shared community relationship. So, unlike any of the legal realms in which custom figures prominently (and which have been considered so far in this part), the law of torts and negligence is typically one where common community values do not count. In the drama that is each instance of a delictual wrong, the players are usually strangers to one another. But does that necessarily mean that they should be estranged from the communities of expectations and norms that each may individually embrace?

In the continuum of private-public law relations and disputes, torts (like criminal law, with which it is always compared) has strong public law overtones. Custom, as a consequence, is often viewed skeptically as a source of legal norms in tort. Yet, ironically enough, it is in the law of negligence or fault that custom-based standards of care find their greatest fulfillment in the law. We take it as an article of faith that community standards establish the relevant duties of care, whether we employ one English judge’s quaint idiom of the “man on the Clapham omnibus,” or simply the “reasonable person.” As one court elegantly put the matter, the customary manner of doing something “would presumably represent the standard of ordinary prudent” people.

Constitutional law is the most public of domestic public-law topics. Indeed, the entire notion of a polity’s fundamental law would seem to be the ultimate exemplar of a legal domain that should, in the construct of writers such as James Coolidge Carter, H. L. A. Hart and T. F. T. Plucknett, be utterly immune from customary influences. Inasmuch as constitutions are a lex scriptum, custom should, under this theory, play no role in their construction, application, or interpretation.

The reality is that customary regimes have not only been a historical feature of English constitutional governance from the Middle Ages, but also have contemporary relevance in separation-of-powers debates within the tradition of U.S. constitutionalism. So, even in this most public of legal arenas, customary regimes can survive and flourish. Any explanation for this ostensible paradox must account not only for the historic origins of custom and the common law (narrated in Chapter 3), but also the continued relevance of the practices of political branches in the resolution of structural or institutional disputes within domestic polities. Constitutional custom is not only a historical construct; it is a phenomenon associated with the common law of government officerial prerogatives, as well as a pragmatic approach to the resolution of separation-of-powers disputes.

Custom continues to permeate almost all realms of contemporary law. This may seem a highly surprising and counterintuitive proposition. The anthropological, cultural, sociological, and economic perspectives revealed earlier might lead to a conclusion that customary law is a historic relic, an artifact of primitive law that has no place in modern, highly sophisticated legal systems. But this would be a profoundly mistaken position to embrace. Rather, customary norms continue to exist side by side with other mechanisms of positive law-making: constitutions as fundamental law, statutes adopted by legislatures, and case precedents decided by judges. More than merely subsisting within established legal orders, customary law regimes reinforce and strengthen existing systems of rules by anchoring them to the experiences and legal judgments of the relevant communities that follow those rules. In short, custom is no mere souvenir of bygone law; it is an integral and coherent part of any healthy, functioning contemporary legal system.

This part unfolds by examining five distinct realms of domestic legal regimes: family law, property, contracts, torts, and constitutional law. This sequence is by no means serendipitous. One possible critique of the continued relevance of customary law is to confine its applicability to purely “private law” topics. For sure, custom in domestic legal orders often has its greatest impact on norms related to the ownership and transmission of property, the enforcement of promises, and redress for civil wrongs. One of the central propositions of this volume is that custom remains a potent force in such purely public law domains as constitutional law and the law of nations. Indeed, custom may have especial force in those legal systems where determinations of status (especially family status), which combine both public and private law elements, are at issue. Within each of these five fields of domestic law, problems of the recognition and application of customary norms will be broadly reviewed in a comparative context. While the focus of the narrative will largely be devoted to the role of custom in common-law systems, in general (and in United States law, in particular), this will not invariably be the case in all instances. The goal here is to provide as wide a conspectus as possible of the character of customary law regimes as a part of the “living law” of contemporary domestic orders.

Custom and commerce in transnational trade have been inextricably linked for millennia. The customary norms of international commerce have been a traditional part of what has been called the lex mercatoria and lex maritima, that body of law respected and followed by merchants and ocean-traders. But international commercial norms present a number of doctrinal puzzles, and these will be considered in this chapter.

The first is the extent to which they can ever be truly independent of domestic legal systems. The traditional historical account is that from antiquity and the Middle Ages, the customary regimes of the law merchant and maritime law were created and enforced in a manner that was utterly free from interference by law-making mechanisms in any one polity. While these customs were often compiled and codified, they still reflected the actual usages of trade by the merchants and mariners involved. Indeed, the way in which disputes under these norms were typically resolved was by expedited recourse to panels of arbitrators selected from the relevant commercial community. Over time, merchant tribunals were co-opted as merchant juries by regularly constituted local courts. With the positivist legal revolution of the mid-nineteenth century, even this last vestige of autonomy for the lex mercatoria disappeared, and international commercial law became the creature of domestic courts and legislatures. But is this standard legal history narrative accurate? Did transnational commercial customs really disappear? And has there today been a renaissance for the international law merchant, reinvigorated and reconceptualized, and now truly globalized and autonomous?