Constitutional rights come in different sizes, shapes, and flavors. Vertical rights structure the relationship between government and individuals. The government may not ordinarily search a person’s home without a warrant or require persons to attend a particular house of worship. Horizontal rights structure the relationship between individuals. No individual may hold another individual as a slave or, if he or she is a public figure in the United States, obtain damages in a libel suit against another individual, unless the statement was false and made with actual malice.

Extending Rights’ Reach: Constitutions, Private Law, and Judicial Power provides a much-needed analysis of horizontal constitutional rights in an academic market saturated with discussions of vertical constitutional rights. Jud Mathews provides a detailed discussion of how horizontal constitutional rights influence and do not influence the private law—the law governing the relationships between individuals—in Germany, the United States, and Canada. His meticulous treatment of constitutional horizontal rights in each regime provides readers with a vital understanding of the different approaches that national high courts take to constitutionalizing private law, as well as important perspectives on why those approaches differ. Students of both comparative and American constitutional law will find themselves much smarter after having read Extending Rights’ Reach.

High courts in Germany, the United States, and Canada offer three distinctive understandings of constitutional horizontal rights. The German Federal Constitutional Court treats rights as values that inform the private law. The constitutional rights to free speech and against discrimination that are enumerated in the constitution of Germany do not mandate particular private law rules. Nevertheless, the German Federal Constitutional Court requires other tribunals when adjudicating disputes between individuals to interpret German private law with German constitutional values in mind. Constitutional prohibitions against gender discrimination must influence decisions on whether persons can leave all their money to their sons rather than their daughters, even if those prohibitions do not compel specific rulings. The Supreme Court of the United States accepts horizontal rights only on an ad hoc basis. For most of American history, Mathews contends, a strict state action rule governed constitutional rights in the United States. Private action unconnected to official conduct was not governed by the constitution of that regime. During the late New Deal and Great Society era, the Supreme Court made exceptions to this practice when adjudicating cases involving the free speech and related rights of participants in the civil rights movement. Once support for the civil rights movement flagged, the justices largely returned to treating state action as an impenetrable barrier to horizontal rights claims. The Canadian Supreme Court is far more inclined to recognize horizontal rights than the Supreme Court of the United States, but does so in a less all-encompassing fashion than the German Federal Constitutional Court. High court justices in Canada are loath to recognize horizontal rights when individuals claim common law rights against other individuals. Instead, they interpret and alter statutes in light of Canadian constitutional law principles. Canadian justices insist, for example, that statutes protecting some rights against private discrimination be expanded in light of the more comprehensive commitment to equality in the constitution of Canada.

Extending Rights’ Reach is largely convincing when detailing the different approaches that different high courts take to constitutional horizontal rights. Specialists in one country, however, may quibble with particular details. The discussion of the United States barely notes that the Thirteenth Amendment has no state action requirement and fails to address how this omission influenced both the Peonage Cases adjudicated at the turn of the twentieth century and contemporary doctrine. Still, if persons who have spent academic lifetimes in a particular vineyard can find fault at the margins, the general picture seems accurate and important. Horizontal rights are different in Germany, the United States, and Canada. This means that constitutionalism—in particular, the scope of constitutional values—is quite different in Germany, the United States, and Canada. This is a substantial achievement of interest to a wide variety of scholars.

The text is less successful when offering a largely strategic explanation for these national differences in horizontal rights. Mathews points out that the German Federal Constitutional Court insisted on horizontal rights in order to gain greater supervision over other national courts. Because, as is often the case in Europe, the high constitutional court is limited to constitutional issues, the justices could gain supervisory power over other federal tribunals only to the extent that the private law is infused with constitutional values. Article III of the U.S. Constitution gives the Supreme Court of the United States a more general jurisdiction that, provided Congress passes the relevant statutes, gives that tribunal the power to supervise all federal courts and substantial power to supervise private law in state courts through diversity jurisdiction. Hence, U.S. high court justices did not need horizontal rights to extend their jurisdiction. Or so Professor Mathews argues.

This explanation is problematic. High courts, and the Supreme Court of the United States in particular, often seek to reduce private law dockets in the name of judicial efficiency. Given the vast increase in the number of appeals that horizontal rights might entail, whether a court is better off with them than without them is a fair question. Other explanations for national differences in constitutional horizontal rights are plausible. The foundations for much American constitutional law were laid in the nineteenth century. Constitutional rights at that time rarely had horizontal application unless, as in the case of slavery, the right was directed at relationships between individuals. Given that state courts in the nineteenth century did not interpret state constitutional rights as having horizontal effects, it is hardly surprising that federal courts interpreting similarly phrased rights focused almost exclusively on vertical rights. Rights jurisprudence in Canada and Germany developed after World War II. By this time, legal thinking had developed clearer notions of how constitutional law might infuse private law (and also developed proportionality analysis, which Mathews maintains with good reason is connected to horizon rights protection). The United States may differ from Germany because rights practices in the United States have nineteenth-century foundations, whereas rights practices in Germany have twentieth-century foundations.

Contingency also plays a role. Extending Rights’ Reach tends to treat high courts as singulars rather than as plurals. Judicial decisions reflect certain enduring features of the regime. In fact, many horizontal rights cases Mathews discusses, at least many state action cases in the United States, were decided by a divided court. More liberal justices were moving toward German horizontal rights practices and, perhaps, to proportionality analysis during the mid-twentieth century. These trends were halted, less because of any general judicial strategy or permanent feature of the United States, but because constitutional politics in the United States became more conservative than constitutional politics in other regimes. Conservative justices simply outvoted liberal justices in such cases as Flagg Brothers, Inc. v. Brooks (1978) and DeShaney v. Winnebago County Department of Social Services (1989).

Good works produce and inspire ideas. Extending Rights’ Reach meets both these standards. Mathews successfully details the development of horizontal rights practices in national courts in ways that advance comparative and American public law scholarship. His explanation for national differences, although incomplete, should lead other scholars following in his wake to improve our understanding of why, in different times and places, national courts are more and less inclined to integrate constitutional law values into private law. Those scholars will not agree with every sentence in Extending Rights’ Reach¸ but they will be standing on Mathews’s broad shoulders.