In his new book The Court and the World, Justice Stephen Breyer acknowledges that the Supreme Court increasingly hears cases that require it to take account of law and circumstances from abroad. With examples relating to commercial law, treaty interpretation, and human rights, Justice Breyer argues that “[i]t is not the cosmopolitanism of some jurists that seeks this kind of engagement but the nature of the world itself that demands it.”

Yet Justice Breyer also shows the Court struggling with a “knowledge gap.” How can the Court, which is not an expert in foreign law or foreign relations, get the information it needs to decide cases that raise international issues?

One way the Court has quietly addressed its need for information is by relying on amicus or friend-of-the-court briefs filed by foreign governments. Justice Breyer cites some of these foreign government briefs in cases involving international commerce and human rights lawsuits. He notes that the Court has relied on these briefs in particular cases and that it is “helpful to receive briefs from other nations.” As I explain in a forthcoming article, however, the Court’s reliance on foreign government amicus briefs has largely flown under the radar and is an even broader and more important phenomenon than Justice Breyer suggests.

The Court has cited foreign governments’ amicus briefs in a range of cases beyond those Justice Breyer references. For example, in Arizona v. United States, a 2012 case invalidating an Arizona law that targeted undocumented immigrants, the Court cited an amicus brief by Argentina and other Central and South American countries for the proposition that immigration policy affects “trade, investment, tourism, and diplomatic relations.”

Attention to foreign government amici also crosses the Court’s ideological divide. Even Justice Scalia, who has criticized citation to foreign and international sources in constitutional cases, embraced foreign government amicus briefs in a 2010 case (Morrison v. National Australia Bank) on the extraterritorial reach of US securities law.

The Justices frequently cite foreign government amici and discuss their briefs at oral argument. As detailed in the article, the Justices have cited foreign government amicus briefs at rates comparable to amicus briefs filed by the Solicitor General and well in excess of briefs filed by well-known amici, like the American Civil Liberties Union. Moreover, in several cases, the Court has taken the unusual step of granting foreign government amici oral argument time — a privilege usually reserved only to the parties and to the United States in its role as amicus curiae.

For a Court struggling to obtain the information it needs on foreign law and circumstances, there is sometimes good reason to rely on foreign government briefs. Foreign governments have particular expertise about some of the issues they address. For example, in Empagran S.A. v. F. Hoffman-La Roche, Ltd., foreign governments explained how extraterritorial enforcement of US antitrust laws would interfere with the governments’ own domestic antitrust regimes. In another case (Crosby v. National Foreign Trade Council), the European Union informed the Court that it would pursue World Trade Organization proceedings against the United States if the Court permitted a state law to stand. In addition to claims about the international effects of the Court’s decisions, foreign governments are also experts on their domestic laws. As Justice Breyer repeatedly notes, the Justices are not experts on foreign law, and neither is any office of the US government. Foreign governments may be the best available source of information about foreign law.

However, the Court’s practice of relying on foreign government amici also carries risks. For example, a foreign government may exaggerate the foreign relations consequences of the Court’s decisions in order to support companies from its country. Or a foreign government may advance a skewed perspective on international law, presenting an outlier position as a consensus view.

To guard against these risks, the Court needs to be more careful, more deliberate, and more transparent in its treatment of foreign government amici. At oral argument, the Court should question the parties to the cases, along with the US government, about the reliability of claims in foreign government amicus briefs. Doing so could introduce a form of adversarial testing for foreign government claims. Also, in its opinions, the Court should explain and justify its reliance on foreign government representations about particular factual and legal issues. Greater clarity on how the Court approaches foreign government assertions will allow litigants and scholars to help the Court assess the credibility of foreign government arguments in future cases.

Closing the “knowledge gap” on global issues is crucial to the Court’s work, and appropriate reliance on foreign government amicus briefs can be part of the solution.