International law is not one homogeneous legal system with a clear hierarchy of norms, but rather consists of general rules as well as a number of specialized legal systems,Footnote ¹ such as trade law, environmental law, or human rights law. None of these systems can claim ‘hierarchical superiority’ over another, as they exist on a horizontal level of mutual equality. This fragmentation of international law raises questions on the interaction between those various specialized systems, and the question whether and how these systems can communicate with each other, thereby contributing to a so-called protection of international law as a whole.Footnote ²

The WTO as specialized legal system is a particularly relevant field of study, as trade cuts across many non-trade interests, such as the environment, human rights, or labour standards. The question whether the WTO should remain a trade organization or whether non-trade concerns beyond the references in the covered agreements should also receive extensive attention from the adjudicating bodies remains popular. Yearwood does not aim to contribute to that debate with his book, but takes on a descriptive and analytical approach to WTO jurisprudence, aiming to apply his theory of constrained openness to the current interaction between WTO law and external international law. While the author has omitted to clearly define ‘external international law’, he seems to target other specialized legal systems under international law, rather than rules of general international law, including general rules of treaty interpretation.

The focus of the book is on the newly proposed theory of ‘constrained openness’, or rather, as Yearwood indicates himself, a prologue to a theory. On numerous occasions the author emphasizes his main aim is to create new thinking space in the current debates on the interaction between WTO law and other rules of international law, without claiming to offer universal answers to such interaction. In the author's own words ‘it is a theory of WTO law in that [he] choose[s] ways for “identifying, interpreting and applying the norms” of WTO law’ (p. 10), as there seems to be a vacuum of explicit theories that explain WTO law as a matter of law. With his theoretical work, the author answers to the ‘no-nonsense empirical method’ of John Jackson,Footnote ³ one of the architects of the WTO, in that an empirical study of trade, based on economic theory of liberal trade, does not attempt to understand the legal structures of WTO law, nor answers questions on value conflicts within WTO law. According to Yearwood, theory (knowing) cannot be separated from practice (doing). Theory is not only a way to test the outcome of practice, but theory is also used (and allegedly necessary) to understand practice and to question the way in which the letter of WTO law develops into practice. In fact, legal theory helps to determine the nature, existence, and purpose of WTO law. Such legal analysis needs to be conducted apart from the economics and politics of WTO law, as instructive and useful as that can be as well. The findings of Yearwood's legal theory (What is WTO law? What is its purpose?) can then serve to explore how WTO law interacts with external international law and how the WTO's purpose is realized in that interaction.

The theory of constrained openness consists of two parts: first, an identification of WTO law (designated by Yearwood as the method); and second, an examination and understanding of the law and interaction with external international law (which Yearwood calls the concept). Yearwood puts a lot of emphasis on defining WTO law (Chapter 1). While everyone will most likely agree that the WTO-covered agreements are WTO law, there can be disagreement beyond that point. What else can count as WTO law and what cannot? The author defines WTO law through the practice of the WTO Dispute Settlement Body (DSB), based on Hart's rule of recognition.Footnote ⁴ The adjudicating bodies produce institutional acts from their internal point of view, which by their practice can be understood and observed as WTO law, ensuring predictability and stability in the WTO legal system (de facto legal precedent). In contrast to Hart's view, however, Yearwood in his analysis of international law acknowledges that not every system in international law has a centralized judiciary and hence that law is not only the practice of the judiciary. Rather, when studying the interaction of legal systems, based on the rule of recognition, the actions of ‘authoritative decision makers’ of the different specialized systems (the DSB in the case of WTO) need to be examined as they serve as points of reference to identify the criteria for recognized manifestations of law (‘counter-factual rule of recognition’).

After having defined what the law is – with regard to WTO law, but with the mentioned possibility to extend the rule of recognition to other specialized legal systems of international law as well – Yearwood examines whether the traditional conflict rules such as lex posterior and lex specialis, and rules on treaty interpretation (Vienna Convention on the Law of Treaties), can offer satisfactory answers for the resolution of conflicts between those different legal systems (Chapter 2). While an interesting summary of the status of treaty interpretation within the WTO, the chapter slighty distracts the reader from the build-up of Yearwood's theory.

In the following chapter (Chapter 3) Yearwood continues fleshing out his theory of constrained openness. The author argues that in their interaction with external international law, the adjudicating bodies (re)construct norms into WTO law, thereby developing their own understanding of what the external norm entails, rather than incorporating the external norms and their ‘external’ interpretation into WTO law. Scholars such as Cheyne have argued through the gateway metaphorFootnote ⁵ that external norms are indeed incorporated into WTO law, an approach criticized by Yearwood for its lack of specificity and its implicit assumption of hierarchy between non-trade and trade values. Rather the author advances Black and Teubner's idea of operational closureFootnote ⁶ as an explanation of the interaction of the fragmented systems of international law. This entails systems being open in their observations of other systems, while being normatively closed in the recognition of norms from other systems as valid. A system hence produces its own ‘constructs of reality’, based on internal validation of norms. The technique on how to introduce external norms is explained by the concept of ‘reflexive law’. It is up to the WTO system to make sense of the external ‘noise’ and reconstruct it to become part of WTO law, which is clearly not a neutral objective process – contrary to what the gateway metaphor depicts. This reasoning leads to the question whether and how external norms can become part of WTO law.

Yearwood goes on to answer this question by referring to the current debate on the openness of the WTO system to other specialized legal systems (Chapter 4). As illustration, he refers to three models which aim to explain the relation between external international law and WTO law, namely the closed model as adhered to by Trachtman (whereby international law can only be included into WTO law by means of specific references in the covered agreements); the open model as supported by Pauwelyn (whereby international law can be incorporated into WTO law based on implied powers); and the privileged model, adhered to by Bartels (related to the open model, however, whereby WTO law is privileged over other international law). Yearwood criticizes (the discussion on) these three models, stating that they appear to be based on the superficial presumption that the questions what WTO law is and what its purpose entails are settled. Neither model offers a satisfactory explanation of the complexity of how the WTO adjudicating bodies apply external international law. With his theory of constrained openness, Yearwood attempts to break open this debate and create new thinking space on how WTO law can interact with other international law. He argues that the adjudicating bodies must ‘capture’ external law and its meaning according to the ‘code of WTO law’ and in such way retain the WTO's critical identity.

In the last part of his book, Yearwood illustrates his reasoning with examples of WTO disputes. According to the author, these are not to test his theory, but serve to use his theory as a lens through which to understand WTO law. Chapter 5 deals extensively with the precautionary principle and the SPS Agreement, arguing that the WTO did not incorporate the precautionary principle as understood in external international law, but rather interpreted it within the limits and the context of the SPS Agreement. Chapter 6 has a broader, but less profound, scope, focusing on five disputes as examples of interaction between WTO and external international law according to the legal form of the external rule (e.g., bilateral agreement or agreement between the WTO and another international organization). The disputes (EC–Poultry, Argentina–Textiles, Korea–Government Procurement, US–Shrimp, and Mexico–Soft Drinks) are discussed from the point of view of the above-discussed models, as well as through the lens of constrained openness. The distinctive element of constrained openness is not always very clear, though, especially with regard to the actual reconstruction of external norms into WTO law as opposed to the open/closed/privileged models. In US–Shrimp, for instance, the Appellate Body (AB) considered a number of environmental agreements to define the term ‘natural resources’ in Article XX GATT, as well as to assess the justification of the discriminating policy under Article XX GATT. Rather than an ‘application’ or ‘incorporation’ of external international law, this is judged by the author as only ‘to further the operation of the WTO to be able to reach credible decisions in its dispute settlement process, as to its purpose in liberalizing trade’ (p. 197). Yearwood did not indicate whether adherents of the open model, for instance, would have argued that the AB incorporated norms here, or how external international law as a ‘guidance tool’ for the interpretation of WTO law would be differently assessed under that theory. From the examples discussed in the book, one would be tempted to say that the AB is itself seeking the best approach to interact with external international law on a case-by-case basis, depending on a number of criteria, such as the legal form of the external rule, the parties to the dispute, the relevance of the external norm, the importance of said norm for the interpretation of WTO law, the value behind that norm, etc. I would believe that the extent of reconstruction of norms, as well as incorporation of norms, depends on that balancing act. However, this ‘scale’ of reconstruction is yet still lacking in Yearwood's theory. Nevertheless the theory of constrained openness seems to be more nuanced than either the open or closed model, which by itself certainly deserves merit. It would be interesting to further the analysis based upon the field of law (e.g., human rights law, environmental law) additional to the legal form of external norms, to see which norms are more easily and fully accepted into WTO law and which are not, and to see to what extent they relate to the purpose of the WTO.

Yearwood has made a valuable contribution to the academic debate on legal theory within WTO law. Rather than on the extent to which international law interacts with WTO law, Yearwood has focused on the more structural question whether it can interact and, if so, how. In his analysis, the author engages in the debate, questioning the validity of other theories, while at the same time being aware that his own proposal of constrained openness is still vulnerable. Mindful that according to the author this is just a prologue to a theory, there are a number of elements that would warrant further inquiry. Constrained openness allows for mutual influence of specialized legal systems, while norms can at the same time develop differently among various systems. To what extent can constrained openness contribute to a more coherent body of international law? Is the current approach as explained by constrained openness desirable? Is this the way the WTO should proceed and develop? The adjudicating bodies focus on the question whether WTO law has been violated but is this the right approach? Can one say that the WTO adjudicating bodies have a too narrow trade interest or are they indeed trying to close the gap between the WTO and other specialized legal systems, and in this way ‘legitimize’ the WTO? These questions remain unanswered in this book and it might take more practical cases to test the theory in order to come to that point.

In short, Yearwood has written an ambitious book with many valid points on the current legal debate on the interaction of WTO law with external international law. The author has clearly recognized and identified the complexity of the international legal system and its fragmentation. I am looking forward to further case examples on the interaction of international specialized systems analysed through the lens of constrained openness. As stated in the beginning, WTO law is a particularly relevant field of study, as trade cuts across many non-trade interests. If constrained openness can help to better reconcile those interests, Yearwood will have made an important contribution extending beyond theoretic debate. Hence, the book will be of particular interest to legal scholars, both from a trade perspective and from a broader public international-law perspective. Because of the theoretical elaborations it will be of less practical value to practitioners, but nevertheless worth a read as it forces the reader to focus on the structural legal challenges of WTO law – which is indispensable for the development of trade law in today's complex society.