14.1 Introduction

The reception of EU rules by national courts, the interplay between EU and domestic judicial actors, and the ensuing process of legal integration are central aspects of EU law as we know it today. Without the collaboration of domestic courts, the constitutional doctrines spelled out by the Court of Justice in foundational rulings would have remained dead letters. So too would the expanding body of EU directives and regulations churned out by EU institutions.

The judicial construction of Europe and the day-to-day operations of the EU multi-level court system have raised many intriguing questions. Nothing, initially at least, was self-evident about domestic courts embracing the agenda pushed by the Court of Justice, which was still a relatively obscure supranational body in the early 1970s. So what was the actual catalyst for judicial cooperation? Another puzzle relates to the depth and robustness of legal integration given the non-hierarchical structure of the EU multi-level judiciary. How far can integration go when domestic rulings cannot formally be appealed to the Court of Justice? How can the effectiveness of EU law be preserved when Court of Justice does not hold the power to strike down non-compliant domestic judgements?

Doctrinal scholars were the first to try and offer answers to these questions. Their perspective emphasised legal rules and their judicial exposition, typically within a normative perspective – for example, when the acte clair doctrine may legitimately be invoked to decline to submit a reference for a preliminary ruling. Much of this doctrinal work has come in the form of idiographic analyses of particular rulings or doctrines which do not aim to build, let alone test, theories, although some doctrinal constructions have been more ambitious, with theories of constitutional and legal pluralism an interesting case in point (Avbelj and Komárek Reference Avbelj and Komárek2012; Kumm Reference Kumm, Dickson and Eleftheriadis2012; MacCormick Reference MacCormick1999; Walker Reference Walker2002).

What initially spurred the development of empirical perspectives on the EU multi-level judiciary was the interest of political scientists and interdisciplinary-minded legal scholars in integration dynamics and the role the Court of Justice seemingly played in fostering the effectiveness of EU rule-making (Burley and Mattli Reference Burley and Mattli1993; Weiler Reference Weiler1994). This, in the 1990s, made the reception of EU law by domestic legal actors a popular testing ground for grand theories of integration, thereby turning it into the cradle of EU empirical legal studies. While empirical analyses have since expanded to other aspects of the EU’s legal architecture, a sizeable literature has accumulated over the years (some of the older literature is surveyed in Conant Reference Conant2007; Krehbiel et al. Reference Krehbiel, Gabel and Carrubba2017; Stone Sweet Reference Stone Sweet2010). Recent work has built off from previous efforts (Brekke et al. 2022; Dyevre and Lampach Reference Dyevre and Lampach2022) but also explored new avenues, including infra-national spatial variations in referral activity at provincial or regional level (Dyevre and Lampach Reference Dyevre and Lampach2021; Kelemen and Pavone Reference Kelemen and Pavone2018), the effect of workload (Dyevre et al. Reference Dyevre, Glavina and Atanasova2020) or the impact of the use of simplified judgments on the domestic judges’ willingness to submit references (Dyevre et al. Reference Dyevre, Lampach and Glavina2022).

This chapter takes stock of this scholarship through a critical lens. Empirical studies of the reception of EU law by domestic courts have undeniably delivered crucial insights, especially about the interlocutory procedure created by Article 267 TFEU. At the same time, though, the field suffers from substantial limitations at several levels and so there is plenty of room to do more and better deploying new empirical legal studies (ELS) methods and smart designs. One limitation arises from the tendency to operate with EU-specific theories instead of drawing from the broader, multidisciplinary litigation and judicial behaviour literature. A second limitation is more methodological. The methodological assumptions underpinning some of the most prominent empirical findings fail to address the so-called ‘credibility crisis’ in quantitative research (Angrist and Pischke Reference Angrist and Pischke2010). As we shall see, there has been far too little attention to the issues plaguing causal identification. Also, practical hurdles stemming from a combination of disparate national publication practices and the resources that parsing large collections of legal texts used to require means that we have at best a very patchy picture of what is happening outside the specific context of Article 267 TFEU.

To address these shortcomings, I suggest how the field may benefit from incorporating theoretical advances from research on judges in other contexts, a stronger emphasis on smart designs and experimental and quasi-experimental methods, and the deployment of data-crawling and Natural Language Processing (NLP) techniques.

14.2 Theories

An oft-cited article by the Czech-born American legal scholar Eric Stein published in 1981 paved the way for the subsequent development of behaviour-centred and, later, empirical approaches to EU law. Stein described the European judicial process as a ‘complex dialectic process’ involving multiple actors and groups, encompassing the judges of the Court of Justice, lawyers in the national ministries and the Commission Legal Service, judges on national courts, the attorneys appearing before them, as well as legal scholars and writers (Stein Reference Stein1981).

Stein hinted at the role of national courts as one of several aspects of legal integration left for future inquiry. The question was picked up a decade later by Joseph Weiler, when he undertook to explain the increasing willingness of national courts to cooperate with the Court of Justice, notably via the preliminary ruling mechanism. Weiler offered an explanation that stood in sharp contrast to the legalistic narrative then dominant in European legal discourse. ‘[N]ormative acceptance of the ECJ constitutional construct and practical utilization through the Article [267] process by national courts’, wrote Weiler, ‘may be rooted in plain and simple judicial empowerment. Has not power been the most intoxicating potion in human affairs?’ (Weiler Reference Weiler1994: 523). What lured domestic courts into acceptance was the prospect of acquiring prerogatives, such as the right to set aside parliamentary legislation, either inexistent in their legal system or which had remained the exclusive preserve of constitutional judges. Weiler’s empowerment theory posited that the incentive for cooperation would be strongest for the courts at the bottom of the judicial pecking order, since those were also the ones standing to gain most (Weiler Reference Weiler1991: 2426).

Karen Alter took Weiler’s insight about the differential institutional incentive of domestic judicial actors one step further. She saw rivalry among domestic courts as an additional factor in the interest calculus of domestic courts. This factor, she argued, gave lower courts the incentive to operate as the engine of legal integration:

Where Weiler seemed to view Community-law-driven empowerment primarily as a means to expand the courts’ power against the other branches of government, Alter’s court competition theory drew attention to the power dynamics inside national court systems.

The neo-functionalist theories articulated by Anne-Marie Slaughter and Walter Mattli (Burley and Mattli Reference Burley and Mattli1993; Mattli and Slaughter Reference Mattli and Slaughter1998) and, later, Alec Stone Sweet and Wayne Sandholtz (Stone Sweet and Sandholtz Reference Stone Sweet and Sandholtz1997) represented another prominent theorising effort. Borrowing insights from the neo-functionalist theory of regional integration advanced by Ernst Haas in the 1950s, legal neo-functionalists recast the infra-state actors as litigants, domestic judges, and, in some instances, even law professors. At the supranational level, the Court of Justice was viewed as the leading advocate of integration (Burley and Mattli Reference Burley and Mattli1993: 58). The Court’s constitutional doctrines conferred upon private individuals rights directly enforceable in domestic courts, effectively creating a pro-EU constituency with a direct stake in promulgation and enforcement of Community law (Burley and Mattli Reference Burley and Mattli1993: 60). According to Burley and Mattli, the preconditions for integration were met when domestic judges, actively courted by their supranational peers and under intensifying pressure from a small but committed group of Community law professors, eventually overcame their initial misgivings as they began to perceive the benefits of integration (Burley and Mattli Reference Burley and Mattli1993). The interplay of these actors kick-started a process of functional and political spillovers, resulting in the self-reinforcing expansion of the substantive domain of Community law and incremental shifts in the expectations of both Member State courts and governments (Burley and Mattli Reference Burley and Mattli1993: 67).

Though embracing most of Burley and Mattli’s theoretical assumptions, Wayne Sandholz and Alec Stone Sweet offered a more rigorous articulation of the hypothesised causal mechanisms, which also put greater emphasis on transnational exchange as the initial catalyst of integration (Stone Sweet and Sandholtz Reference Stone Sweet and Sandholtz1997: 306). Because of the Rome Treaty’s initial focus on the creation of a Common Market and the removal of trade barriers, those engaged in cross-border economic exchange would form the constituency with the strongest incentive to actively seek the enforcement of Community law. Sandholz and Stone Sweet specified the following basic causal mechanism: transnational exchange > litigation > dispute resolution. Assuming that dispute resolution through the Court of Justice would, by removing national regulatory barriers, in turn foster transnational exchange, they added the feedback loop: dispute resolution > transnational exchange. The assumption that the growth in transnational exchange would spur demands for supranational regulation, which in turn would serve as basis for more litigation, motivated the specification of an additional causal pathway: transnational exchange > legislation > litigation. The hypothesised set of interlocked pathways provided a clear, potentially testable account of how the self-reinforcing dynamics of cross-border exchange, litigation, dispute resolution, and supranational rule-making would work to propel and self-reinforce the integration process.

In their ambitious theorising efforts, Stone Sweet and Sandholtz, although more rigorous than most of what had preceded it, were surprisingly short on detail regarding the motives driving domestic judges to collaborate and supply cases to the Court of Justice. In a related contribution, though, Alec Stone Sweet and Thomas Brunell provided some elaborations of the motives which, they assumed, spur Member State courts to apply Community law and refer cases to the Court of Justice. De-emphasising the logic of empowerment, they argued that efficiency concerns had more explanatory power:

If anything, Stone Sweet and Brunell were even more sceptical vis-à-vis Alter’s court competition theory, which, they argued, overlooked the division of labour inherent in national court systems: because a core function of appellate judging is to resolve disputes involving legal interpretation and conflict of law, we would expect the appellate courts to be far more involved in the construction of the legal system than Alter imagines them to be (Stone Sweet and Brunell Reference Stone Sweet and Brunell1998b: 90).

Some scholars approached the EU legal regime from the perspective of international relations. They typically paid scant attention to domestic judicial dynamics (Carrubba Reference Carrubba2005; Carrubba and Gabel Reference Carrubba and Gabel2015; Garrett and Weingast Reference Garrett, Weingast, Goldstein and Keohane1993; Moravcsik Reference Moravcsik1998). A very different line of research drew on the work of French sociologist Bourdieu to investigate the emergence of an EU-law field comprising law professors, judges, and legal advocates as a crucial force in the consolidation of the Court of Justice of the European Union (CJEU)’s authority and power (Cohen and Vauchez Reference Cohen and Vauchez2011; Vauchez Reference Vauchez2016; see also the pioneering work of Schepel and Wesseling Reference Schepel and Wesseling1997). Until the 2010s, though, the literature exhibited a marked tendency to treat EU judicial dynamics as a sui generis phenomenon without meaningful equivalents in the rest of the world. Rare were the contributions drawing on or explicitly seeking to contribute to research on judicial behaviour conducted in other contexts.

The account of inter-judicial cooperation set forth by Stone Sweet and Brunell, incomplete as it may be, bears some affinity with the team model of adjudication elaborated by the American law and economics scholar Lewis Kornhauser (Kornhauser Reference Kornhauser2012). The team model emphasises the desire of resource-constrained judges to maximise the number of ‘correct’ case resolutions. Judicial work is presumed to consist of three basic tasks: fact-finding, law-finding, and law-creation. These tasks are typically divided among the higher and lower echelons of the judicial hierarchy. Lower courts specialise in fact-finding but rely on the guidance of higher courts for law-finding and law creation. Rather than as manifestations of ideological divergence, appeals and reversals are themselves viewed as expression of informational asymmetry as appellate courts apply their greater resources to correct the legal and factual errors of the lower level (Kastellec Reference Kastellec2017; Kornhauser 1999). Franciso Ramos Romeu was the first to apply this model to the reception of EU law by national courts in a little-noted contribution (Romeu Reference Romeu2006). Dyevre et al. (Reference Dyevre, Glavina and Atanasova2020) also draw on the team model of adjudication. They argue that, since requests for a preliminary ruling to the CJEU may only address points of law, higher courts will be more active interlocutors of the CJEU within the preliminary ruling mechanism. They further supplement this division-of-labour argument with a theoretical elaboration of the work–life balance incentives arising from variations in the judges’ workload, thereby drawing on the labour market theory of judging developed by Epstein et al. (Reference Epstein, Landes and Posner2013) in the US context. To the extent that writing a reference for a preliminary ruling typically adds to a judge’s workload, a larger caseload increases the opportunity cost of submitting a preliminary reference.

In parallel with greater reception of theoretical advances arising from research on domestic courts, there have been attempts to develop a more general understanding of compliance partnerships involving domestic and international adjudicators to allow meaningful comparisons between the European experience and other legal regimes such as the Andean Community and the Inter-American Convention of Human Rights (Alter and Helfer (Reference Alter and Helfer2010) and Huneeus (Reference Huneeus2011)).

Overall, the empirical literature on the domestic dimension of EU law still draws on a somewhat restricted set of theoretical perspectives, even compared to other aspects of the EU legal regimes, such as the relationship between the Court of Justice and other EU decision-makers. Aside from policy goals and strategic considerations, the broad, multidisciplinary field of judicial behaviour research is now considering a much wider spectrum of influences on judging. These include the cognitive heuristics and biases affecting how judges evaluate facts, evidence, and legal arguments (Guthrie et al. Reference Guthrie, Rachlinski and Wistrich2007; Spamann et al. Reference Spamann, Klöhn and Jamin2021; Wistrich et al. Reference Wistrich, Guthrie and Rachlinski2005). Deserving mention, too, is a growing literature attempting to determine the extent to which legal rules actually constrain judicial decision-making (Bartels and O’Geen Reference Bartels and O’Geen2015; Fischman Reference Fischman2021; Pang et al. Reference Pang, Friedman, Martin and Quinn2012; Richards and Kritzer Reference Richards and Kritzer2002).

These theoretical innovations have found little echo in EU ELS. The flipside is that, going forward, there is considerable room to enrich the field’s theoretical understanding of judicial conduct in the EU court system and to spell out new hypotheses for future research. Although controversial judicial policies occasionally originate in seemingly mundane cases, comprehensive content analyses of CJEU cases indicate that they mostly pertain to technical issues such as VAT, tariff nomenclatures, and the mutual recognition of driving licenses (Dyevre and Lampach Reference Dyevre and Lampach2021a). This suggests that the judges who pass on cases to the CJEU are not primarily driven by competition with other courts or the desire to escape their domestic political constraints but, more plausibly, by the desire to satisfy prevailing norms of ‘correct’ dispute resolution which require that judges minimise legal errors (Kastellec Reference Kastellec2017; Kornhauser 1999). On that account, the CJEU may be seen as having developed such a degree of expertise that national courts, especially higher courts specialising in law-finding and law-creation, see it as efficient to refer legal questions. While reflections along these lines can be found here and there in the literature (see Dyevre et al. Reference Dyevre, Glavina and Atanasova2020; Dyevre et al. Reference Dyevre, Lampach and Glavina2022; Romeu Reference Romeu2006; Stone Sweet and Brunell Reference Stone Sweet and Brunell1998b), a systematic theorisation of judicial decision-making in routine cases is still largely missing from its overall picture of judicial decision-making.

Even in salient cases, and despite efforts to theorise overt defiance (Dyevre Reference Dyevre2016a, Reference Dyevre2016b), more attention should be given to the principal–agent problems potentially stemming from the absence of an appeal procedure between domestic and EU courts. In appeal-structured judicial hierarchies, losing parties have the strongest incentive to appeal when they expect that the appeal court will come to a different determination. This litigant selection effect help detect instances of agency drift, that is, decisions by judicial subordinates deviating from the preferences of judicial superiors (Cameron and Kornhauser Reference Cameron and Kornhauser2005). By contrast, in a system structured around an interlocutory procedure like the preliminary ruling procedure, such litigant selection effect does not exist. On the contrary, since preliminary references are submitted at the behest of domestic courts, they have stronger incentives to refer legal questions when they expect to like the CJEU’s answer. This implies that judicial misalignment between EU and national courts might be substantially more widespread than if the EU judiciary were structured around an appeal mechanism.

Also, susceptibility to cognitive biases and their impact on responsiveness to EU law arguments in briefs and pleadings or willingness to submit references may differ widely depending on the workload and resources of EU and domestic judges. The cognitive strain experienced when encountering unfamiliar EU law arguments, for example, may spur domestic judges, particularly those with large caseloads, to prioritise domestic law arguments. Knowing this, litigants also have the incentive to deploy domestic law arguments to which judges will be more receptive, thereby favouring a domestic law equilibrium. Research along these lines would help better understand obstacles to the reception of EU law in domestic judiciaries.

To sum it up, these theoretical perspectives, which were born in different corners (economics, political science, psychology) of the social science community, offer promising ways to renew how we think about the reception of EU law in the domestic realm. Ultimately, though, they will only prove fruitful if accompanied by improvements in methodology and data collection.

14.3 Data: Existing Limitations and Potential Fixes

Until the late 1990s, researchers relied principally on qualitative evidence – semi-structured interviews, case studies, and small, non-random samples of CJEU and national court rulings – to back their narratives of a field where integration was still the central research question (Alter Reference Alter, Slaughter, Stone Sweet and Weiler1998; Burley and Mattli Reference Burley and Mattli1993; Weiler Reference Weiler1991; see also Naurin, Lindholm, and Schroeder, Chapter 2 in this volume).

While preceded by Golub (Reference Golub1996), the efforts of Stone Sweet and Brunell to collect and encode data on referral activity (Stone Sweet and Brunell Reference Stone Sweet and Brunell1998b, Reference Stone Sweet and Brunell1998a) marked a major quantitative turn. Subsequently, referral rates have served as the primary source of empirical evidence to claim or deny support for the neo-functionalist hypothesis (Hornuf and Voigt Reference Hornuf and Voigt2015; Lampach and Dyevre Reference Lampach and Dyevre2017; Stone Sweet and Brunell Reference Stone Sweet and Brunell1998a); to criticise Alter’s court competition hypothesis (Dyevre et al. Reference Dyevre, Glavina and Atanasova2020; Stone Sweet and Brunell Reference Stone Sweet and Brunell1998b); to document the effects of workload and hierarchy implied by team and labour market models of adjudication (Dyevre et al. Reference Dyevre, Glavina and Atanasova2020); and to assess the influence of monist and dualist legal traditions on the domestic reception of EU law (Carrubba and Murrah Reference Carrubba and Murrah2005).

Most studies using Article 267 referral data aggregate the data at the country-year level, which, given the level of heterogeneity among Member States and the strong temporal trend in the data, severely limits its usefulness for testing causal hypotheses (see discussion in Section 14.4). More recent studies, starting with Kelemen and Pavone (Reference Kelemen and Pavone2016), break down countries in subnational units, which does represent an improvement but still constrains the ability of researchers to isolate the motives of courts and individual judges from the influence of litigants and other factors. The GEOCOURT Dataset (Dyevre and Lampach 2021b) and the National Courts component of the CJEU IUROPA Database (Brekke et al. Reference Brekke, Fjelstul, Lyder Hermansen and Naurin2023) have data collected at court level but they do not compile information on the courts that have never interacted with the CJEU via the preliminary ruling procedure. Mayoral and Wind (Reference Mayoral and Wind2022) conduct a court-level analysis but restrict the scope of their study to a subset of sixty-eight high courts. Likewise, Dyevre et al. (Reference Dyevre, Lampach and Glavina2022) only consider the courts that have submitted at least one reference at some point in time.

The problem with studying only the courts that participate in the preliminary ruling mechanism is that it may hide a major selection effect: it is quite possible that the courts that do not send reference to the CJEU differ fundamentally from the courts that do. Unless resources are invested in the construction of a comprehensive database identifying all judicial bodies across the EU, from the most modest first instance tribunal up to appellate courts, this selection effect will continue to inhibit progress on the domestic dimension of EU judicial decision-making.

Going one step further in investigating domestic judicial dynamics would necessitate judge or case-level data. Even at supreme court level, there has been little effort to compile information on panel composition over time, although the datasets assembled by Hamann (Reference Hamann2019) and (Lindholm et al. Reference Lindholm, Derlén and Naurin2022) on, respectively, the German and Swedish supreme court judges represent important exceptions.Footnote ¹ Garnering systematic case-level data on EU law outside the preliminary ruling mechanism is made arduous by the sheer quantity of domestic rulings as well as by linguistic barriers and vast disparities in reporting practices and online availability across courts and Member States. There has been no comparable attempt to Chalmers (Reference Chalmers2000)’s manual annotation of all reported judgments in UK over the period 1971–1998. The Dec.Nat. database covers a large number of cases and courts, going back to 1959 (Hübner Reference Hübner2015) but data collection did not follow explicit, replicable criteria, making its exploitation for quantitative analysis questionable at best. Proxies for EU law use, such as citations to Court of Justice rulings or EU law acts (see, e.g., Romeu Reference Romeu2006), although easier to extract from the mass of case reports, are no real substitutes. Not only is their frequency likely to be distorted by norms governing opinion writing, but they do not capture the nature and direction of citations.

US researchers have collected information on millions of federal appeal court and state supreme court cases, allowing the investigation of behavioural hypotheses about judges’ cognitive biases and attitudes (Ash and MacLeod Reference Ash and Bentley MacLeod2015; Ash et al. Reference Ash, Chen and Lu2018, Reference Ash, Chen and Galletta2022). Getting anywhere close to this mark in the EU context will necessitate an unprecedented data-collection effort. However, advances in NLP and data-crawling methods along with increasing ability to access digitalised legal databases via Application Programming Interfaces (APIs) shall greatly facilitate this enterprise (Dyevre Reference Dyevre2021; see also Ovádek in this volume). NLP methods are increasingly able to operate across languages, and supervised classification models trained in one language show convincing performance on unseen documents written in another (Artetxe and Schwenk Reference Artetxe and Schwenk2019; Chalkidis et al. Reference Chalkidis, Fergadiotis and Androutsopoulos2021). Even with a large sample of human-labelled documents, though, training a model to detect EU law use in domestic rulings is not a trivial task. The baseline rate at which domestic rulings refer to EU law appear low. It is probably well below 5 per cent of all reported domestic decisions (and this rate is likely much lower even in some Member State judiciaries). This entails that a classifier with 95 per cent sensitivity (i.e., correctly labelling 95 per cent of the cases that do refer to EU law) and 90 per cent specificity (i.e., correctly labelling 90 per cent of the cases that do not refer to EU law) would still produce many false positives. In fact, assuming a 5 per cent baseline prevalence (which, in most Member States, is likely to be the upper bound of visible EU law use), a case classified as referring to EU would only have a 33 per cent probability of truly referring to EU law.Footnote ²

14.4 Methods: Promise and Pitfalls

EU empirical scholars have deployed a variety of methods to study dynamics on and around the reception of EU law by national courts.

Case studies constitute an important source of knowledge about the EU multi-level legal system, as shown by studies examining landmark rulings such as Van Gend en Loos, Cassis de Dijon and Costa v. ENEL (Arena Reference Arena2019; Alter and Meunier-Aitsahalia Reference Alter and Meunier-Aitsahalia1994; Dyevre Reference Dyevre2011; Ovádek Reference Ovádek2022; Rasmussen Reference Rasmussen2014). Much of the best and most insightful work on the domestic reception of the Court of Justice’s constitutional jurisprudence in Germany, France, and Italy relies on case studies and qualitative analysis, including semi-structured interviews (Alter Reference Alter2001; Davies Reference Davies2012; Pavone Reference Pavone2022). Some seminal work on the role of professional networks in the emergence of a CJEU-supporting EU law field (Cohen and Vauchez Reference Cohen and Vauchez2011; Vauchez Reference Vauchez2016) similarly relies on largely qualitative analysis. By applying qualitative interview methods EU empirical scholars have also generated important insights about the beliefs and attitudes of domestic judges in Poland, Germany, the Netherlands, Slovenia, Sweden, and Croatia towards EU law and engagement with EU courts (Glavina Reference Glavina2021; Jaremba Reference Jaremba2012; Leijon Reference Leijon2024; Nowak et al. Reference Nowak, Amtenbrink, Hertogh and Wissink2011).

Quantitative descriptive studies have also made significant contributions to our understanding of the domestic dimension of legal integration. While prominent theories, including Weiler’s empowerment and Karen Alter’s court competition theory emphasised the role of lower courts (Alter Reference Alter, Slaughter, Stone Sweet and Weiler1998, Reference Alter2001; Weiler Reference Weiler1991), descriptive analyses of referral activity have shown that higher courts do in fact account for the bulk of references (Gerring Reference Gerring2004). This is true in relative terms (higher courts are far less numerous) and, since the turn of century, in absolute terms too (Dyevre et al. Reference Dyevre, Glavina and Atanasova2020; Stone Sweet and Brunell Reference Stone Sweet and Brunell1998b). Likewise, Dan Kelemen and Tomasso Pavone singled out the geographic concentration of referrals in France by comparing French courts to Italian and German ones (Kelemen and Pavone Reference Kelemen and Pavone2018). Examining data for all Member States, Dyevre and Lampach, however, later showed that centralisation is the common pattern across the EU (Dyevre and Lampach 2021b). Large-scale descriptive accounts in the form of thematic attention in cases involving EU law can also be informative. Dyevre and Lampach (Reference Dyevre and Lampach2021a), for instance, apply topic modelling and other text-mining techniques to compare issue attention across preliminary ruling, infringement, and annulment proceedings.

Empirical methods come with their strengths and weaknesses. Case studies and qualitative methods are well suited for exploration (especially when there are many unknown unknowns) to map the institutional environment or to illustrate a process or a mechanism. Their limitations are also well discussed. One is external validity. A case study describes what is in essence a single data point and the danger exists that too much is inferred from an overfitting description. Nor do case studies provide the strongest basis to test theories, unless the theory solely proffers existential propositions (Gerring Reference Gerring2004; King et al. Reference King, Keohane and Verba2021). Selection effects and even the aspects that the researcher picks out for description may induce a confirmation bias – particularly if the theory’s development was itself motivated by the case under study.Footnote ³

This said, making valid causal inferences with quantitative methods is no easy feat either, even when the data is complete and accurately measured (see the recent discussion in empirical constitutional studies (Chilton and Versteeg Reference Chilton and Versteeg2021; Petersen and Chatziathanasiou Reference Petersen and Chatziathanasiou2021). Looking back, early quantitative studies seem methodologically naïve. Stone Sweet and Brunell (Reference Stone Sweet and Brunell1998a), for instance, claimed to have found evidence for neo-functionalism in the strong correlation between intra-EU trade and referral activity. Their paper reported regression results with a cross-section of eleven Member States (taking average annual trade and references) and a cross-pooled analysis with only one control (with N = 246 and staggering adjusted explained variance R² = 0.77!). Their analysis ignored concerns of reversed causality and omitted variable bias as well as dependencies between observations (the same Member States are observed over time, not multiple random samples). CJEU rulings may stimulate trade, thus reversing the direction of causality (a point made by Pitarakis and Tridimas Reference Pitarakis and Tridimas2003), while Member State courts and judges differ in many observed and non-observed ways which might plausibly affect the rate at which courts use the preliminary ruling procedure.

Models reported in subsequent publications adopted assumptions better suited to the data structure (e.g., panel data analysis with negative binomial distribution for referral activity per country over time) and often included many more controls for potential confounders (see, e.g., Carrubba and Murrah Reference Carrubba and Murrah2005; Kelemen and Pavone Reference Kelemen and Pavone2016). Some cross-country studies report models with country and year fixed effects (e.g., Kelemen and Pavone Reference Kelemen and Pavone2018) which go some ways towards capturing unobserved country-specific time-invariant effects as well as unobserved short-term events and shocks affecting entire cross-sections of countries. However, while this mitigates the problem of causal identification, it does not definitely solve it. Referral rates, for example, exhibit a strong upward trend without pronounced peaks and troughs. This means that any variable exhibiting a temporal trend is bound to be (either positively or negatively) correlated with referral activity. For this reason we should take claims about the effect of EU-law specialised firms or the intensity of cross-border trade (as measured by EU trademark registrations) on referral rates (Kelemen and Pavone Reference Kelemen and Pavone2018; Lampach et al. Reference Lampach, Wijtvliet and Dyevre2020) with a grain of salt.

Problems of this sort have precipitated the credibility crisis in economic research (Angrist and Pischke Reference Angrist and Pischke2010). Because randomisation ensures that treatment and control groups share similar observed and non-observed characteristics, making causal identification comparatively neat and straightforward, the upshot of the methodological debate has been to elevate the randomised controlled experiment, whether in the field or in the lab, to the gold standard for causal identification (Rubin Reference Rubin2007).

The credibility crisis did not sound the death knell of observational studies. Instead, it has spurred efforts to develop smart research designs. Researchers have increasingly turned to natural and quasi-natural experiments (DiNardo Reference DiNardo, Durlauf and Blume2010), instrumental variable estimation (Angrist et al. Reference Angrist, Imbens and Rubin1996) and methods that mimic experimental settings by constructing control groups and synthetic counterfactuals (Abadie Reference Abadie2021; Abadie et al. Reference Abadie, Diamond and Hainmueller2015; Sekhon Reference Sekhon2009; Stuart Reference Stuart2010). Dyevre et al. (Reference Dyevre, Glavina, Lampach, Ovádek and Wijtvliet2018), for instance, seek to estimate the impact of the Brexit referendum on British courts by creating a synthetic UK judiciary to serve as counterfactual.

However, such methods are still far from being mainstream. Inevitably, as methodological standards for causal identification improve, many past findings will be revisited and questioned. Ultimately, this shall help place our understanding of EU judicial behaviour on stronger empirical foundations.

14.5 Conclusion

The literature surveyed in this chapter has substantially advanced our understanding of how domestic courts engage with EU law, particularly through the lens of the preliminary ruling mechanism under Article 267 TFEU. Empirical research has mapped out patterns of interaction between national judiciaries and the CJEU, revealing valuable insights into the dynamics of judicial cooperation and conflict within the EU’s multi-level legal system. By analysing referral activity, scholars have illuminated the roles that institutional incentives, court hierarchies, and legal traditions play in shaping the reception of EU law across Member States.

Yet the empirical literature remains constrained by EU-specific theoretical frameworks, often overlooking the broader disciplinary advances in judicial behaviour and institutional analysis. This insularity limits the field’s ability to generate comparative insights and test hypotheses that might apply beyond the EU context. Methodologically, the literature has not always lived up to the highest standards, notably with respect to causal inference, casting doubt on some prominent findings.

While the study of legal integration can also be viewed as the field’s specific contribution to judicial behaviour research, there is considerable room to enrich the theoretical debate with insights from research on judicial hierarchies, jurisprudential regimes, or the influence of cognitive biases. Research shall also benefit from a greater emphasis on randomisation, natural experiments, and synthetic control methods. Data is still a challenge. But as this chapter has tried to show, there are solutions there too. NLP, machine learning, and web-crawling methods promise to improve data collection and facilitate research on the domestic dimension of the EU court system.