Why this debate matters now

The European Union’s legal architecture rests on the supremacy of Union law, articulated through the doctrines established by the European Court of Justice (ECJ) in Costa v. ENEL (1964) and Simmenthal (1978). These doctrines enshrine a positivist approach to law, whereby legal validity derives from formal sources rather than cultural or customary legitimacy. Yet, within the EU’s diverse membership, national legal cultures continue to preserve elements of customary law. Such elements are visible in land tenure practices, in family and inheritance rules where long-standing customary norms still shape marital property or succession regimes, in community-based systems of resource management such as fisheries and forests, and in constitutional traditions where courts invoke the idea of “constitutional identity”—for example, the German Federal Constitutional Court’s reliance on Verfassungsidentität—to safeguard historically rooted customs and principles regarded as essential to the national constitutional order. The persistence of these customary influences highlights a structural tension within the EU’s legal system.

This executive summary distills a longer academic study that proposes Relative Universalism as a workable middle path between strict universalism and cultural relativism. The core claim: Europe can preserve universal commitments—human dignity, equality, rule of law—while recognizing context-sensitive applications rooted in legitimate local traditions.

In this article, customary law is understood in its legal sense as the body of rules and practices that derive their binding force from long-standing usage and social acceptance, rather than from formal legislative enactment. While it often operates in conjunction with codified norms, customary law remains a recognized source of legal authority in many European jurisdictions, particularly in areas such as land tenure, inheritance, family-related law (such as 1. inheritance and succession e.g., customary preference for male heirs or first-born sons, which has historically influenced property distribution rules 2. marital property regimes: e.g., customs about dowry, joint property, or separate property in marriage, sometimes recognized in local law 3. Guardianship and parental authority: e.g., traditions regarding the father’s or extended family’s role, which may still inform interpretation in certain jurisdictions.), and communal resource management (Zweigert & Kötz, 1998; Watson, 2001; Helmersen, 2018; Popa, 2013). This understanding forms the basis of our analysis, distinguishing customary law from mere social convention and situating it as a continuing component of national legal systems that may come into tension with the positivist framework of EU law.

Problem statement and the proposed avenues towards resolution

The ideas proposed center upon the problematic summed up in the following question: How can the EU reconcile the positivist supremacy of Union law with the enduring role of member states’ customary laws—without sacrificing either integration or legitimacy?

1. Positivist EU Law: Legal validity flows from formal sources; supremacy and direct effect are essential for coherence and predictability (Kelsen, 1967; Hart, 1994; Costa v. ENEL, 1964; Van Gend en Loos, 1963).
   
   
2. Customary Law: Law as cultural practice—longstanding, embedded, and often locally efficient (Zweigert & Kötz, 1998; Watson, 2001; Helmersen, 2018; Popa, 2013).
   
   
3. Constitutional/Legal Pluralism: Europe as a multi-level constitutional order already juggling sovereignty claims (MacCormick, 1999; Walker, 2002; Kumm, 2005).
   
   

The gap: Pluralist accounts often describe coexistence but rarely justify a stable normative balance between the EU’s need for uniformity and communities’ need for cultural legitimacy.

Relative Universalism: the bridge

Relative Universalism argues that a limited set of universal principles is indispensable for order and rights protection, but their instantiation must be context-aware. In legal terms, that means upholding EU-wide benchmarks while permitting calibrated, transparent, and reviewable variation in how those benchmarks take form across different legal cultures (Bilgin & Yaycı, 2025). Think of it as “universal ends, contextual means.” The ends—dignity, equality, rule of law—remain non-negotiable. The means—legislative design, transposition pathways, remedial structures—can adapt to historically legitimate and contemporarily institutionalized practices, provided they converge on the ends and do not undercut them.

Relative Universalism, therefore transforms conflict into dialogue: EU law establishes the robust foundational framework of binding principles —a floor that secures the Union’s authority and coherence—while local customs contribute legitimacy, knowledge, and compliance pathways that adapt and enrich this framework in line with diverse legal cultures.

Where conflict bites

Addressing these issues, the bottom line is that Europe faces a recurring choice between legal certainty and cultural legitimacy. Picking one over the other is short-sighted; lasting integration needs both. Here are a few contexts for this idea:

1) Supremacy vs. constitutional identity

Supremacy is vital to avoid fragmentation, but courts like Germany’s BVerfG insist on Verfassungsidentität as a principled limit (BVerfG, 1974; 1986; 2020). Relative Universalism treats this as a legitimate safeguard rather than rebellion when it protects universal ends through local constitutional means.

2) Direct effect vs. private-law custom

Direct effect empowers citizens, but can collide with customary land tenure or communal resource practices, particularly in Mediterranean contexts. A binary “override” model risks compliance deficits and social backlash (Watson, 2001).

3) Uniform directives vs. family/religious traditions

EU equality directives are indispensable for universal rights, yet abruptly displacing customs in inheritance or marital regimes can undermine legitimacy and slow social change. Sequenced, dialogical adaptation sustains both equality and buy-in.

4) Environmental quotas vs. community stewardship

Nordic customary rights in fisheries or communal forests often align with sustainability, yet formal quotas may ignore embedded stewardship and erode local cooperation (Helmersen, 2018).

What Relative Universalism changes in practice

1. Reinterpreting supremacy: Not as absolute uniformity, but as “framework supremacy” consistent with subsidiarity and proportionality, which already signal EU respect for competences and necessity (TEU; TFEU; Charter);sSupremacy holds on ends, while means may vary within principled limits (European Union, 2012a, 2012b, 2012c).
   
   
2. Using directives as flexibility engines: Directives already allow discretion in transposition. Relative Universalism makes that discretion principled—context-sensitive adaptations are welcome if they converge on universal ends and survive proportionality review.
3. Importing a calibrated “margin of appreciation” logic: The European Court of Human Rights has long used a margin of appreciation to respect diversity while protecting core rights (Handyside v. UK, 1976; Letsas, 2006). An EU-tailored version can offer structured leeway for customary practices without abandoning uniform benchmarks.
4. Embedding custom in proportionality: Customary stewardship in fisheries/forestry can become evidence in proportionality analysis—not an exception, but a means to the EU’s sustainability ends, where local knowledge improves regulatory fit.
5. From zero-sum to dialogue: National resistance should not be reflexively treated as disobedience; it can be signal—a plea for calibrated pathways that uphold universal rights with rather than against local legitimacy.

Three illustrative case clusters

Germany’s Solange and PSPP

The BVerfG’s line—from Solange I and II to PSPP—insists the EU order must protect fundamental rights comparably to the Basic Law and remain within conferred competences (BVerfG, 1974; 1986; 2020). In Relative Universalism terms, this is a guardrail: it keeps the EU’s universal ends intact while demanding demonstrable justification and proportionality.

Mediterranean land and water customs

Centuries-old practices of communal grazing, irrigation, or iterative land-use rights can clash with EU competition or environmental rules. The proposal is not to exempt these customs, but to integrate them into tailored transposition and impact assessments so policies converge on EU aims with local legitimacy (Watson, 2001).

Family and inheritance norms vs. equality

Equality is non-negotiable. But reform that phases in changes, co-designs remedies with communities, and builds local jurisprudence can accelerate genuine compliance and reduce backlash (Letsas, 2006).

Institutional and policy implications

For the Commission

• Differentiated transposition guides: Publish model pathways showing how diverse customary regimes can lawfully meet directive objectives.
• Impact and legitimacy audits: Add a “customary law and legitimacy” module to impact assessments; require community consultation where customs are salient.
• Capacity building: Fund legal-technical interfaces (clinics, advisory panels) to help administrations translate EU ends into viable local means.

For the ECJ and national courts

• Structured proportionality with custom as evidence: Treat coherent customary regimes as prima facie relevant in the suitability/necessity/balancing stages—never to dilute ends, but to improve fit.
• Dialogic reference procedures: Encourage iterative preliminary references on custom-sensitive questions; consider reasoned margins where universal ends are fully protected.

For the Council and Parliament

• Recitals that anticipate diversity: Where customs are foreseeable, legislate explicit room for contextualization plus evaluation criteria for convergence on ends.

• Periodic review clauses: Build in review points to replace blunt uniformity with learning-based harmonization.

For member-state administrations

• Map customs that are governance-relevant (land, water, fisheries, family); identify where they advance or hinder EU ends.

• Design transition plans that sequence reform, pairing legal change with education, local jurisprudence building, and monitoring.

What this buys the Union

1. Deeper compliance: Laws that resonate are obeyed; contextualization builds ownership.
2. Democratic legitimacy: The EU’s so-called democratic deficit shrinks when people recognize themselves in the law (Weiler, 1991).
3. Resilience without fragmentation: Pluralism, if principled, becomes a strength rather than a risk (Walker, 2002; Kumm, 2005).
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Five-point checklist for a Relative-Universalist EU measure

1. Universal end clearly stated (e.g., equality, sustainability, market fairness).
2. Context pathways specified (acceptable variants in implementation).
3. Proportionality test operationalized (custom as evidence for suitability and necessity).
4. Measurement and review (benchmarks, timelines, learning loops).
5. Judicial dialogue channel (structured preliminary references; reason-giving for margins).

The practical philosophy of integration

The EU must never abandon its universal rights architecture. But it equally must not ignore the cultural infrastructures that give law meaning and traction. Relative Universalism’s promise is precisely this double fidelity: fidelity to principles and fidelity to people.

Seen this way, constitutional identity claims are not exit ramps but feedback loops; customary stewardship is not an obstacle but a knowledge asset; family-law traditions are not immutable barriers but living practices that can evolve toward equality through engagement rather than erasure.

Integration endures when it persuades. A Europe that insists on universal ends and respects context will integrate more completely—because it integrates honestly.

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Adem Bilgin is the chair of Association of Digital Ecosystem Governance and Development Research in Ankara and also an environment negotiator in the Ministry of Agriculture and Forestry. He was the chief negotiator of Türkiye to UNCBD between 2008 and 2020. He was the vice president of European Landscape Convention of the Council of Europe between 2011 and 2013, and also, he is one of the founder Bureau members of Intergovernmental Platform on Biodiversity and Ecosystem Services representing East Europe.  He has B.S. in biology and M.S. in Bioeconomics and M.S. in Political Sciences and International Relations. He worked in Turkish government, EU (EEA) and UN (FAO).