Published On: October 15, 2026
Authored By: Priscille Mabwa Mbongo
Abstract
The growing use of non-binding international instruments has raised an enduring question in international law: when, if ever, does soft law acquire the normative force of hard law? This article examines the role of United Nations General Assembly resolutions, declarations, guidelines and other non-binding instruments in the identification and development of customary international law. It argues that soft law does not become legally binding merely through repetition, political endorsement or widespread acceptance. Rather, its legal significance depends on its interaction with the established requirements of customary international law, particularly State practice and opinio juris. Drawing on the jurisprudence of the International Court of Justice, the article examines how non-binding instruments may provide evidence of legal conviction, influence the interpretation of existing norms, and contribute to the progressive development of international law. It ultimately proposes a contextual approach that recognises the normative value of soft law while preserving the distinction between political commitment and binding legal obligation.
Keywords: soft law; customary international law; opinio juris; State practice; United Nations General Assembly resolutions.
I. Introduction
International law is increasingly shaped by instruments that deliberately operate below the threshold of formal legal obligation. United Nations General Assembly resolutions, declarations, guidelines, principles and political commitments may lack binding force, yet they can influence how States articulate expectations, justify conduct and respond to emerging legal questions.[1] This creates a difficult doctrinal boundary: if an instrument is expressly non-binding, by what process can it contribute to the formation of a rule that is binding as customary international law? The answer cannot rest simply on repetition or political endorsement. Customary international law requires both general State practice and acceptance of that practice as law (opinio juris), and the evidentiary significance of international organisations’ resolutions must therefore be assessed within, rather than outside, that framework.[2]
This article asks when and to what extent non-binding international instruments can contribute to the identification and development of customary international law. It adopts a doctrinal and critical methodology, examining primary sources, particularly the International Law Commission’s conclusions on customary international law and the jurisprudence of the International Court of Justice, alongside scholarship on soft law and international norm formation.[3] It argues that soft law should be understood neither as disguised hard law nor as legally irrelevant political expression. Rather, its principal significance lies in its capacity to provide evidence of opinio juris, influence State practice and facilitate normative development.[4] The article first examines the place of soft law within international law, then analyses its relationship with customary law and judicial practice, before assessing when normative influence acquires legal significance and the legitimacy risks of treating soft law as an autonomous source of obligation.
II. Soft Law and the Sources of International Law
2.1 What Is ‘Soft Law’?
‘Soft law’ is generally used to describe international instruments that influence conduct or expectations without possessing the formal binding force of treaties or customary international law.[5] The category includes United Nations General Assembly resolutions, declarations, principles, guidelines, codes of conduct and political commitments. Their legal character is contested precisely because their practical influence may exceed their formal status. Abbott and Snidal explain soft law partly through the deliberate choice of States to formulate commitments that are less precise, less obligatory or less enforceable than traditional legal rules.[6] Boyle similarly cautions against treating soft law as legally irrelevant: non-binding instruments may shape behaviour, clarify expectations and contribute to the emergence or interpretation of legal norms.[7]
The difficulty, however, lies in distinguishing normative influence from legal obligation. A General Assembly resolution may express a substantial degree of international agreement, but the fact that States have endorsed a text does not, without more, establish that they regard its content as legally binding. As Droubi observes, the role of United Nations instruments in the formation of customary international law must therefore be assessed through their relationship with State practice and opinio juris, rather than through their institutional form alone.[8] Soft law is consequently better understood as part of the evidence and process of norm development, rather than as an additional autonomous source of law.
2.2 Where Does Soft Law Fit within the International Legal Order?
This distinction becomes clearer under Article 38(1) of the Statute of the International Court of Justice, which identifies international conventions, international custom and general principles of law as sources to be applied by the Court, while treating judicial decisions and scholarly writings as subsidiary means for determining rules of law.[9] Soft law does not appear as an independent category within this framework. Its significance therefore depends upon whether, and how, a particular instrument contributes evidence relevant to an established source.
Accordingly, soft law is not synonymous with customary international law. Even repeated resolutions cannot automatically transform political agreement into legal obligation. The decisive question remains whether the underlying conduct and legal conviction of States satisfy the requirements of custom.[10]
III. The Path from Soft Law to Customary International Law
3.1 The Two Elements of Custom
The starting point is that customary international law cannot be created simply by declaring that a particular principle ought to be legally binding. Under the approach reflected in the International Law Commission’s 2018 Conclusions, the existence of a customary rule requires a general practice that is accepted as law (opinio juris).[11] The two elements are analytically distinct, although they may be established through overlapping evidence. In North Sea Continental Shelf, the International Court of Justice emphasised that practice must be sufficiently extensive and representative, while the conduct must occur in circumstances indicating a belief that the practice is legally required.[12] Thus, the relevant question is not merely how many States support a proposition, but what States do and the legal reasons they attach to that conduct.
This creates an important evidentiary problem for soft law. A resolution, declaration or guideline may record considerable international support, but support for a political objective is not necessarily acceptance of a legal obligation.
3.2 Soft Law as Evidence of Opinio Juris
Non-binding instruments may nevertheless be valuable evidence of opinio juris. The ICJ has recognised that the content and voting patterns surrounding General Assembly resolutions may provide evidence relevant to determining whether a customary rule exists. In Nicaragua, for example, the Court considered General Assembly resolutions alongside State practice when examining the customary status of principles concerning the use of force and non-intervention.[13] Similarly, in its Nuclear Weapons Advisory Opinion, the Court examined General Assembly resolutions as evidence of the emergence of a normative position, while remaining cautious about treating resolutions themselves as constitutive of law.[14]
The distinction is therefore between political endorsement and legal conviction. A State may vote for a resolution because it considers the proposed norm desirable, without believing that international law already requires it. Conversely, a resolution may reinforce opinio juris where States’ explanations, subsequent conduct and legal statements demonstrate that their support reflects a sense of legal obligation. As Boyle argues, soft law can consequently operate as a bridge between political commitment and legal development, without erasing the distinction between them.[15]
3.3 Soft Law as Evidence of State Practice
The evidentiary value of soft law must also be considered in relation to State practice. Voting for a resolution is not, by itself, equivalent to State practice: a vote expresses a position, whereas practice concerns conduct. Implementation of the principle contained in an instrument may therefore carry greater weight. Repeated reliance upon a resolution in diplomatic statements, legislation, official policy or international proceedings may likewise strengthen the evidentiary connection.
This also explains why inconsistent conduct matters. A State that endorses a principle but repeatedly acts contrary to it weakens the inference that its support reflects opinio juris. Silence cannot automatically be treated as acceptance either; its significance depends upon the circumstances and whether the State had an opportunity and reason to respond.[16] Ultimately, soft law becomes legally significant not because it commands obedience, but because States’ conduct and legal convictions surrounding it may supply evidence from which customary law can be identified or progressively developed.
IV. The Jurisprudential Treatment of Non-Binding Instruments
The jurisprudence of the International Court of Justice (ICJ) demonstrates that non-binding instruments occupy an intermediate position in the identification of customary international law. The Court has neither treated them as legally irrelevant nor accepted that their adoption automatically creates binding rules. Their significance depends instead on what they reveal about the practice and legal convictions of States.
4.1 ICJ and United Nations General Assembly Resolutions
In Nicaragua v United States, the ICJ gave particular significance to General Assembly resolutions concerning the principles of non-intervention and the prohibition of the use of force. The Court did not treat the resolutions themselves as sources that independently imposed obligations on States. Rather, their content, together with State conduct and other evidence, supported the conclusion that corresponding principles existed in customary international law.[17] This approach is important because it locates the legal significance of the resolution outside the resolution itself: its value lies in what it reveals about the normative position of States.
The same distinction appears in the Nuclear Weapons Advisory Opinion. The Court considered the voting patterns and substance of General Assembly resolutions addressing nuclear weapons, recognising that their repeated adoption could demonstrate the existence of an emerging normative expectation. Yet the Court stopped short of concluding that the resolutions, merely by being adopted, established a customary prohibition.[18] The jurisprudence therefore suggests that repetition can strengthen evidentiary weight, but cannot substitute for the requirements of custom.
Other cases reinforce this contextual approach. In Western Sahara, the Court relied upon General Assembly practice and resolutions in assessing the international legal status of self-determination, but situated that material within the broader development of international law.[19] Likewise, East Timor illustrates the Court’s willingness to recognise the significance of General Assembly resolutions in tracing the development and recognition of legal principles, while maintaining the distinction between political recognition and binding legal obligation.[20]
4.2 The ICJ’s Caution
The jurisprudence therefore reveals a deliberate judicial restraint. General Assembly resolutions may confirm, reinforce or provide evidence of customary law, but they do not possess an automatic law-making effect. This distinction protects the integrity of the customary process: if a resolution alone could create a customary rule, the separate requirements of State practice and opinio juris would become largely unnecessary.
The better reading of the ICJ’s approach is consequently one of normative influence rather than independent law-making authority. As Boyle argues, soft law may influence the formation and development of international law precisely because it can shape expectations and provide evidence of emerging legal conviction.[21] The Court’s jurisprudence thus leaves room for soft law to matter considerably, while refusing to allow political consensus to become binding law without the evidentiary foundation required by custom.[22]
V. When Does Normative Influence Become Legal Obligation?
The central difficulty is identifying the point at which a non-binding instrument moves beyond political influence and becomes legally relevant to the formation of customary international law. Formal adoption cannot answer that question. What matters is the conduct and legal reasoning that follow. On that basis, this article proposes five cumulative but context-sensitive indicators for assessing the legal significance of soft-law instruments.
1. Degree of State participation. Widespread and representative support for an instrument may indicate that a principle has acquired international acceptance, particularly where the participation reflects the interests of States most directly affected by the subject. Yet numerical majorities should not be treated as a substitute for legal consensus. A resolution supported by many States remains insufficient if significant States expressly reject its legal character.[23]
2. Subsequent State practice. Subsequent State practice is decisive. States must translate normative language into conduct before a stronger claim to customary status can be made. Legislation, diplomatic conduct, official policy and conduct in international proceedings may therefore be more revealing than the initial vote itself. The key issue is whether practice is sufficiently general and consistent, rather than merely whether States have endorsed the instrument.[24]
3. Evidence of opinio juris. Evidence of opinio juris must accompany that practice. States must act because they regard the relevant conduct as legally required or permitted, rather than simply desirable. Statements accompanying votes, diplomatic explanations and subsequent legal positions can therefore be particularly valuable.[25] This prevents political enthusiasm from being mistaken for legal conviction.
4. Subsequent reliance. Repeated invocation of an instrument by States, courts or international organisations may demonstrate that its principles have acquired continuing normative significance. Such reliance, however, should corroborate rather than replace the underlying evidence of practice and opinio juris.[26]
5. Contrary State conduct. Finally, contrary State conduct must be taken seriously. Persistent objection, express rejection or sustained contrary practice may prevent an alleged rule from crystallising, or demonstrate that consensus remains incomplete.[27] This is particularly important where soft-law instruments address politically sensitive or rapidly developing areas of international law.
Taken together, these factors suggest that soft law does not become hard law at a single identifiable moment. Its legal significance develops through a cumulative process in which political expression is reinforced, or undermined, by subsequent State conduct and legal conviction. The decisive consideration is therefore not the formal status of the instrument, but what States subsequently do with it and why they do so.
VI. The Risks of Treating Soft Law as Hard Law
The normative value of soft law should not obscure the risks of giving non-binding instruments excessive legal weight. The first concern is State consent. If repeated endorsement of a resolution were sufficient to create a binding rule, States could become legally bound by an instrument they never intended to accept as law. This would weaken the distinction between political commitment and legal obligation that underpins the consensual character of international law.[28]
A second difficulty is majoritarianism. A resolution adopted by a substantial majority of the UN General Assembly may demonstrate broad political support, but numerical support cannot, by itself, establish opinio juris. The ICJ’s jurisprudence instead requires attention to the conduct and legal convictions underlying such support.[29]
There is also a question of institutional legitimacy. Treating international organisations as indirect law-makers risks allowing political bodies to generate obligations without the procedural safeguards associated with treaty-making or the evidentiary requirements of customary law. Yet rejecting soft law entirely would be equally problematic, since international instruments can provide valuable evidence of emerging normative expectations and facilitate gradual legal development.[30]
The final danger is normative inflation: if every declaration, guideline or resolution is treated as evidence of an emerging customary rule, the boundaries between political aspiration, soft law, emerging norms and binding custom become increasingly indistinct.[31] The appropriate response is therefore not to disregard soft law, but to impose analytical discipline: its significance should remain dependent on independently demonstrated State practice and opinio juris.
VII. Conclusion
This article has argued that soft law does not become hard law merely because it is repeated, widely endorsed or normatively persuasive. Its legal significance emerges when it interacts with the established requirements of customary international law, particularly State practice and opinio juris.[32] Soft-law instruments can accelerate normative development, record emerging expectations and assist courts in identifying or interpreting customary rules, but they cannot substitute for the evidentiary requirements of custom.
Two recommendations follow. First, international courts and States should assess soft-law instruments through clearer, context-sensitive criteria rather than treating institutional adoption as evidence of legal obligation. Second, States should more expressly distinguish political endorsement from acceptance as law when adopting such instruments. Future research should examine whether these criteria operate consistently beyond UNGA practice, particularly in regional organisations and emerging fields such as climate governance and artificial intelligence.
Ultimately, the value of soft law lies not in transforming political agreement into law, but in revealing when political expectation has matured into genuine legal conviction.
Footnotes
[1] Kenneth W Abbott and Duncan Snidal, ‘Hard and Soft Law in International Governance’ (2000) 54(3) International Organization 421, 421–22.
[2] International Law Commission, ‘Draft Conclusions on Identification of Customary International Law, with Commentaries’ (2018) UN Doc A/73/10, conclusions 2–3.
[3] Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) (Merits) [1986] ICJ Rep 14; Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion [1996] ICJ Rep 226. The General Assembly subsequently took note of the ILC’s conclusions in resolution 73/203.
[4] Sufyan Droubi, ‘The Role of the United Nations in the Formation of Customary International Law in the Field of Human Rights’ (2017) 19(1) International Community Law Review 68, 68–97.
[5] Alan Boyle, ‘Soft Law in International Law-Making’ in Malcolm Evans (ed), International Law (5th edn, OUP 2018) ch 5.
[6] Kenneth W Abbott and Duncan Snidal, ‘Hard and Soft Law in International Governance’ (2000) 54 International Organization 421, 421–23.
[7] Boyle (n 5).
[8] Sufyan Droubi, ‘The Role of the United Nations in the Formation of Customary International Law in the Field of Human Rights’ (2017) 19 International Community Law Review 68, 68–72.
[9] Statute of the International Court of Justice, art 38(1).
[10] International Law Commission, ‘Draft Conclusions on Identification of Customary International Law, with Commentaries’ (2018) II(2) Yearbook of the International Law Commission 65, conclusions 2–3; North Sea Continental Shelf (Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands) [1969] ICJ Rep 3, 44–45.
[11] International Law Commission, ‘Draft Conclusions on Identification of Customary International Law, with Commentaries’ (2018) II(2) Yearbook of the International Law Commission 65, conclusions 2–3.
[12] North Sea Continental Shelf (Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands) [1969] ICJ Rep 3, 44–45.
[13] Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) (Merits) [1986] ICJ Rep 14, 98–100.
[14] Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion [1996] ICJ Rep 226, 254–55.
[15] Kenneth W Abbott and Duncan Snidal, ‘Hard and Soft Law in International Governance’ (2000) 54 International Organization 421, 421–23.
[16] Sufyan Droubi, ‘The Role of the United Nations in the Formation of Customary International Law in the Field of Human Rights’ (2017) 19 International Community Law Review 68.
[17] Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) (Merits) [1986] ICJ Rep 14, 98–101, 107–08.
[18] Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion [1996] ICJ Rep 226, 254–55.
[19] Western Sahara, Advisory Opinion [1975] ICJ Rep 12, 31–33, 37–38.
[20] East Timor (Portugal v Australia) [1995] ICJ Rep 90, 102–03.
[21] Kenneth W Abbott and Duncan Snidal, ‘Hard and Soft Law in International Governance’ (2000) 54 International Organization 421, 421–23.
[22] Sufyan Droubi, ‘The Role of the United Nations in the Formation of Customary International Law in the Field of Human Rights’ (2017) 19 International Community Law Review 68, 68–72.
[23] International Law Commission, ‘Draft Conclusions on Identification of Customary International Law, with Commentaries’ (2018) II(2) Yearbook of the International Law Commission 65, conclusions 8–9.
[24] North Sea Continental Shelf (Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands) [1969] ICJ Rep 3, 43–44; International Law Commission (n 23), conclusions 4–8.
[25] Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) (Merits) [1986] ICJ Rep 14, 98–109; International Law Commission (n 23), conclusion 9.
[26] Alan Boyle, ‘Soft Law in International Law-Making’ in Malcolm Evans (ed), International Law (5th edn, OUP 2018) ch 5; Sufyan Droubi, ‘The Role of the United Nations in the Formation of Customary International Law in the Field of Human Rights’ (2017) 19 International Community Law Review 68, 68–72.
[27] International Law Commission (n 23), conclusions 9–10; Oliver Urrutia, ‘Unregulated Fishing on the High Seas and the Formation of Customary International Law’ (2022).
[28] Kenneth W Abbott and Duncan Snidal, ‘Hard and Soft Law in International Governance’ (2000) 54 International Organization 421, 421–23.
[29] Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion [1996] ICJ Rep 226, 254–55; Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States of America) (Merits) [1986] ICJ Rep 14, 98–101.
[30] Alan Boyle, ‘Soft Law in International Law-Making’ in Malcolm Evans (ed), International Law (5th edn, OUP 2018) ch 5.
[31] Eric M Ahmadov and Elchin M Ahmadov, ‘Customary International Law and OSCE Documents: The Potential for Mutual Influence’ (2024) 84 Uzhhorod National University Herald. Series: Law; International Law Commission, ‘Draft Conclusions on Identification of Customary International Law, with Commentaries’ (2018) II(2) Yearbook of the International Law Commission 65, conclusions 2–3.
[32] International Law Commission, ‘Draft Conclusions on Identification of Customary International Law, with Commentaries’ (2018) II(2) Yearbook of the International Law Commission 65, conclusions 2–3; Alan Boyle, ‘Soft Law in International Law-Making’ in Malcolm Evans (ed), International Law (5th edn, OUP 2018) ch 5.
Bibliography
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