Customary International Law and Human Rights — Human Rights Law Notes

Customary International Law and Human Rights

Somalia never ratified the Convention Against Torture — yet no lawyer would say Somalia is free to torture. That is customary international law at work: some human-rights norms bind every State, treaty or no treaty, because the world treats them as law.

Custom is one of the sources under Article 38 (see Topic 4), and it matters especially for human rights because it reaches States that never signed the treaties. A rule of custom needs two elements together:

State practice — a general, consistent pattern of how States actually behave.
Opinio juris — the belief that they behave that way because the law requires it, not out of mere habit or courtesy.

Where both are present, the rule binds all States. The core human-rights prohibitions — against torture, genocide, slavery and prolonged arbitrary detention — are customary, and the strongest of them are jus cogens [peremptory norms] from which no derogation is ever permitted. Much of the UDHR, though a non-binding declaration in 1948, is now accepted as customary law.

🧩 WORKED EXAMPLE — binding a non-party State

Facts. A State has ratified no human-rights treaty and claims it is therefore free to practise slavery.

Rule. A customary norm binds even a non-party if State practice plus opinio juris establish it; slavery’s prohibition is custom and jus cogens.

Apply. Nearly every State prohibits slavery (practice) and does so believing it is legally bound (opinio juris); the norm is therefore customary and peremptory.

Conclusion. The State is bound regardless of any treaty — the “we signed nothing” defence fails.

Definition of custom (ICJ Statute Art 38(1)(b)): “international custom, as evidence of a general practice accepted as law.”

In Simple Terms: If States generally act a certain way and do so because they think the law demands it, that becomes binding custom — and the ban on torture, genocide and slavery binds everyone this way.

flowchart TD
    A["CUSTOMARY INTERNATIONAL LAW"]:::root
    A --> B["State practice<br/>consistent, general behaviour"]:::leaf
    A --> C["Opinio juris<br/>belief it is legally required"]:::leaf
    B --> D["Both together = binding custom"]:::leaf
    C --> D
    D --> E["Binds even non-party States<br/>torture, genocide, slavery = jus cogens"]:::warn
    classDef root fill:#FFF8DC,stroke:#000,stroke-width:1px,color:#000;
    classDef leaf fill:#E6F3FF,stroke:#1E3A8A,color:#000;
    classDef warn fill:#FDE7E7,stroke:#B00020,color:#000;
    linkStyle default stroke:#888,stroke-width:1px;

Case Laws

Prosecutor v Furundžija (ICTY, 1998) — the prohibition of torture is customary and jus cogens, binding all States.
Filártiga v Peña-Irala (US, 1980) — the ban on official torture is a settled norm of customary international law.

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