Lex Posterior

Lex posterior derogat legi priori ("the later law repeals the earlier law") is one of the classical maxims used to resolve conflicts between legal norms of the same hierarchical rank. It sits alongside two companion principles: lex superior (the higher-ranking norm prevails) and lex specialis (the more specific norm prevails over the more general). Together, these three rules form the standard toolkit for handling normative conflicts in both domestic and international law.

In international law, lex posterior is codified in Article 30 of the Vienna Convention on the Law of Treaties (1969), which governs the application of successive treaties relating to the same subject matter. Under Article 30(3), when all parties to an earlier treaty are also parties to a later one, and the earlier treaty is not terminated or suspended, the earlier treaty applies only to the extent its provisions are compatible with the later treaty. Article 30(4) addresses the more complex situation where the parties to the two treaties differ.

The principle has important limits:

• It does not override jus cogens (peremptory norms), which prevail regardless of timing.
• It typically yields to lex specialis: a specific earlier rule may still trump a general later one.
• Article 103 of the UN Charter gives Charter obligations priority over conflicting treaty obligations, irrespective of which came first.
• Many human rights and humanitarian treaties contain explicit clauses preserving more favourable earlier protections.

In domestic systems, lex posterior is routinely invoked when a parliament passes legislation inconsistent with prior statutes without expressly repealing them — producing what is called implied repeal. Courts in common law jurisdictions, including the UK, have long applied this doctrine, though constitutional or "constitutional-status" statutes are often treated as immune to implied repeal.

For MUN delegates and treaty researchers, identifying whether lex posterior, lex specialis, or lex superior controls a given conflict is often the decisive analytical step.