Peter principle
Etymology
Formulated by the Canadian educator Laurence J. Peter and popularized in The Peter Principle (1969), co-written with Raymond Hull. Its core line — "in a hierarchy every employee tends to rise to his level of incompetence" — was framed as satire but struck readers as uncomfortably true. It has since been taken seriously by management scholars, with later studies finding real evidence for the effect.
noun phrase · /PEE-ter PRIN-suh-pul/
The idea that people in a hierarchy tend to be promoted until they reach a role they are no longer competent to perform, and then stay there.
What it names
Why organizations fill up with people stuck one rung above their competence — the reward for doing a job well is a different job you may be bad at.
Example
Their best engineer became a mediocre manager — a textbook Peter principle promotion that cost the team its strongest builder and gained it a reluctant boss.
Related idea
Parkinson’s law explains why bureaucracies swell in size; the Peter principle explains why they swell with people mismatched to their roles.