English Reference
Idiom idiom

cui bono

C2 Formal Roman origin
Meaning

Who benefits? — asked to find out who is really behind something.

Examples
SIMPLE

“Before you believe the story, ask cui bono.”

CONTEXTUAL

“The reporter started where reporters usually start — cui bono — and followed the ownership of the land.”

COMPLEX

“Cui bono is an excellent way to decide what to investigate and a poor way to decide what happened, since the person who gains most from an event is very often just the person who was standing nearest to it.”

Origin

Cicero attributes the question to an earlier judge, Lucius Cassius Longinus, whom he describes as the shrewdest and most reliable judge the Roman people had. Cassius put the same question in every trial he heard: cui bono fuerit — to whose benefit was it?

Cicero uses it twice in surviving speeches, and his point is procedural rather than clever. Before you weigh anybody's testimony, work out who stood to gain, and let that direct where you look. The Latin is a dative of advantage — for whom was it an advantage — not the loose "who benefits" of English translation, though the difference rarely matters in practice.

Detective fiction, investigative journalism and conspiracy theory have all inherited the question, along with its weakness: benefiting from something is not the same as having caused it, and cui bono argues far better as a starting point than as a conclusion.

The Law's Latinconnected storiesSee all Roman origin →

Roman legal Latin that never left the courtroom — or the boardroom.

  1. 01

    bona fide (idiom)

    Genuine and honestly meant — the real thing, not a pretence.

    The Roman standard of good faith.
  2. 02

    caveat emptor (idiom)

    Let the buyer beware — checking what you are buying is your own problem.

    What Rome did not actually say.
  3. 03

    quid pro quo (idiom)

    Something given in return for something else, usually as an understood trade.

    From the apothecary's substitute.
  4. 04

    de facto (idiom)

    In practice, whatever the official position says.

    The question of fact.
  5. 05

    de jure (idiom)

    According to the law, whether or not that is what actually happens.

    The question of law.
  6. 06

    ipso facto (idiom)

    By that very fact — the consequence follows automatically, with no further step.

    Consequences that need no hearing.

Last updated August 3, 2026