"Cui bono" (/kwiː ˈboʊnoʊ/), literally "for whose benefit?", is a Latin phrase which is still in use as a key forensic question in legal and police investigation: finding out who has a motive for a crime. It is an adage that is used either to suggest a hidden motive or to indicate that the party responsible for something may not be who it appears at first to be.
The phrase is a double dative construction. It is also rendered as "cui prodest" ("whom does it profit?").
Commonly the phrase is used to suggest that the person or people guilty of committing a crime may be found among those who have something to gain, chiefly with an eye toward financial gain. The party that benefits may not always be obvious or may have successfully diverted attention to a scapegoat, for example.
The Roman orator and statesman Marcus Tullius Cicero, in his speech Pro Roscio Amerino, section 84, attributed the expression "cui bono" to the Roman consul and censor Lucius Cassius Longinus Ravilla:
L. Cassius ille quem populus Romanus verissimum et sapientissimum iudicem putabat identidem in causis quaerere solebat "cui bono" fuisset.
The famous Lucius Cassius, whom the Roman people used to regard as a very honest and wise judge, was in the habit of asking, time and again, "To whose benefit?"
Another example of Cicero using "cui bono" is in his defence of Milo, in the Pro Milone. He even makes a reference to Cassius: "let that maxim of Cassius apply".
In his 1962 book Formal Organizations, American sociologist Peter Blau used the concept of "cui bono" to differentiate organizations by who primarily benefited – whether it was owners, members, specific others or the general society.