Cui bono (/ /), literally "to whose profit?", 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.
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?'