quis est beneficium 

Latin, "where is the benefit?" A type of logical fallacy in which one claims one didn't do something bad because it was not in one's interests to do so. An example would be, "Why would I steal from the cash register? It's going to hurt the business if I do, and then I might lose my job."

The argument is usually used on behalf of someone else: for example, Ludo Martens (1995) argues that Stalin could not possibly have massacred millions of Russians because he needed them to fight WW2; Fogel & Engermann claimed* that American slavery was not very bad because it was in the best interests of slaveowners to have content slaves.

The argument is a fallacy because it assumes that all relevant motives of the actor are well-established, and lead away from the act. It does not account for motives like personal hatred, shame, fear, spite, ideology, and so on.
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* In *Time on the Cross* (1971); the book was conclusively debunked by David & Stampp, *Reckoning with Slavery* (1976).

One frequently encounters *quis est beneficium?* arguments among Holocaust deniers of all stripes. Among such worthies it is claimed that Hitler/Stalin/Enver Pasha could not possibly have wanted to massacre all those millions because it was a nuisance to try.

quis est beneficium by Abu Yahya February 14, 2009