Cui bono

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?'

Qui bono (properly Cui bono?) is Latin for "who benefits?"

It is the single most important question to ask when evaluating any (usually political or criminal) incident. After all, humans are inclined to support, if not cause, actions that benefit themselves. The phrase itself dates back to antiquity. The Roman orator and statesman Marcus Tullius Cicero, in his speech Pro Roscio Amerino, attributed the expression cui bono to the consul and censor Lucius Cassius Longinus Ravilla. Cicero, who was fond of using the phrase, also quoted it in another of his speeches, Pro Milone, in which he defended his friend Milo against accusations that he had murdered a political opponent, stating: "let that maxim of Cassius apply". As Cicero himself pointed out, "no one pursues a crime without the hope of some profit."

However, this is not a particularly wise course of logic to follow when answering a question of a scientific or medical nature, as these fields are based on fact and empirical evidence. Special interest groups benefit from scientific advancements, for sure, but the underlying assumption, in this case, is that everyone else will as well. This has not stopped some from questioning motives anyway. Take, who got a significant part of the world afraid that cell phones and power lines cause cancer because people do not understand the difference between ionizing and non-ionizing radiation.

Misuse of cui bono
[W]ere who benefited the primary question, Aristotle Onassis would be a legitimate suspect [in the assassination of JFK]

While it's always good to ask "Cui bono?", it can be taken to ridiculous extremes. One particularly fine example is found in the anti-vaccination movement, where the mundane fact that pharmaceutical companies expect to be paid for the vaccines they produce is somehow blown up into a line of reasoning extending way beyond the sanity horizon: to protect the huge profits that they ostensibly make with vaccines, Big Pharma does everything they can to bribe or intimidate scientists, doctors, and health officials into silence, because otherwise, the word would come out that vaccines are Bad, which would, of course, be bad for profits. It doesn't occur to the anti-vaxxers that simply making good products is far easier, more profitable, and rather less risky than selling crappy stuff and then spending absolute fortunes to keep a lid on that indefinitely.

Many other conspiracy theories are also built using this line of reasoning and then work backward from the conclusion by relying on post hoc reasoning. A classic example is 9/11 conspiracy theories. Cui bono? The Bush administration, of course, so they must have done it! Isn't it suspicious that 9/11 just happened to give cover to their plans for war?

Cui bono should also never be the only question to ask, as benefits from an event are never enough to directly implicate one with an action. If it is only asked who benefits from something and other factors are not taken into account, ridiculous conclusions can be extrapolated. Take the following example:

It could — and should — have been asked whether emperor penguins even have the means to make it cold in Antarctica, which they don't.

A different example would be the Moon landing hoax theories, which get increasingly complicated until landing on the Moon is the easier proposition.