The fallacy, in a nutshell, arises when some controversial practice--like the exclusion of some people from access to social goods made available to others--is justified by appeal to social problems that the practice may actually help to cause. The controversial practice is touted as the solution--or at least the most fitting thing to do given the problem. But any kind of thoughtful investigation will show that there is good reason to think this proposed "cure" is actually making the problem worse.
Consider, as an example, some of the ideas on education espoused by the namesake of the building I'm in right now. Murray Hall, home of the Oklahoma State University Philosophy Department and several other departments in the College of Arts and Sciences, is named after Alfalfa Bill Murray--who presided over Oklahoma's Constitutional Convention, served as Oklahoma's first Speaker of the House, and was governor during the Great Depression.
Murray, like many white politicians in his day, was a racist. In Murray's case, he wasn't just a racist in private. He spearheaded efforts to make sure Oklahoma was a Jim Crow state. And at one point during a speech at Oklahoma's Constitutional Convention, he argued forcefully against providing higher education to blacks.
On what basis did he make his case for excluding blacks from higher ed? Here's a snippet of what he said:
He must be taught in the line of his own sphere, as porters, bootblacks and barbers and many lines of agriculture, horticulture and mechanics in which he is an adept, but it is an entirely false notion that the negro can rise to the equal of a white man in the professions or become an equal citizen to grapple with public questions…Murray's claim that this is "an entirely false notion" is, of course, an entirely false notion. But he got away with asserting this falsehood. I'm sure most of privileged white people in his audience nodded the heads in blithe agreement--and would happily point to the rarity of accomplished black professionals, the relatively fewer displays of high academic achievement, etc., as evidence that what Murray said was true.
But, of course, we all know that intellectual excellence is cultivated through education. If you systematically provide fewer educational opportunities to a class of people, they're likely to show less aptitude for law and medicine and scholarly research, and more aptitude as "porters, bootblacks and barbers." If you use this fact--which we know to be an effect of being denied educational access--as a justification for denying it, you are guilty of the cause-cure fallacy.
I should point out that in this case we know that Murray was dead wrong. We know that when afforded the same educational opportunities, blacks and whites are equally capable of achieving intellectual excellence.
Did Murray know this, too? Was he just in denial about it? I can't say. But that's not the problem with Murray's thinking. Or, perhaps better: His thinking would be problematic whether or not he knew. The problem with Murray's thinking is that anyone who reflects on the matter can see that such an exclusionary educational policy could be the cause of the difference in intellectual achievement that's observed. And because the exclusionary policy could be the cause of the difference, you can't appeal to the difference as a justification for the exclusionary policy.
Put more broadly, you commit the cause-cure fallacy any time you justify a controversial practice based on considerations that, reasonably, could be caused by that practice.
Consider the following example, which has some relevance to the recent conflicts in Ferguson, MO. Suppose you want to justify a militarized style of policing, in which outsiders to a community patrol its streets in military gear, maintain their distance from the community they patrol, are quick to draw their weapons as a way of enforcing their authority, and routinely shoot to kill when they feel threatened. And suppose you justify this approach on the grounds that the people in the community show little respect for the law.
Here's the problem: If a community finds itself in an antagonistic relationship with law enforcement, that tends to generate an attitude of hostility towards the agents of the law. Respect for the law predictably erodes under such conditions. Militarized policing by outsiders without deep community ties thus could be an important contributing cause of eroded respect for the law. As such, such eroded respect can't be invoked as a reason for militarized policing tactics. To do so is to fall prey to the cause-cure fallacy.
Or here's another example. Conservative opponents of same-sex marriage like to point to the promiscuity of the gay community. They talk about the "gay lifestyle" as if acting on same-sex attraction were essentially bound up with a kind of sexual free-for-all which disdains sexual constraints. Many conservatives point to this "licentiousness" as a reason why same-sex relationships should continue to be stigmatized and same-sex marriage withheld.
The problem, again, is that being systematically stigmatized and denied access to marriage could readily explain why there is so much more promiscuity and impermanence in the gay community. Denied access to society’s primary tool for encouraging and supporting monogamy, the gay subculture is understandably less monogamous. Furthermore, since society has historically declared that homosexual intimacy is sinful regardless of how it is expressed, conservative norms entail that a faithfully monogamous gay relationship is just as much a “sin” as promiscuity and casual sex. Since lesbians and gays see no place for themselves within a society ruled by such norms (those norms lay out no legitimate expression of their sexuality), they predictably end up adrift in a marginalized space outside the scope of those norms--a situation that can readily lead to a collapse of sexual restraints.
Add to that the depression and sense of uprootedness that comes with social rejection, and the way in which superficial and self-destructive pleasure-seeking often follows on the heels of such depression, and you can see why it is reasonable to suppose that stigmatization of gay sex and exclusion from the chief social model of responsible sexuality (marriage) could be major causes for the promiscuity and "licentiousness" of the gay community. To justify such stigmatization and exclusion based on these phenomena is therefore fallacious.
As these examples hopefully indicate, the cause-cure fallacy is a real issue. It's a pattern of thinking that rears its head again and again--and it seems to function especially in contexts where privileged groups seek to preserve their privilege and justify the continued marginalization of others. They point to the effects of marginalization as if they were some essential problem with marginalized people, and invoke those effects as a justification for continuing to marginalize.
Insofar as this sort of thinking obscures and even seeks to vindicate social injustice, it's important to call it out. The first step to defusing its power is to become aware of it, and to pass that awareness on.