Asserting that if we allow A to happen, then Z will consequently happen too, therefore A should not happen.
Used mainly as a fear-mongering tactic, the slippery slope is used most often by politicians in order to quell any support for a position they oppose.
Recent uses include:
Marijuana is a gateway drug! First our kids will smoke pot, then they will try the harder stuff. Next thing you know they’ll be a prostitute, selling their bodies to buy smack and stealing for their next hit.
While, logically, it is possible to go from smoking pot to, say, being a no-toothed meth junkie, the odds are stacked against it happening as there are a host of other factors ignored in this overly simplistic and paranoid progression.
Using the same logic, a kid drinking milk will get a taste for drinking and will soon be drinking anything liquid, including bleach and formaldehyde. In reality, many factors are involved and the odds of this exact progression playing out as predicted is highly unlikely to the point of reasonable dismissal.
Regarding this example, numerous studies have shown a great majority of marijuana smokers, even heavy smokers, never move on to dangerous drugs let alone becoming no-tooth meth junkies or prostitutes, proving this particular doomsday prediction fallacious.
It should be noted such slippery slopes, especially those unduly focusing on children, tend to also appeal to emotion, another fallacy.
If I allow my son to become an atheist, he’ll start murdering people for fun! Nothing will stop him from doing bad things, because he’ll have no morals at all!
This is considered a compound fallacy as it incorporates both a slippery slope and a straw man into one fallacious statement. Not only are morals not based in religion, if he were to murder people for fun it is unlikely a holy book would keep him from his desires. Given a little bit of cherry picking, one of the more popular holy books may even aid him in rationalizing his murderous actions, compounding the problem. This argument obviously misses the mark by glossing over why the person would suspect such desires out of his son, redirecting the focus on a mostly irrelevant topic, atheism.
If I give him an inch, he’ll take a mile
This is an oft used slippery slope regarding inevitable entitlement issues and possible theft when in reality these issues are the rare exception, not the rule.
US Republican lawmakers and pundits are popular with this particular slippery slope fallacy. In making such an assertion they also commit a fallacy of equivalence, attempting to compare such disparate things as different species and, more absurdly, comparing people to cars. Which brings up the question, how would a car let you know it loves you? Do you propose to it, or it to you? Is a warranty the same thing as a prenuptial?