FallaciesCausal
Slippery Slope
Also called Thin end of the wedge · Domino fallacy · Camel's nose
Claiming that a first step will set off a chain of events ending in disaster, without showing the chain is likely.
Definition
A slippery slope argues against a modest step by predicting that it will lead, link by link, to an extreme outcome, without showing that each link is likely.
The argument gets its force from the end of the chain, which is usually vivid and frightening. The weak points are in the middle, where each “and then” is asserted rather than shown. Slippery slopes are tricky because some slopes really are slippery. The question is never whether a chain of consequences is possible, but whether this one is probable.
Form
- If we do A, then B will follow.
- If B, then C, and so on, until Z.
- Z is terrible.
- Therefore we must not do A.
Each step needs its own evidence, and the odds multiply. Five links that are each 80% likely leave only about a one-in-three chance of reaching the end.
How to spot it
“Where does it end?” or “Next thing you know…” stands in for an explanation.
The argument skips straight from a small first step to the worst possible outcome.
Nobody explains why one step would cause the next.
Existing lines that have held for years are ignored, as if no one could ever stop partway.
The same warning was made about an earlier step that didn’t lead where it predicted.
Examples
01“If we allow same-sex marriage, people will be marrying their pets”
Opponents of same-sex marriage often argued that redefining marriage once would mean it could no longer be limited at all.
What went wrong
The argument never explains how one change would force the others. Marriage law kept requirements like consent between adults, and the predicted chain never came. Each proposed change has to be argued on its own merits, and it was.
02“Facial recognition at airports today, a total surveillance state tomorrow”
A civil-liberties group argues that letting airports match faces against passport photos will inevitably lead to cameras tracking every citizen everywhere.
What went wrong
Concerns about expanding surveillance are legitimate, and some technologies do spread past their original use. But the argument jumps from one use to every use without naming the steps, the actors, or what would stop them. A stronger argument would point to specific mechanisms, like data-sharing agreements or weak legal limits, and target those.
03“If I let you stay up fifteen minutes late, you’ll never go to bed on time again”
A parent turns down a one-night exception for a special occasion.
What went wrong
One exception doesn’t remove the parent’s ability to say no next time. Rules can include exceptions without collapsing, and most households manage this every week.
Why it persuades
The end of the chain is concrete and alarming, while each step on the way sounds only mildly plausible. People judge the whole chain by the vividness of the ending, not by multiplying the odds of each link. Fear of losing control also does a lot of work: it feels safer to refuse the first step than to trust that you could stop later.
How to counter it
1Ask for the links
Make the speaker spell out each step and what causes the next one. Weak links usually show up as soon as they’re said out loud.
Walk me through it. What actually makes the first step lead to the second?
2Do the math on the chain
Grant that each step is fairly likely, then point out what that means for the whole chain.
Even if each step is likely, the odds multiply. How confident are you in every single link?
3Point to where the line holds
Most slopes have a place where society already stops. Ask why that stopping point would suddenly fail.
We already draw a line right here, and it’s held for years. What would make it stop holding now?
When it's not a fallacy
Some slopes are real. Legal scholar Eugene Volokh showed that one decision can make the next one easier: it changes attitudes, sets legal precedent, lowers the cost of further steps, or creates pressure to enforce a rule more widely. A slippery-slope argument is reasonable when it names a mechanism like these and gives evidence that the mechanism is likely to operate here.
Precedent arguments in law are a common legitimate version. A court ruling really does make similar rulings easier. The fallacy is in treating “could lead to” as “will lead to,” with nothing in between.
Sources & further reading
- Douglas Walton, Slippery Slope Arguments (Clarendon Press, 1992).
- Eugene Volokh, “The Mechanisms of the Slippery Slope,” Harvard Law Review 116 (2003): 1026. The mechanisms by which slopes can be real.
- Bradley Dowden, “Fallacies”, Internet Encyclopedia of Philosophy.
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