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Saturday, June 1, 2024

Is the Slippery Slope ‘Fallacy’ Really a Fallacy?

Sometimes slopes are actually slippery.

Image Credit: Mindmatrix via Wikimedia | CC BY SA 2.0

If you participate at all in online discussions, particularly on social media, you’ve likely seen someone discuss the idea of the “slippery slope” fallacy.

Quite often, people will call out “fallacy!” any time someone expresses concern about one thing leading to another. But it’s not always the case that slippery slope arguments are fallacious. It’s sometimes right to expect that doing one thing at the very least increases the chance that some related thing will happen. So when is it a fallacy and when is it not? The Wikipedia entry on the matter sets the record straight: “When the initial step is not demonstrably likely to result in the claimed effects, this is called the slippery slope fallacy.”

In other words, slippery slope arguments are not fallacious if you can show the initial step can improve the likelihood that the claimed effects will come about. The fallacy occurs when you claim there’s a slippery slope but then have no good reasons to expect the first step to increase the likelihood of the effects.

How much does the initial step need to increase the likelihood of claimed effects for the argument to not be fallacious? It’s unclear. Wikipedia argues the initial step should be “demonstrably likely” to result in the claimed effects, but I’d argue that if the claimed effects are bad enough, we can imagine even a slight increase in probability as being too slippery of a slope.

Part of the reason for this ambiguity of definition is that the slippery slope fallacy is not a formal fallacy. In other words, you can’t express the logical mistake it makes purely with reference to formal logical symbols. I owe this insight to my colleague professor Justin Clarke. Put simply, whether or not a slippery slope argument is fallacious depends on if the slope is in fact slippery. But this is a judgment call. In the real world, some slopes are slippery, and some are more slippery than others! So we have to examine the context to determine whether the argument is truly fallacious.

To give an example of a real argument about slippery slopes, let’s say Karl favors price controls to keep the price of gasoline down. We’ll call this Policy A. Fred responds that he opposes centrally planned economies, which we’ll call Policy B. Fred says that Policy A has a good chance of bringing about Policy B, so he opposes Policy A on those grounds.

But he’s made a fatal mistake. Karl quote-tweets him with the caption “slippery slope fallacy!” The debate is over, right? Wrong. If Fred can demonstrate that there is, in fact, a slippery slope, there is nothing fallacious about his reasoning. 

Relative Costs

Have you ever noticed that many “slippery slope” predictions do come to fruition? I have, and although anecdotal evidence doesn’t prove that slippery slope arguments have their merits, we can use economic logic to see why there’s probably something to them.

Let’s say you’re at home, and you live four miles away from an ice cream store. You’re craving chocolate ice cream, and you’d be willing to walk three miles to get a cone, but four miles is just a bit too far.

In other words, the benefit of the ice cream is less than the cost in terms of money and time spent walking.

But now let’s say your friend tells you he’s driving one mile in the direction of the ice cream store. There isn’t enough time for your friend to drop you at the store itself, but your friend does offer to take you one mile closer. At this point, your walk shortens from 4 miles to 3 miles, and now you’re willing to make the trip.

Economists would describe you here as being “on the margin.” You were right on the edge of deciding to get an ice cream cone, and being one mile closer pushed you over the margin into purchasing the cone.

Look at the structure of what happened. You wanted to go from one place—let’s call it Point A—to another place which we’ll call Point C. Your friend had no interest in going to Point C, but his decision to bring you to Point B pushed you over the edge to travel to Point C. Your friend never intended to go to Point C, but his destination was “on the way,” and as a result the relative cost of your traveling to Point C fell.

This is a general truth in economics. When you lower the cost of some action, people will do more of that action, all else equal, because those who were previously “on the margin” now have the resources or desire to take the action.

Notice how this situation could mirror the logic of the policy world. It’s possible that there is an interest group out there that wants, for example, government-paid college tuition. We’ll call that Policy C in this case. However, the interest group currently doesn’t have adequate resources to advocate Policy C. They fall just a little bit short. Perhaps it’s too expensive for them to hire the lawyer they would need to argue for the constitutionality of the policy.

Now let’s say a different interest group comes along and advocates student loan forgiveness. We’ll call this Policy B. This separate group successfully gets the policy passed and argues for its constitutionality successfully.

If the group who supported Policy B was able to establish precedents which make it cheaper to argue for the constitutionality of Policy C, it’s possible—arguably even likely—that the advocates of government-paid tuition would now have enough resources to lobby politicians to get the policy passed.

Similar to the ice cream example, Policy C is a different destination from Policy B, but it’s imaginable that the latter being passed can push supporters of Policy C over the margin.

Whenever a policy you advocate lowers the cost of another sort of policy, it increases the probability that the other policy will change as a result. And if one can demonstrate that the increase in probability is considerable, then that is a very real slippery slope.

Capital Comes Home

Let’s look at another example of how one policy can lower the relative cost of a different, unintended policy. Consider foreign affairs. It’s often the case that hawkish foreign policy leads to aggressive policy and tyranny domestically.

It’s easy to imagine that someone would claim this observation is based on a slippery slope “fallacy”. But it is not fallacious. In line with the logic above, we see that imperialist foreign policy lowers the relative cost of tyranny at home.

Economics professors Chris Coyne and Abigail Hall chronicle examples of this in their book Tyranny Comes Home.

For one example, the professors highlight that modern SWAT teams have their origin in US war efforts. John Nelson, a Vietnam War veteran, was instrumental in their development. Nelson was a member of a “Force Recon” special unit who were “trained to engage and kill, and they did so efficiently.”

Coyne and Hall argue that the special skills, or human capital, which Nelson developed in Vietnam helped him develop SWAT teams. He partnered with inspector Daryl Gates who is noted as saying, “Nelson became our specialist on guerilla warfare… [We attended several marine sessions on guerilla warfare… We brought in military people to teach [the SWAT units].”

This is only one of Coyne and Hall’s examples. They find similar evidence that the US military anti-drug operations in Afghanistan lowered the cost of the US “War on Drugs.” They also highlight how police units have access to military surplus equipment.

Again, the point here is not that one policy necessarily leads to another. The point is simply that it can create a slippery slope. Advanced military operations generate physical and human capital which lower the cost of police using military-like tactics and equipment domestically, thus making aggressive policing much more likely on the margin. There’s no fallacy there.

The Middle-of-the-Road Policy

There’s another example of an apparent “slippery slope” in action. Ludwig von Mises famously argued that interventionist economic policy, if taken to its logical conclusion, would lead to socialism. This is similar to the thought experiment in this article’s introduction. If Karl favors price controls and Fred argues this could lead to a centrally planned economy, Mises introduces logic which suggests Fred isn’t totally off base.

Mises did not say that every economic intervention would automatically transform a free market into a centrally planned socialist economy. Instead, he highlighted that in order for, say, a price control to be successful, politicians would have to implement more policies to sustain it.

In the case of a price control on milk, he argues that if the government wants to lower the price of milk for consumers, this lower price will mean that some producers of milk go out of business as their profits fall below their next best opportunity. Milk producers dropping out of the market means that less milk is available, thereby undermining the original intention of making milk available for all.

To fix this, the government is going to have to intervene in another market, such as by putting price controls on inputs to milk production, to make sure that milk producers don’t go out of business. The problem is that this creates the same issue in the market for those inputs. The government must then interfere with their input market, and so on. To summarize with a quote from a previous article:

Mises’s point is that when the government imposes certain controls on the market, the market reacts in a way that undermines the goal of the controls. In order to fix this, the government must take even more control of the market again and again until eventually there is no market left.

It’s also important to highlight what Mises is not saying. Mises is not saying that every government that intervenes in the economy will become socialist. He is saying that if they want to succeed at their stated ends then they will need to take over increasingly large parts of the economy.

What this argument points out is that sometimes certain policies bring about other policies, given that lawmakers want those initial policies to be successful. In the world, some slopes are, in fact, slippery.

So, am I claiming that every policy is a slippery slope for some worse thing? Of course not. I only mean to say that sometimes policies lead to or at least lower the cost of other policies that are worse. When this is the case, there is nothing fallacious about pointing it out.

Additional Reading:

Tyranny Comes Home by Christopher Coyne and Abigail Hall

The Middle of the Road Leads to Socialism by Ludwig von Mises


  • Peter Jacobsen is a Writing Fellow at the Foundation for Economic Education.