Logical Fallacies: Slippery Slope Arguments

All these family members are thankfully still alive and well. But their misadventures have lent me a healthy dose of respect for winter and gravity. And my own “slippery slope” experiences, while less glamorous, have proven themselves instructive reminders that decisions have consequences. For example:

Decision #1: Agree with a friend that it’s a good idea to try the most difficult route on the ski hill.

Decision #2: Ride the chairlift uphill, knowing that what goes up must come do-o-o-o-own.

Decision #3: Ski to the route’s entrance, behold the steep, icy mounds (moguls) below, and contemplate whether to back out . . . Nah.

Decision #4: Start skiing down the moguls, no turning baaaAAAAaaaAAAAaaaAAAAaaaAAACK! *crash*

Once we’d peeled ourselves off the snow, found our skis, and assured wide-eyed onlookers that we were still alive, my friend and I made one final decision: “Let’s never try that again.”

Slippery Logic

Just as one decision launched me and my friend into a chain of events from which there was eventually no return, slippery-slope arguments suggest that one choice, action, or belief will inevitably (or at least, very likely) lead to another . . . and another . . . resulting in dire consequences. For example:

If you leave the gate open, then the dog will escape.
If the dog escapes, it will attack the mail carrier.
If it attacks the mail carrier, then important mail will be delayed.
If important mail is delayed, society will collapse.
Therefore, if you leave the gate open, society will collapse.

This is a logically valid argument because if the premises are true, then the conclusion must be true. However, the problem with slippery-slope arguments is that it’s sometimes hard to tell how true the “if-then” premises really are. Will society collapse if the mail gets delayed? That doesn’t seem likely.

Because of the uncertainties involved, slippery-slope arguments aren’t usually meant to be deductive so much as inductive.1 So, slippery-slope arguments are only considered fallacies (faulty lines of logic) if the outcome isn’t necessarily likely, given the premises. Two questions can help you discern the outcome’s likelihood, and therefore, the argument’s strength:

1. How probable is each “if-then” statement?

For example, how likely would the dog escape if the gate were left open, and how likely would the mail carrier be attacked if the dog escaped? To estimate outcomes’ likelihoods, it’s often helpful to question how logically each event connects to its supposed consequence and to consider whether other factors could likely alter or prevent that consequence. The less probable each consequence, the weaker the argument.

2. How many “if-then” statements are there?

The longer the chain of events, the less likely the argument’s conclusion will be if each event has an associated uncertainty. For instance, if it’s only 50% likely the dog would escape if the gate were open and 50% likely that the mail carrier would be attacked, then there’s only a .5 x .5, or 25% chance the mail would be delayed. Conversely, if you can show that each “if-then” statement is highly likely, the resulting argument is inductively strong, and therefore, not a fallacy.

Slippery Worldviews

Now, let’s see how slippery slope arguments can play out in the creation-evolution controversy. I’ve read skeptics say creationists use slippery slope fallacies when pointing out consequences that are consistent with an evolutionary worldview, including relative morals and decreased regard for every human life. These consequences, in turn, can extend (and have extended) to the justification of outcomes including racism, school violence, abortion, infanticide, euthanasia, and eugenics.

So, do creationists use slippery slope fallacies when making such observations? Only if an evolutionary worldview is not logically consistent with these outcomes. But scholars, including Dr. Jerry Bergman, have documented in detail the links between an evolutionary worldview and negative social outcomes.2 After all, if humans are merely evolved animals, there’s no reason for humans to have greater rights than any other evolved animals. If we destroy animals we don’t believe are worth keeping alive, then we should be able to destroy humans we don’t believe are worth keeping alive.

Naturally, not every evolutionist will want to act on such reasoning. So, it would be a slippery slope fallacy to claim, “If someone holds an evolutionary worldview, then they are guaranteed to behave consistently with that worldview and murder others to further their own evolutionary success.” A more reasonable statement would point out, “If someone holds an evolutionary worldview, then they may have no reason not to act consistently with that worldview, opening the door to negative outcomes.”3

Looking at secular cultures today, we can indeed see the outcomes of people acting consistently with an evolutionary worldview. In Belgium, for instance, which ranks among the world’s 10 most atheistic cultures,4 89.1% of doctors support the killing of disabled newborns. Such actions diametrically oppose the biblical truth that every human life has inherent value and dignity because God made us all in his image, loves us, died for us, and has a plan for us. But infanticide is logically consistent with an evolutionary worldview, where human rights are animal rights, morals are social constructs, and “personhood” is ours to define. So, demonstrating the social ills that unfold when people act in line with their worldviews does not entail using a slippery slope fallacy.

A Firm Foundation

In the end, slippery-slope arguments contain fallacies if their final outcomes are not necessarily likely, given the preceding chain of events. But they do not contain fallacies if their final outcomes can be shown as likely and logical. In the case of worldview, negative social outcomes demonstrably can (and do) flow from the belief that humans—and their morals—are products of evolutionary happenstance.

Showing that evolutionary worldviews may lead to serious consequences does not, of course, prove the story of evolutionary origins is false.5 But it does illustrate the societal importance of defending a biblical worldview, beginning in Genesis. The foundation of God’s Word offers the surest footing to avoid a steep, bumpy ride down a genuine “slippery slope” of moral decline.