In critical-thinking, logic, and philosophy classes, students are often taught to detect and avoid something called “the slippery-slope fallacy.”
Such warnings are sometimes justified. But at other times they are actually misleading—so much so that rejecting certain arguments as slippery-slope fallacies is itself fallacious. Understanding why is important morally, politically, and psychologically.
A search on the phrase “slippery-slope fallacy” yields a variety of definitions that are not all equivalent to each other. That alone should give one pause. But in its barest bones, a slippery-slope argument is of the following form:
“If A, which some people want, is done or allowed, then B, which most people don’t want, will inevitably follow. Therefore, let’s not do or allow A.”
The fallacy occurs when that form is not fleshed out by sufficient reasons to believe that B will inevitably follow from A, such as in the following examples:
- "The US shouldn't get involved militarily in other countries. Once the government sends in a few troops, it will inevitably send in thousands to die."
- "We've got to stop them from banning pornography. Once they ban one form of literature, they will never stop. Next thing you know, they will be burning all the books!"
In these examples, the conclusion does not follow deductively from the premise. Nor is any reason given to believe that a chain of events set in motion by the act described in the premise will inevitably lead to what’s described in the conclusion. So the above examples are not even good inductive logic. That’s why they are slippery-slope fallacies.
But not all slippery-slope arguments are fallacies.
In some cases, that’s easy to see. You’ve heard the saying: “Give ‘em an inch and they’ll take a yard.” That isn’t true of everybody, but it’s definitely true of some people—and I’m sure you know at least one.
Another example is more academic and bears greater social significance. In a famous, hotly debated paper “Defining Deviancy Down” (1993), the now-late New York Senator Daniel Patrick Moynihan (who was a sociologist by training) documented how several social trends that were at first thought healthy or harmless were actually quite destructive. Each trend had begun over objections that they would tilt behavior down a slippery slope. The stats before and since Moynihan wrote have proven that they did just that—the most important being those about the social effects on children of divorce and single-parenthood.
Of course Moynihan was not widely praised for being right. That’s because his conclusions had political implications that the majority of people found unwelcome. Indeed, it sometimes happens that good slippery-slope arguments are dismissed because their conclusions are so unwelcome that it’s socially or politically unacceptable to state them.
The lesson? What greases the slippery slope is not deductive logic, but human psychology. That happens more often than many are comfortable admitting. But it’s not reasonable to expect reason always to prevail in human affairs. Sometimes the slope down is slippery indeed.