Plausibility Does Not Equal Proof

A common mistake in critical thinking.

Posted Jun 28, 2018

I’ve written before about my use of POT papers in my first-year seminar called “How to Think Like a Psychologist.” POT stands for “proof of thinking.” Students write about readings or topics of their choice, but must apply some critical thinking to evaluate what they’ve read, experienced, or thought. For example, they must consider what evidence or proof exists for claims being made, either by authors or by the students themselves. Students learn (I have evidence!) that psychologists favor empirical evidence over such forms of evidence as supposition, individual experiences that may not be typical, or value judgments masquerading as facts.

One mistake—one type of bad critical thinking—that I’ve encountered frequently in student papers is what I will call the plausibility fallacy: Students assume that because an argument is plausible, it is true. Let me show you two forms of the plausibility fallacy:

Fallacy, Part 1:  A plausible explanation for a phenomenon is a true explanation.

The word because is a favorite among students. As with all human beings, they love to explain things—to find reasons. The difference between psychologists (at least, psychologists at work) and other human beings is that psychologists will demand or collect empirical evidence, whereas human beings will often settle for other forms of proof—or a lack thereof.

For example, a student might write this about a political advertisement they saw on TV: “This ad did not convince viewers to vote for the candidate because it aroused too much fear.” What comes after the word “because” is not evidence. It is merely a plausible hypothesis about why the ad is ineffective, albeit dressed up to sound like a fact. By the way, it doesn’t matter what the explanation is, as long as it’s plausible. For example, the student might argue that the ad was ineffective because “it did not contain any new information” or “only men were interviewed,” or “it did not arouse enough fear.” Students who write these kinds of statements have not considered evidence—they have merely made assertions.

Lots of researchers, in lots of studies, have given participants a “fact of human behavior” and then asked them to (a) come up with an explanation for why that phenomenon occurs, and (b) rate how sure they are that the phenomenon is true. Some participants receive an old adage, such as “Absence makes the heart grow fonder,” and other participants received an apparently conflicting adage, such as “Out of sight, out of mind.”  Participants came up with reasons for whatever “facts” they received, and believed their given facts more strongly. Of course, both of these adages can be correct in various circumstances. The point is that first-year students (like the rest of us) are very smart—they can come up with plausible-sounding explanations for just about everything! They are smart, just untrained in critical thinking.

Fallacy, Part 2:  A plausible explanation for a phenomenon proves that the phenomenon exists.

In Part 1 we explored explanations students might provide for why a given ad was ineffective, but Part II was hiding in plain sight. Notice the insidious, sneaky assumption that we made as we considered all kinds of plausible explanations: We assumed, without any proof, that the ad really isn’t effective! How do we know that? What’s the evidence, and how good is it?  (These two questions are central to critical thinking.) To say, for example, that only men appeared in the ad might be true. In addition, it might be a plausible explanation for why the ad is ineffective, if the add is ineffective. But we haven’t provided any evidence for the ineffectiveness of the ad!  It doesn’t necessarily follow that because only men were interviewed, the ad was not effective. We would have to test empirically our assertion about the effect of the ad. Maybe it was effective! IF we find (via surveys, experiments, etc.) that the ad did not change opinions, only THEN we can answer the question about WHY. We would still need to do some empirical, systematic investigation to find reasons. (By the way, another mistake that students often make is to assume that there is only one reason for a given phenomenon or behavior!  More on that in another post….)

Here’s another example: Let’s say I ask my students, “Does living together before marriage reduce the likelihood of divorce for those couples?”  They might answer, “Yes, because the couple will get to know each other’s idiosyncrasies and get used to them.” Notice the fallacy: Students are answering a question about the existence of a fact only by presenting a plausible reason why that fact might exist. However, the answer to the question is no! The empirical data show that living together may have the opposite effect—the divorce rate is often higher among couples who marry after living together than among couples who do not live together. Why might the divorce rate be higher? If we try to explain this (true) finding, given that we’re human beings, I’m sure we can all come up with the reason why this might be the case!  

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© 2018 by Mitchell M. Handelsman. All Rights Reserved

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