The idea that many, if not most, people exhibit physical signs – tells – when they lie is an old idea – one that has been extensively studied by psychologists, and is of obvious practical interest to fields as otherwise disparate as gambling and law enforcement. Some of the tells that indicate someone is lying include:

Pauses in speech.

Providing too much information.

Breathing heavily.

Covering one’s face.

Excessive finger pointing.

Throat clearing.

Not blinking.

Swallowing.

Shuffling feet.

Tugging on ears.

Licking lips.

Cleaning glasses.

Grooming hair.

The psychological research on tells is interesting and important, and knowledge of tells also makes high-stakes poker that much more fun to watch. But what hasn’t been appreciated until now is the fact that philosophy, and in particular, the study of paradoxes, has something to offer with respect to our understanding of tells.

Now, in real life, tells are general and not absolute – in other words, people are generally more likely to exhibit one or more of the behaviors listed above (or other tells) when they are lying than when they are telling the truth. There is no evidence that there are any absolute tells – in other words, there is no evidence that any of these symptoms is such that a person will exhibit that symptom if, and only if, he is telling a lie. And for good reason, since we can prove that an absolute tell is impossible.

Let’s be a bit more precise. First, it is worth recalling a distinction that has come up in previous posts in this column that is relevant here: the distinction between telling a lie – that is, making an assertion that one either believes to be false, or is intended to deceive the listener, or both – and the mere assertion of a falsehood, which need involve neither the speaker believing that the claim is false nor the speaker intending to deceive anyone. Although discussions of tells in psychology and elsewhere are rarely explicit about this, presumably a tell indicates that the speaker is lying, not that he or she is merely asserting a falsehood.

Second, we need to say a bit more about what we mean by “absolute tell”: A physical symptom (such as covering one’s face) is an absolute tell for a person if and only if the person in question will exhibit that symptom when lying, and not exhibit that symptom when not lying. Thus, an absolute tell for a person is a completely reliable indicator of whether that person is lying or not.

We can now prove that absolute tells are impossible, since the existence of an absolute tell leads to paradox. Imagine that behavior X is an absolute tell for person P. Then ask person P to say:

“I am exhibiting X right now”.

Let’s assume that we are in a friendly laboratory environment, where the speaker has every reason to follow your directions if possible, rather than a police interrogation room, poker table, or some other environment where the speaker might have reasons not to cooperate. Further, let’s assume that the room is generously equipped with mirrors, so that both you and the speaker are immediately aware of the speaker’s physical behavior, and in particular of whether or not they exhibit symptom X.

Now, one of two things will happen:

Person P will find themselves unable to utter the sentence in question. Person P will utter the sentence.

Option (1), however, would be even more mysterious than the existence of absolute tells, since it is utterly unclear why the existence of a uniform relationship between one’s speech and one’s physical behavior (i.e. a tell) would imply constraints on what one can say. After all, it’s a simple sentence, and simple to say. So let’s set aside (1) and concentrate on (2).

Now, when person P utters the sentence in question, either they exhibit behavior X or they don’t. Further, given the set-up, they will also know whether or not they exhibited X, and you will know whether they exhibited X, and so on. So, we have two further cases:

Person P exhibits symptom X. Person P does not exhibit X.

In either case, however, if X is an absolute tell then we obtain a contradiction.

In case (1), person P exhibits X, knows that they exhibit X, and says that they exhibit X. In addition, you know that they exhibited X, and they know you know. So person P can’t be asserting something that they believe to be false, and they can’t be intending to deceive you. Hence they are not lying. But if X were an absolute tell for person P, then they should not be exhibiting X, since they are not lying. Contradiction.

In case (2), person P is not exhibiting X, knows that they are not exhibiting X, yet says that they are exhibiting X. If person P is not exhibiting X, then they are not lying, so they must believe their assertion and must not be intending to deceive you when uttering it. But they know that they are not exhibiting X, so they can’t believe they are. Again, a contradiction.

So we’ve made some philosophical progress: Absolute tells are impossible!

Note: The exact details of this argument might differ depending on the exact details of one’s philosophical account of what it is to tell a lie – see here. But some version will work on any reasonable account of what it is to tell a lie.

The impossibility of absolute tells is a purely logico-philosophical matter, depending solely on the conceptual analysis carried out above, and is independent of any empirical inquiry. But this impossibility result suggests some related questions in empirical psychology that are worth wondering about.

First, what would happen if we carried out the above scenario with subjects who had very reliable, even if not perfect (i.e. absolute), tells? In other words, what would happen if we took a bunch of subjects who exhibited some characteristic behavior X almost every time they lied, and didn’t exhibit X almost every time they didn’t lie, and then asked them to assert the sentence above. Now we know they would either not exhibit their tell and hence be lying, or exhibit their tell and not be lying. But which one? Would one outcome be more common than the other?

Second, it is worth noting that most people are not aware of the tells that (reliably, even if not perfectly) indicate when they are lying. Thus, it is worth asking whether informing subjects of their tells – that is, telling them that they generally exhibit symptom X when they are lying, and don’t exhibit it when they are not lying – before carrying out the experiment sketched above would affect the results. In other words, would a person’s knowing that X is a reliable tell for them affect whether or not they would exhibit that tell when forced to say the sentence in question?

These are interesting questions, but I’ll leave it up to the psychologists to determine their answers.

Featured Image Credit: ‘Abstract background wallpaper’ by tommyvideo via Pixabay. CC0 Public Domain.