Bryan Caplan  

Silent Signals

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Elephants Clashing... Proposals for Liberalization...

I didn't know logicians had a name for it, but they do: it's called the Argument from Silence.

The argument from silence (also called argumentum a silentio in Latin) is that the silence of a speaker or writer about X proves or suggests that the speaker or writer is ignorant of X. Whether such an argument is reasonable is subject to some interpretation or debate -- in general, the argument from silence does not offer a rigorous logical proof of a premise, although it may potentially offer some circumstantial evidence for a position.

Here is an easily recognizable example:

Bobby: I know where you live.
Billy: Where?
Bobby: I'm not telling you!
Billy: You're just saying that because you don't know!

Obviously, the Argument from Silence has limits. Wikipedia offers this logically parallel, but unconvincing, argument:

John: Do you know your wife's email password?
Jack: Yes, I do as a matter of fact.
John: What is it?
Jack: Hey, that's none of your business.

So when does the Argument from Silence work? Signaling models can light the way. The Argument from Silence is probative (though not decisive!) if, on average, people who know are better off speaking, and people who don't know are better off silent.

On exams, for example, we reason that if a student knows the answer to a question, he will write it down, and if he doesn't know the answer, he won't. So we infer ignorance from silence. In contrast, if a student fails to mention Tolstoy on a math exam, we don't infer ignorance of Tolstoy, because talking about Tolstoy on a math exam takes time and won't raise your grade.

An obvious extension: In a trial, does the failure to take the stand in one's own defense indicate guilt? While you could concoct a elaborate story about why an innocent man would be worse off telling his side of things, it wouldn't be very plausible. Silence is, probabilistically, an admission of guilt, whatever the law says.

But can you really believe what I've just written? After all, I have an incentive to say it even if I don't believe it - it might get me get out of jury duty one day!


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COMMENTS (9 to date)
Anonymous2 writes:

Wow, what a coincidence! Here's an even better application of the argument:

1) The infoshop.org faq offers a detailed criticism of capitalism, and is well-known among anti-capitalists online.

2) Bryan Caplan has never attempted to refute the main points of the faq.

3) THEREFORE, Bryan Caplan is unable or unwilling to refute the contents of the faq.

choupt writes:

The extension to non-testifying defendants is not very persuasive. The prosecution has the burden of proof - if the defense believes that the prosecution (which goes first) hasn't proved guilt beyond a reasonable doubt, it may conclude that there is no reason to take the risk of the defendant testifying. Lots of unexpected things can happen on cross-examination, especially of a nervous witness, and even an innocent defendant will be nervous.

Furthermore, on appeal, the question is whether a reasonable jury could have found guilt - meaning, approximately, "if we ignore the evidence in favor of the defendant and only look at evidence that could be construed in favor of the prosecution, is that evidence adequate?" The more evidence that is in the record, generally the worse for the defendant.

Anyway, it is true that, for whatever reason, plenty of defense lawyers are very reluctant to put their clients on the stand, even if they believe they are innocent.

Jason Briggeman writes:

Possibly the best refutation of the Argument from Silence is the fact that Anonymous2 clearly wants Bryan to break his silence with regard to the FAQ at infoshop.org!

tom writes:

If I wrote the Coke-a-Cola company and asked them to give me their secret formula to Coke and they replied saying they would not tell me the secret formula, then I should conclude that the Coke-a-Cola company does not know the secret formula to Coke.

Ragerz writes:

I agree that the point about testifying in trial is extremely invalid. Perhaps Caplan would be better off keeping his comments a little closer to his area expertise.

Often, innocent people are advised by their lawyers not to testify. That someone follows the advice of their lawyer is not a very good indicator of guilt.

Anonymous2 writes:

Since Caplan has thus far made no effort to address the arguments against capitalism made on infoshop.org, I think we can safely conclude he can't refute them and simply doesn't want to embarrass himself by mentioning the injustices of the capitalist system.

Lauren writes:

Hi, Anonymous2.

You said:

Since Caplan has thus far made no effort to address the arguments against capitalism made on infoshop.org, I think we can safely conclude he can't refute them and simply doesn't want to embarrass himself by mentioning the injustices of the capitalist system.

I think what we can safely conclude is that Dr. Caplan is busy with classes that have started up in the last few weeks, and responding to students, and scholarly research. The infoshop faq you reference is described thus on the infoshop.org website:

An infoshop is a cross between a radical bookstore and a movement archive. Activists go there to read or buy movement literature; buy paraphenalia such as stickers, masks and spray paint; attend meetings, lectures or films; or just plain hang out.
[http://www.infoshop.org/infoshops_defined.html]

Buying stickers, masks, and spray paint, hanging out, etc., are not top priorities or well-spent time if one wants scholarly interaction. Capitalism is well-worth discussing. Short, dismissive fuses don't contribute to effective dialog on the subject.

An "Argument from Silence" is not about failure to address a topic altogether, but from running circles around the topic in the midst of appearing to address it. It's a deliberate sleight-of-hand, sufficiently deliberate that the absence is palpable--in fact, usually noticeable to one's opponents. Accidentally not noticing or not contributing to a discussion that is going on is not a good example of an argument from silence. Wikipedia is not always the best source for definitions of terms in philosophy, logic, economics, math, etc. EconLog should not be providing examples even more specious!

[Addendum: Anonymous2's followup comment in this thread has been removed for crude name-calling.]

Rodriguez12 writes:

Since apparently "crude name-calling", i.e. insinuating someone is too lazy to read a reference, is apparently off-limits, I'll more briefly state that Lauren's objections are empty since she seems to have missed the location of the faq in question: www.infoshop.org/faq. I've seen many arguments against liberty, but claiming that their is only about selling T-shirts or hanging out is a new one to me. :)

Lauren writes:

Rodriguez12:

Crude name-calling in any form is not acceptable on EconLog.

Perhaps you are the person who also has posted comments as Anonymous2? Would you like to identify yourself to our readership? Your frequent comment references to the same page from the infoshop website make you look like the same person.

Name-calling is not usually about "insinuating someone is too lazy to read a reference." You associated these two matters for what reason?

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