Why jury duty is bad poker (Caro blog)

Poker1.com default content graphicEntry #41 (2015-01-14)

This morning I’ve been summoned to serve on a jury. Here’s wishing Stone County, Missouri good luck with this adventure.

I’ve always wanted to serve, but I was disqualified in Los Angeles. And I might see a similar sad fate here in the Ozarks, too. I hope not, and I’ll let you know. Anyway, my disqualification for a jury had nothing to do with my profession as a poker player. It had everything to do with my approaching decisions logically. Let me explain.

Jury flaw

I call it the “Great American Jury Flaw.” Our system is so obviously distorted that if it were a poker game, you couldn’t rationally play. It would be as if pots could only be awarded if 12 impartial observes had to unanimously vote first on whether you played the best shown-down hand correctly “beyond a reasonable doubt.”

Your chances of winning wouldn’t depend so much on whether the observers thought you probably played the hand well. It would depend on how each observer defined “reasonable doubt.” Maybe some observers would leave it to whim to decide what a reasonable doubt was emotionally.

Now, you and I know that poker decisions during the game itself aren’t built on whim. They’re built on logic and evidence. And they’re always made in consideration of the pot size — both now and projected beyond this decision point, the prospects of the hand winning, the immediate bet size, and the estimated cost of pursuing the pot further. Great players then try to gauge whether it’s most profitable to fold, check, bet, or raise, and — when betting or raising — how much. Fine. It’s complicated, but at least we have guidelines.

The meaning

What are the guidelines for jurors in a criminal case? “Guilty beyond a reasonable doubt.” Huh? What does that mean?

Well, in Los Angeles, I posed that question during jury selection. I wasn’t trying to be cute or difficult, as I may have been perceived. It was simply that I couldn’t ethically swear to decide beyond a reasonable doubt without knowing what one was. I teach that you should try to make all important decisions — in poker and in life — in accordance with probability. So, I’d need to know how reasonable doubt translates statistically. Clearly 51 percent probability of guilt leaves reasonable doubt. What about 75 percent, 83 percent, 99 percent. Is one chance in a million reasonable doubt? One chance in ten? What?

And so I was honest and said to the judge that I wanted to pledge to carry out this mission, but needed clarification on what a reasonable doubt was.

He said — I’m sure correctly in accordance with law — that he couldn’t advise me about that. It was whatever it meant to me.

So, I said something like: “Then, for the sake of this case, I’m going to define reasonable doubt as one chance in 20. Please tell me if that’s too high or too low and I’ll adjust it.” Bam! I was immediately disqualified by one of the lawyers selecting the jury.

Not only that, when I was summoned again years later, I called in to explain what had happened and was disqualified on the phone! Apparently I had made the equivalent of the “No fly” list for Los Angeles juries.


In truth, I believe a one in 20 chance of innocence is a scary standard to impose. That might mean you’d be condemning someone to prison who you thought had a five percent chance of going there in error. But I’m wise enough to know that the system doesn’t assume “beyond a reasonable doubt” to mean one chance in 10 million of innocence. So, if you’re on a jury in a criminal trial, you need to live with the haunting notion that you might have done wrong if you vote to convict. But I was happy to fulfill my civic obligation — if only some guidance were given on what reasonable doubt meant.

If you’re a little confused about why this is a big deal, let me give you an example. Suppose that Mary decides to use probability to decide guilt or innocence and that a reasonable doubt to her is a 20 percent chance of innocence. After hearing the evidence, she estimates that there’s only a 10 percent chance the guy didn’t kill his girlfriend, so she obviously votes guilty. But Sam thinks a reasonable doubt is two percent. He thinks there’s a five percent chance the guy didn’t do it, but that’s too much doubt by his standard.

Now you have the absurd situation where Sam is more convinced of guilt than Mary, but votes not guilty, while Mary — who thinks it’s less likely the guy did it — votes guilty. And that isn’t just theoretical. It’s probably happening often among juries. And it’s all because of the Great American Jury Flaw.


If you want Mike Caro to decide “beyond a reasonable doubt,” you have an obligation to tell him what one is. Otherwise, assign him to a civil case where the “preponderence of evidence” guideline means anything over 50 percent.

Do you see the issue here? It isn’t silly or picky. One thing I know for sure is that if you don’t understand it, you’re unlikely to play poker profitably. You’ll make too many decisions at whim, rather than analytically. Please think about it. Meanwhile, I’m off to the Stone County Courthouse. They need me. — MC

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Mike Caro

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Known as the “Mad Genius of Poker,” Mike Caro is generally regarded as today's foremost authority on poker strategy, psychology, and statistics. He is the founder of Mike Caro University of Poker, Gaming, and Life Strategy (MCU). See full bio → HERE.


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  1. I was on one jury – assault by several – we couldn’t tell who was the guilty party based on testimony, there was no physical evidence, we would have said not guilty, but judge directed acquittal before we had to decide.

    I was on another jury – violation of restraining order (not by contact but by drinking) – we couldn’t tell if he had been drinking because they gave no test results – only cops testifying he smelled like alcohol. We found not guilty. He was still taken away in handcuffs for some other charges.

    I think jurys do not automatically go with the prosecution

  2. In Arizona reasonable doubt is defined as follows:

    The government has the burden of proving the defendant guilty beyond a reasonable doubt. In civil cases, it is only necessary to prove that a fact is more likely true than not true, or that its truth is highly probable. In criminal cases such as this, the government’s proof must be more powerful than that. It must be beyond a reasonable doubt.

    Proof beyond a reasonable doubt is proof that leaves you firmly convinced of the defendant’s guilt. There are very few things in this world that we know with absolute certainty, and in criminal cases the law does not require proof that overcomes every doubt. If, based on your consideration of the evidence, you are firmly convinced that the defendant is guilty of the crime charged, you must find the defendant guilty. If, on the other hand, you think there is a real possibility that the defendant is not guilty, you must give him the benefit of the doubt and find the defendant not guilty.

    You must decide whether or not the government has proven the defendant guilty beyond a reasonable doubt. You must start with the presumption that the defendant is innocent. The government must then prove the defendant guilty beyond a reasonable doubt. This means that the government must prove each element of the charge(s) beyond a reasonable doubt. If you conclude that the government has not met its burden of proof beyond a reasonable doubt, then you must find the defendant not guilty of that charge.

    Dan Colmerauer

    1. Hi, Dan —

      That’s very similar to many other legal definitions of “reasonable doubt” that I’ve seen. However, it does nothing to resolve the issue I raised in the entry above.

      Under the definition that you cited, it’s possible that a juror who believes the defendant is 99 percent likely to have done the crime votes NOT guilty, while a juror who believes the defendant is only 75 percent likely to have done the crime votes guilty. This isn’t a stretch, either. You can safely bet that this sort of thing happens regularly.

      Using this absurd, established system, guilt is determined largely by how jurors define “reasonable doubt” and the advisory does nothing to clarify it. So, the jury system is significantly luck based, rather than logic based. The “luck” part is how jurors decide what “reasonable doubt” means to them. And that is usually decided by whim, rather than by mathematical guidelines.

      No defendant, under that system, ever got a fair trial.

      Straight Flushes,
      Mike Caro

      1. Mike,

        Great response! Would you say that a defendant would be better off opting for a trial to the court (the judge alone hears the case then decides) than trial to a jury?


  3. Jury duty is a tournament, not a cash game. I think you’d do well as a foreman of the current jury to which you’re assigned. Otherwise, you’re going to have to make the same argument with everyone on your jury at different times.

  4. We are on the same page Mike! Every time I read your post it, it confirms we have a meeting of the mind. Whenever Gregory was called to a jury, he was pretty much bounced off the first one. Wondered why they never called me.

    Anyway thanks for sharing Mike. Always a super read.

    Was great spending time with you in Daytona. See you next time soon!


  5. Mike, just a grammar error at the end of this paragraph: Well, in Los Angeles, I posed that question during jury selection. I wasn’t trying to be cute or difficult, as I may have been perceived. It was simply that I couldn’t ethically swear to decide beyond a reasonable doubt without know what one was.: <— Without knowing is what I think you meant. No nit-picking, I know you want them brought to your attention. Great read! Thank you.

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