Real-World Prisoner's Dilemma from France

This is a real story of a pair of identical twins who are suspected in a crime. There is CCTV and DNA evidence that could implicate either suspect. Detailed DNA testing that could resolve the guilty twin is prohibitively expensive. So both have been arrested in the hope that one may confess or implicate the other.

Posted on February 13, 2013 at 1:39 PM43 Comments


Jordan Brown February 13, 2013 2:15 PM

It is also an interesting demonstration of the implications of “innocent until proven guilty”. Absent a confession or additional evidence, both men must go free even though it’s known that one is guilty.

(Yes, I know that presumption of innocence is not a law of nature, but it applies in most places and in particular applies in France.)

JacobM February 13, 2013 2:23 PM

Of course, unlike the usual formulations of the Prisoner’s Dilemma, in this case if neither defects, they (presumably) both go free. So there’s no incentive to defect.

Geoffrey Kidd February 13, 2013 2:28 PM

The Prisoners’ Dilemma depends on the reward setting for the payoff matrix. So change the payoff matrix.

Both confess, both get jail.

One confesses (or testifies against the other), the innocent brother goes free.

Neither confesses, or each accuses the other, the test is run, the guilty party goes to prison but BOTH brothers are held responsible for paying for the test.

Or provide some other large incentive (or dis-incentive) for betrayal.

David February 13, 2013 2:36 PM

It is absolutely NOT true that absent a defection both will go free. In the US there have been many cases where multiple individuals have been charged and convicted for the same act because neither defected.

This happens fairly frequently. Two suspects, one gun, one bullet, both could not have pulled the trigger, but both can be prosecuted (and convicted) as if they had. Two different juries seeing two different presentations of the same evidence can come to different conclusions about who did the crime. Its not going to convince too many juries to say “there is this other guy who might have done it” unless you can either provide some evidence that you didn’t do it, or that they did.

In fact this is so common that to avoid putting prosecutors in an awkward position its codified as part of the law. If you are an accomplice (say the get-away driver) in an armed bank robbery where someone is killed, you can be prosecuted for murder even if you never held a gun, or even entered the building.

Fabrice Roux February 13, 2013 2:39 PM

This story happens in my own town (Marseille).

There is one possibility that is likely… BOTH are guilty. DNA and CCTV shows that at least one of them is guilty.

agesilaus February 13, 2013 2:43 PM

If I understand correctly, the two conviction one bullet event occurs since most states have laws saying that when one perpetrator commits murder then all are guilty. This can even occur if one or the criminals is killed during the crime event.

The above case is hardly equivalent, one of the two may not even have known or been present when the crimes occurred.

Junk Science Skeptic February 13, 2013 2:48 PM

I have a bigger problem with the notion of the test’s “prohibitively expensive” status being used as an excuse to jail the innocent twin.

Most of the resources available to the typical LEO/prosecutor are prohibitively expensive for the typical defendant. The cost of the court system itself is already prohibitively expensive.

But suddenly it’s ok to throw out the presumption of innocence because the prosecution is cheap/lazy?

We spend millions on an OJ trial for likely killing his ex, because celebrity trials are rare. How rare are DNA dependent cases involving twins?

David February 13, 2013 2:52 PM

My point is that there is nothing about innocent until presumed guilty which suggests that you should not be convicted for a crime just because there is some interpretation of the evidence the leads to the conclusion that someone else did the crime.

Prosecutors have charged individuals for crimes (like rape/murder) based on newly available DNA evidence, while at the same time leaving individuals convicted for those crimes in prison and fighting their appeals to be let out.

Derek February 13, 2013 3:13 PM


My point is that there is nothing about innocent until presumed guilty which suggests that you should not be convicted for a crime just because there is some interpretation of the evidence the leads to the conclusion that someone else did the crime.

Of course, but we have another term for that: Reasonable Doubt. If the prosecutor cannot prove beyond a reasonable doubt that you committed the crime, then the jury should acquit. If all the evidence points to someone else just as much as it points at you, that’s reasonable doubt.

Prosecutors have charged individuals for crimes (like rape/murder) based on newly available DNA evidence, while at the same time leaving individuals convicted for those crimes in prison and fighting their appeals to be let out.

Yes, and it’s a terrible miscarriage of justice, not something to be held up as a good example of modern justice.

Ed Davies February 13, 2013 3:18 PM

This conversation is bizarre; there’s nothing in that report about any sort of “prisoners’ dilemma” or about anybody being sent to prison unless it can be proved beyond reasonable doubt that that person was guilty of a particular crime.

It seems to me that the police investigation is more likely to be along the lines of “where we you on the night of the 21st” – i.e., looking for some evidence or alibi which eliminates one or other of the brothers from at least one of the crimes for which DNA evidence is available thereby showing that the other one dun it (unless there’s reasonable doubt about them being only identical twins, not two out of three identical triplets, of course 😉 ).

David February 13, 2013 3:36 PM

I’m really responding more to comments like the second:

Absent a confession or additional evidence, both men must go free even though it’s known that one is guilty.
Which is just not true.

So I should rephrase:
“there is nothing about innocent until presumed guilty [or reasonable doubt] which guarantees that”

One juror seeing both presentations should come to a logically consistent conclusion as to who committed the crime and should have reasonable doubt as to the other individual (or perhaps reasonable doubt as to both).

But that doesn’t mean it will happen, especially with multiple jurors (and potentially multiple prosecutors).

Yes, and it’s a terrible miscarriage of justice, not something to be held up as a good example of modern justice.

I’m not arguing the ethics of the situation, although I think the bigger issue with these cases is probably representation and the fact that juries are likely to side with the prosecution rather than the defense when they are uncertain. Prosecutors who charge (and convict) multiple individuals for the same crime are really relying upon the fact that the Jury just won’t believe anything offered as defense or is relying upon a defense that just isn’t trying at all.

Cae February 13, 2013 4:15 PM

Anyone of them have a cell phone.

Check their cell phone location history and you should be able to determined the location of each person at a specific time.

He who is present at the crime scene at the time when the crime occurs is the criminal.

Jordan Brown February 13, 2013 4:49 PM

When I said “both men must go free”, I meant in theory… alas, indeed, in practice they might not. If you believe in presumption of innocence and Blackstone’s formulation, the fact that they might not go free is a bug.

aaaa February 13, 2013 5:11 PM

@Junk Science Skeptic but also others

Just a note: French law enforcement and justice systems do not work the same way as American one. Most importantly, it does not share weaknesses and strong points as American system.

For instance, it does not have adversarial system, so it is not really prosecutor fighting defense and both throwing money on opposing experts. It is also cheaper so the defense is not that prohibitively expensive.

I may be wrong, but that expensive test would go out of court pockets and it is up to judge to decide whether it is going to be done (not only prosecution). Basically, fact finding is on court and judge, not on fight between opposing sides experts.

Juries are reserved only for some (worst) crimes and play very different role then juries in united states. The panel of judges also gets to have say in deciding quilt or facts or whatever is technically decided by jury in states.

Also, just because united states have laws that make all perpetrators of an act guilty of murder if one of them did it does not mean that France has the same law. It does not, their laws and procedures are very different.

You can not simply take what you know about American justice system and expect different country to act consistently with it.

KevinC February 13, 2013 5:13 PM

I am somewhat skeptical of the million-plus-euro price tag.

Commercial services will carry out full-genome sequencing for 5,000-10,000 USD at most (and even lower prices are available for larger orders).

I get the impression that someone, somewhere is tacking on some costs that shoudn’t be there. You could spend a million euros if the forensics lab decided to buy a sequencer of their own and build a new laboratory to put it in, I guess.

Heck, I would be shocked if you couldn’t find a sequencing company that was willing to do the whole thing pro bono, just for the publicity.

Steven Metke February 13, 2013 6:27 PM

This doesn’t seem to be prisoner’s dilemma, there doesn’t seem to be a way for the innocent twin to defect.

Jay February 13, 2013 6:37 PM


They’re identical twins. The chances of finding DNA differences between them would have to be about the same as finding DNA differences between their heads and their feet (or, even more relevantly, between their bone marrow and their gonads).

That might explain the expense. (Line item: breaking the laws of physics.)

Another interesting question is how they’d present it in court (“Is this test certified? Has it ever been tried on identical twins before?” “Nooo…” “Next!”)

MikeB February 13, 2013 6:42 PM

What would happen if both are charged with the original crime AND with obstructing justice? With the prospect of a jury or judge finding both liable for punishment on at least one of those charges, I would expect the incentive to lead one or both of the twins to spill the beans (even if both are guilty).

Vicki February 13, 2013 7:33 PM


I don’t know whether “obstruction of justice” is a crime in France the way it is in the United States, but if one of the brothers is innocent of all the attacks, he is also innocent of obstructing justice. It’s not obstructing justice to truthfully say “I didn’t commit any of those horrible crimes. It must have been my brother.”

Joe February 13, 2013 9:25 PM

All this attention to terrorist is so tiresome. In the 2003-2004 flu season 48,000 people died of flu in the US. I don’t have the stats on how many more died from food borne illnesses but that was in the thousands. However we will focus on fighting terrorism rather than fixing health care or regulating the food industry. Wasn’t there a wise man who said something like “you strain out the gnat and swallow the camel”. Could have been talking about our gov’t.

Ann Onymous February 13, 2013 10:57 PM

Since it’s France, “innocent until proven guilty”, which is part of English common law doesn’t apply. French law is “civil law”, based on the Napoleonic Code, so it is my (probable mis-)understanding that the accused has to prove his innocence rather than the State proving his guilt.

Somebody February 13, 2013 11:07 PM

@Ann Onymous:

Article 6 of the European Convention on Human Rights requires a presumption of innocents.

Steve Jones February 14, 2013 1:16 AM

It is an interesting case, certainly, and shows a serious weakness in current law enforcement techniques in these extreme cases.

If they both refuse to testify, there might be an angle along the lines of “perverting the cause of justice” that would apply. This can carry a long prison sentence, up to life in the UK, I believe.

aaaa February 14, 2013 1:42 AM

@Steve Jones Accused has a right to keep silent. That right is quite strong in France, you can not go to prison for refusing to testify against yourself.

phanmo February 14, 2013 3:09 AM

@Mikeb @aaaa
Both of you are right. Everyone in this conversation seems to be under the assumption that the French system of justice is the same as the US/common law system. Nothing could be further from the truth.
I am currently in the midst of a assault with deadly weapon case here in Nantes, France, (I was stabbed with a broken bottle without provocation) and it is absolutely nothing like the common law system. For one, I will be represented at the trial by my own lawyer, due to the fact that criminal cases in France also have a civil part. As has already been mentioned, there won’t be a jury either. There are numerous other differences as well.

Regarding the case, most of the newspapers here are reporting that one (who is deaf or hard of hearing, although I don’t see the relevance) fits the description better than the other : “Yohan, malentendant, est celui qui correspondrait le plus aux descriptions faites par les victimes”.

Very few of the news sources here mention a cost for the tests; the only one I could find gives a figure of approximately 500 000 euros per test.

ewan February 14, 2013 6:18 AM

“If they both refuse to testify”

Even if they could be ‘forced’ to testify, they could simply each say “I’m innocent, it must have been the other guy.” For one of them, that would be perfectly true[1], so there’s no ‘refusal’ or ‘obstruction’ there.

[1] Assuming they didn’t both do it, of course.

Mitch February 14, 2013 7:11 AM

Reminds me of when my son was diagnosed with leukemia. His younger sister was tested and found to be an almost perfect marrow donor match (something which later became a liability). The doctor jokingly said that they’d both better grow up to be honest adults, as a crime committed by one could be blamed on the other.

Anselm Lingnau February 14, 2013 7:12 AM

Identical twins can provide for difficult situations in court. Here in Germany, a boy (now 14) sued to find out which of two identical twins was his father – apparently both guys had slept with his mother around the time in question. This went all the way up to constitutional court (which said that the boy had a right to find out who his father was and bounced the suit back to the state court) but the state court decided recently that, with no witnesses available, there was currently no scientific way of making sure who the actual father was, as the usual method does not work in the case of identical twins. Therefore there would be no point in forcing the twins to provide DNA samples, as »basic genetic research is not within the scope of trials«.

The two »suspects« themselves, sensibly, aren’t cooperating because whoever is identified as the father would have to reimburse the state for considerable child support and other social-services type expenses.

Bradley Evans February 14, 2013 7:55 AM

You have to consider first the strategy that police and prosecution could use. They would probably separate the twins and then could lie. They might say that the other twin had already confessed, the other twin said that this one had done it, or that both were going to jail, one for doing it and one for being an accomplice.

vwm February 14, 2013 11:34 AM

The “right to remain silent” extends to close family in France (as it should anywhere, IMHO). I guess that rules out any “obstruction of justice”-accusation.

Matt February 14, 2013 1:40 PM

Bruce: We all know you read this story and slammed your fist in the air screaming “DAMN IT! Perfect example for Liars and Outliers!”

Prohest February 14, 2013 8:48 PM

<a href=”>Chang and Eng Bunker were Siamese twins. Chang was once convicted of general assault on a member of the audience during one of the twins’ variety acts. However, the judge in the case could not hold Eng in prison as well, so he set them both free. The Bunkers created the term “Siamese twins” for people who are conjoined, because they were originally from Siam. They lived till the age of 63 and married a pair of sisters and had 21 children between them. On the journey from Siam, one of the twins wanted a cold bath and the other didn’t, so the captain had to separate them. Chang was a drunk and died first, so Eng woke up waiting for a doctor to separate them. Eng then died an hour later as he wrapped himself round his twin. It is believed he died from a broken heart, because he had no reason to die.


bob February 15, 2013 11:58 AM


Something went wrong at the end of your story. The cold bath bit non sequiturs oddly between the start and Apple have released an update to the Macbook Pro. Also, Eng presumably died from live failure – although they were functionally independent, they were still joined and, presumably, exchanged blood.

Wayne February 15, 2013 1:12 PM

@Ann Onymous I’m going on how the US civil trial system works but you still don’t have to prove your innocence, you are still innocnet till proven otherwise just that the bar for proving guilt or partial guilt is much lower.

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