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Stopping the innocent from pleading guilty

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Written by Dr John Danaher.

Dr Danaher is a Lecturer in Law at NUI Galway. His research interests include neuroscience and law, human enhancement, and the ethics of artificial intelligence.

A version of this post was previously published here.

Somebody recently sent me a link to an article by Jed Radoff entitled “Why Innocent People Plead Guilty”. Radoff’s article is an indictment of the plea-bargaining system currently in operation in the US. Unsurprisingly given its title, it argues that the current system of plea bargaining encourages innocent people to plead guilty, and that something must be done to prevent this from happening.

I recently published a paper addressing the same problem. The gist of its argument is that I think that it may be possible to use a certain type of brain-based lie detection — the P300 Concealed Information Test (P300 CIT) — to rectify some of the problems inherent in systems of plea bargaining. The word “possible” is important here. I don’t believe that the technology is currently ready to be used in this way – I think further field testing needs to take place – but I don’t think the technology is as far away as some people might believe either.

What I find interesting is that, despite this, there is considerable resistance to the use of the P300 CIT in academic and legal circles. Some of that resistance stems from unwarranted fealty to the status quo, and some stems from legitimate concerns about potential abuses of the technology (miscarriages of justice etc.). I try to overcome some of this resistance by suggesting that the P300 CIT might be better than other proposed methods for resolving existing abuses of power within the system. Hence my focus on plea-bargaining and the innocence problem.

Anyway, in what follows I’ll try to give a basic outline of my argument. As ever, for the detail, you’ll have to read the original paper.


  1. Plea Bargaining and the Innocence Problem

Plea bargaining is the common practice whereby by a defendant charged with a particular offence will plead guilty to a lesser offence, in an effort to reduce their expected punishment. Radoff’s article describes how the practice currently operates in the US. Similar practices operate in other countries, though they are possibly less extreme than that found in the US.

Plea-bargaining is attractive to both prosecutors and defendants. It is attractive to prosecutors because they are incentivised to achieve the maximum amount of punishment for the minimum expenditure of prosecutorial resources. Plea bargaining enables them to do this by eliminating the costs associated with lengthy trials. It is attractive to defendants because they are incentivised to minimise their expected amount of punishment. Going to trial is risky from their perspective because it carries with it a higher possible sentence. If they are being prudent, pleading guilty to a lesser offence is often going to be the safer bet.

In fact, things are probably skewed more heavily in favour of entering a guilty plea than I am letting on. As Radoff points out in his article, certain changes to sentencing law (mandatory minimums) coupled with differential power as between prosecutors and (most) defence lawyers, will make entering a guilty plea nearly always the sensible thing to do. For example, most defence lawyers are at a considerable informational disadvantage when they first meet with the prosecutors. They will have had limited opportunities to talk to their clients and discuss a defence, whereas the prosecutor will have a full police report, witness testimony and forensic evidence (assuming there is any):

Against this background, the information-deprived defense lawyer, typically within a few days after the arrest, meets with the overconfident prosecutor, who makes clear that, unless the case can be promptly resolved by a plea bargain, he intends to charge the defendant with the most severe offenses he can prove…If, however, the defendant wants to plead guilty, the prosecutor will offer him a considerably reduced charge—but only if the plea is agreed to promptly (thus saving the prosecutor valuable resources).

(Radoff 2014)

Under these conditions who wouldn’t be inclined to plead guilty?

“The truly innocent”, you might respond. But that is not the case. Again, as Radoff points out in his article, studies have shown that a number of innocent defendants have opted to plead guilty in order to avoid more serious charges. The Innocence Project, which seeks to exonerate innocent defendants on the basis of DNA evidence, has identified 30 people (approx. 10% of their total) who pleaded guilty despite later turning out to be innocent. Similarly, the National Registry of Exonerations (in Michigan Law School) has found that 10% (or 151 cases) of legally acknowledged exonerations since 1989 have involved false guilty pleas. Of course, the real number is difficult to know since many of those who plead guilty despite their innocence will never be uncovered. But the 10% figure from these sources looks worrisome.

This is plea bargaining’s innocence problem: the incentives are such that innocent defendants are persuaded to plead guilty more often than we would like.


  1. The Innocence Problem as a Signalling Problem

There are many possible causes of the innocence problem. Long-standing structural and political issues are part of the problem, as are the idiosyncrasies of particular cases and personalities. But at the heart of them all is a basic signalling problem. Innocent defendants feel the pull of the guilty plea because they know they have no way in which to credibly signal their innocence to the prosecutors.

The classic signalling problem can be found in the biblical tale of King Solomon and the two women. According to the traditional version of the story, two women came to King Solomon with a dispute as to parental rights. Apparently, each woman had recently had a child. One woman had rolled over her child while sleeping and the child suffocated and died. She then stole the other woman’s child and claimed it as her own. This is what led to the dispute coming before King Solomon.

The problem for Solomon was that the signals sent to him by the women were the same. They both claimed to be the mother with equal vigour, and in the absence of further evidence there was no reason to believe one over the other. Economists sometimes refer to this as a pooling equilibrium: both the fake mother and the real mother are incentivised to adopt the same signalling strategy. The claim I’m making here — and I’m certainly not the only one to make it — is that a similar sort of pooling takes place in the typical criminal case. It doesn’t matter how much the truly innocent defendant protests their innocence. Their signals will tend to be pooled with the signals of guilty defendants who also protest their innocence. In the absence of overwhelming evidence to the contrary, there is no reason for the prosecutors to believe either.

How can the signalling problem be resolved? Well, speaking in very abstract terms, you need to change the incentives so as to avoid the pooling equilibrium. That’s exactly what King Solomon did in the case of the two women. He decreed that the child be cut in half and shared equally between the them. The false mother was happy to go along with this (she had already lost her child and wished to punish the true mother), but true mother was not (she didn’t want her child to die). Consequently, she was incentivised to concede the dispute to the false mother, which allowed Solomon to work out her real identity. The signals were suddenly separated.

Can something similar be done in the case of the innocence problem? Can we change the incentives so that there is some signal that innocent defendants are more likely to send to the prosecutors than guilty ones?


  1. The P300 CIT as a device for Credible Signalling

In a lengthy article, which I covered last year, Russell Covey has argued that the signalling problem can be solved by introducing a “subwager” into the pre-trial bargaining game that is being played between prosecutors and defendants. The subwager is akin to a bet that a truly innocent defendant would be willing to take, while a guilty one would not. In other words, it is a bet with asymmetrical risks: it is high risk to the guilty defendant but low risk to the innocent. My claim is that the willingness to undergo a voluntary P300 CIT could count as such a subwager.

Now, you may be wondering, what exactly is a P300 CIT and how can it count as a subwager? In brief, a P300 CIT is a type of brain-based lie detection. Actually, no, scrap that: it’s not really a form of lie detection. Rather, it is a type of memory or recognition detection test. It provides evidence for whether or not a suspect recognises information that was present at a crime scene. Thus, it can be used (as part of an appropriate inductive inference) to either link a guilty defendant to a crime scene or separate an innocent defendant from a crime scene. It does so by detecting the presence or absence of a particular brainwave known as the P300. Hence the name. The assumption underlying the test — and backed up by experimental tests thereof — is that this brainwave is detected when a suspect — or, perhaps more correctly, a suspect’s brain — recognises information.

I don’t want to get into the evidence supporting the reliability of the P300 test here. I cover that at some length in my article, and there is an excellent review paper covering all the experimental evidence in favour (and against) the version of the test that I think stands the best chance of actual forensic use. Suffice to say, I think the evidence for the test is more impressive than you might think (though certainly not without its flaws). It can be used to distinguish those who recognise crime-relevant information from those who do not at a rate that is far better than chance (with several experimental tests reporting accuracy levels above 90%). To be sure, there have been dubious uses of the test in past — for instance, Lawrence Farwell’s use of a P300 “brain fingerprinting” test has been criticised — and we should guard against dubious uses in the future, but I nevertheless believe that with more extensive field testing this technology could be used in forensic contexts.

But, as I say, I don’t want to dwell on the evidence in favour of the P300. Instead, I want to highlight how it could be used to resolve the innocence problem. In brief, I think the test provides a way for innocent defendants to credibly signal their innocence to investigators and prosecutors of crimes. Why so? Because the test has the asymmetrical risk profile needed for a successful subwager. It presents a low-risk to innocent defendants (indeed, one of the nice things about the P300 test is its low rate of false positives, particularly when compared with classic forms of lie detection), but a high-risk to guilty defendants. Innocent defendants could thus voluntarily submit to such a test and credibly signal their innocence to prosecutors. In the article, I develop this argument in more detail, explaining why it is important that the use of the test be truly voluntary and why it is important not to simply infer guilt from an unwillingness to undergo such a test. To summarise the argument:

  •  (1) The innocence problem arises from a signalling problem: signals sent by innocent defendants are indistinguishable from the signals sent by guilty defendants.
  •  (2) Introducing a subwager into the pre-trial bargaining game can help solve this signalling problem by giving those with private knowledge of innocence a credible way to distinguish themselves from others.
  •  (3) Giving defendants the option of voluntarily submitting to a P300 CIT provides them with just such a subwager.
  •  (4) Therefore, giving defendants the option of voluntarily submitting to a P300 CIT can help solve the innocence problem.

Don’t read too much into the wording of this conclusion. I don’t think that my proposal will fully “solve” the innocence problem. At best it will provide a partial solution, applicable in a certain range of cases. But I think this is nothing to be sniffed at and could be considered seriously in the not too distant future.


  1. Criticisms of my proposal?

This initial argument for my proposal will probably seem unpersuasive in and of itself. That’s why I insist upon developing the argument within a comparative advantage framework. In other words, within a framework that explicitly compares the proposal to other possible solutions to the innocence problem. When considered in this light, I believe it becomes a good deal more persuasive. I’ll try to explain by considering three other possible solutions to the innocence problem (there are more — for example other types of forensic evidence like DNA testing can be used and have been used by the Innocence Project — but in my analysis I’m limiting my focus to cases in which these other forms of evidence are not available).

The first solution is the one proposed by Russell Covey, from whom I got the idea of the subwager. He thinks that voluntary submission to interrogation functions as a credible signalling device for innocent defendants. In other words, if I am truly innocent, I should forego my right to silence and submit myself to robust questioning by the authorities. Since I am innocent, I am more likely to “pass” the interrogation test than a guilty defendant. The asymmetry of risks needed for the subwager is present in this decision. To be fair, Covey adduces some empirical evidence to suggest that innocent defendants really are better off if they voluntarily submit to interrogation. But I think we should be cautious about this proposal. Interrogation, particularly if the methods become more robust, is open to abuse and comes with no known error rates. The P300 CIT has an advantage over interrogation in that it is a scientifically based test, with known error rates, that has to be administered in accordance with strict protocols.

Another possible solution would be to use other methods of lie detection — e.g. fMRI lie detection. The reasoning would be similar: they represent a low risk to innocent defendants and a high risk to guilty defendants. But, again, I think we should be cautious about such a proposal. Other methods of lie detection tend to follow a control question test (CQT) format, which is open to abuse and has been used, in the past, as little more than an interrogation prop. Also, I think we should be much more suspicious of the evidence claimed on behalf of fMRI-based tests: the signals can be overinterpreted, and it is much more difficult to test whether someone is lying in a lab setting than it is to test whether they recognise certain information. I think the P300 CIT has the advantage once more.

Finally, there is what I call the “sousveillance” solution. This isn’t a subwager-like proposal. This is something far more radical. The idea behind it is that everybody wears veillance technologies at every moment in their lives. This technology will allow them to record and detail everything they have ever done. This will provide them with reliable and credible documentary evidence of their movements and, if they are truly innocent, it should provide them with a way to document their innocence. I accept that this may resolve the innocence problem. And I accept that the evidence produced by such veillance technologies may be more reliable than that produced by a P300 CIT. But, again, I think the P300 has some advantages over the sousveillance solution. For one thing, the sousveillance solution would require prospective implementation, i.e. everyone would need to be using such technologies before any crime were committed. The P300 CIT can be implemented retrospectively, i.e. to investigate crimes after they have taken place. Since we may not be able to guarantee the widespread use of sousveillence technologies, the P300 CIT seems like it could be more useful. For another thing, the widespread use of sousveillance would have a range of other social costs (and benefits) associated with it. It should not be adopted as a targeted solution to the innocence problem. (Note: The sousveillance solution is something I wanted to discuss in the article but the editor asked me to remove the it before publication. I am grateful to have the opportunity to add it in here)

When considered in light of these other possible solutions, the P300 CIT “solution” to the innocence problem looks more promising. There are other objections to the proposal too, but I’ll leave you read about those in the article itself.


  1. Conclusion

To briefly sum up, there is an innocence problem inherent in existing systems of plea-bargaining. The incentives of the system are such that innocent defendants are sometimes persuaded to plead guilty. Ideally, we should avoid this problem. Although there are many possible causes, one of the chief ones is the inability of innocent defendants to credibly signal their innocence to prosecutors. I have argued that a brain-based recognition detection test — specifically the P300 CIT — may help to correct for that inability. The technology is not ready for this use just yet, but may be in the near future.

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5 Comment on this post

  1. A very enjoyable read. I have been concerned with neuroprivacy, but this seems to be exactly the kind of use of brain-reading technology that is moral: it occurs on the request of the suspect, it can improve fairness, the technology is epstemically on far firmer footing than any deception detector.

    However, I am concerned about the imperfect use of a perfect technology by imperfect people. For P300 CIT to work one needs to be careful not to prime the suspect with information that can bias their responses – it is rather plausible that a deviation from a strict protocol could happen if it was run by inexperienced or biased law enforcement officials, for example. And that way the ethical benefit of the approach becomes much weaker: even the merest hint that the prosecutor could get erroneous but court-admissible “evidence” for guilt from it would make the suspect loath to try it and unravel the correction of the plea-bargaining incentives.

    Clearly for this system to work we need to ensure integrity in testing to a high degree.

  2. Thanks Anders! I quite agree on the need to ensure integrity in the process. It might be worth noting here that Japanese police forces have used autonomic nervous system based versions of the CIT on a regular basis. So there is a body of evidence to draw upon in relation to how this might be implemented in practice.

    One point I would like to make, which I did not in this post, is that I’m not considering one other obvious solution to the plea-bargaining problem: getting rid of it altogether. I don’t do so because I’m not sure complete elimination would be desirable.

    1. There is also the problem that changing structural features of legal systems is often hard: the routines become entrenched. Cognitively speaking getting rid of jury deliberations and replacing them with a ballot would be better, but people care more about the symbolic representative aspect of juries than their actual correctness, it seems.

      This suggests that when a new epistemic technology is introduced, it is very important to do it right.

  3. Thanks for the summary of the piece – the article is really interesting. I had three semi-related worries, though, that I think weren’t addressed.

    1) The strategy only works if P300 CIT test results are admissible – and routinely given significant credence – in court. Are they though? I understand traditional lie detectors are not admissible, and there’s considerable suspicion surrounding these new brain-based versions. If they aren’t admissible and taken seriously, you may just say they should be made so (on account of super-low false positive rates). But then a) that’s a big legal shift, making Covey’s alternative much easier to implement and b) the strategy may be redundant, since the tests will be a new, reliable form of forensic evidence that should lower innocent conviction rates anyway. (and only having results be used as evidence avoids the below problems with having refusal be taken as a signal of guilt)

    2) Many innocents will distrust the reliability of the tech, or be uncomfortable with the sort of neuroprivacy violation Anders notes. Innocents who refuse to participate on those grounds would be unfairly punished with a worse bargaining position. You may say such refusal is irrational, but that’s a further flaw with the system – it presumes significant rationality of the agents involved, when technophobia is a reality that must be taken into account. (this will also reduce confidence in guilt based on refusal, further exacerbating the already-high false negative rate of the test itself)

    3) While it doesn’t violate the letter of the ‘refusal to speak will not be used against you in court’ principle, it violates the spirit of that principle. Refusal is not strictly being used as evidence, but it has a similar effect under your strategy – to increase the expected sentence. This is a soft form of punishment, and one that is in fact quite intentional. In some ways, it is part of a troubling movement to replace trials (with strict protections and procedures) with cheaper, more state-favourable interrogations that lack those protections – including the right to remain silent.

    1. Thanks for the comments. One general point about my purpose in writing this: I don’t know if the use of technologies like this would be good all things considered, and I certainly don’t think it is something we should rush into. I’m just trying to push people away from what I perceive to be a status quo bias against this type of technology.

      1. The admissibility of the polygraph varies a bit from jurisdiction to jurisdiction. I’m assuming that the test would have to pass whatever admissibility test is present in the given jurisdiction before this system would work (be that Daubert or some modification thereof). I’m not sure that admitting something like this would be a big legal-shift if it passes such a test. I guess it depends on how deeply-embedded that test is in the given legal culture. (England and Wales might be different insofar as a Daubert-style admissibility test has only recently been adopted). When it comes to comparing the relative merits of different proposals, I’m trying to focus on the epistemic and normative merits, not on their relative ease or cost-effectiveness (though such things would, no doubt, in practice be factored in). In this sense, my comparative advantage test is somewhat idealistic.

      2. I agree, technophobia could be a serious practical impediment to the solution. Need to think about this more.

      3. In the paper I make clear that my proposal only works within a system in which their is robust protection of the right to silence. In this sense, I operate within the terms of the argument made in Seidmann and Stein ‘The Right to Remain Silent Helps the Innocent’. This right is under threat in many jurisdictions and there is a sense in which the dependency between my argument and Seidmann and Stein’s argument helps to bolster the commitment to that right (though I don’t know if this is strong). I agree that the proposal has the effect you mentioned, and I agree there is a worrying trend to replace trials with cheaper systems and this should be resisted. I guess my point would be that voluntary interrogations (as advocated for by Covey) are an extant type of solution and that the P300 CIT would have the advantage over them that it would require some strict protocols etc.. But the cost and the training involved may put many governments off (as per Anders’ point above)

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