Psychology Weekends: What Does “Reasonable Doubt” Really Mean to a Jury?

There are some concepts in the law that are frustratingly amorphous. One of those principles is “proof beyond a reasonable doubt.” What does that even mean? Jury instructions purport to explain it but have the tendency to just complicate the matter. We know from past precedent that it can’t be quantified into some percentage of certainty and that it’s not a requirement of absolute, 100% certainty. But that’s about it…leaving one to wonder how jurors grapple with the concept and apply it in actual criminal trials.

Today, I’m gonna tell you how.

1. Jury Instructions Lower The Amount of Certainty Jurors Need to Feel When Deciding to Convict

There are two pieces of research that we should look at. The first, by Daniel Wright and Melanie Hall, is an interesting attempt to determine what level of confidence may actually be the threshold for reasonable doubt in the real world. The design of the study is fairly simple: have participants read a fact pattern and make a decision about the criminal defendant’s guilt. Next, have the individual rate his or her level of confidence in the guilt. Finally, determine the confidence “point” at which 50% or more of participants will vote to convict. Essentially, this is applying statistical methods most common to pharmacology and biochemistry in general—we look to see what the “LD-50” or lethal dose for which 50% of the population would die.

The study examines a far more intriguing question. Namely, can a judge’s instructions defining reasonable doubt have an effect on the belief of guilt itself? Or, put a different way: would a jury that’s been given no definition of reasonable doubt tend to have a different threshold for what’s enough evidence than a jury instructed on the concept? The researchers theorized that jury instructions telling jurors that they didn’t need to be absolutely certain would have the effect of increasing conviction rate.

To investigate this inquiry, the researchers conducted 2 experiments. In the first one, participants were put into two conditions: no jury instruction given and jury instruction given. Half of the participants were told only to find the fictitious defendant in a fact pattern guilty if they believed him guilty beyond a reasonable doubt. The other half of participants were told this and then given a jury instruction clarifying that “beyond a reasonable doubt” does not require 100% certainty. Lastly, participants rated their confidence in their decision and provided a brief, written explanation for why. The second experiment used the same methods as the first experiment but a substantially larger group of participants.

Some interesting results came out of the experiments. There was not statistically significant evidence to support the theory that detailed jury instructions will make people more likely to convict. However, the instruction did affect the “LD50” for how much proof is needed. That is, when jury instructions were given that told people they needn’t be absolutely certain of guilt, the confidence level at which 50% of jurors chose to convict decreased.

This is both an intuitive and remarkable finding if you think about it. The results tell us that when people are given a jury instruction saying that total certainty isn’t necessary, people will convict on a 63% level of confidence. However, when no jury instruction is given explaining reasonable doubt, people will not convict unless they are 77% convinced of guilt.

How do we square this what I said earlier—that jury instructions don’t make people more likely to convict? It’s a little tricky but the authors explain it as follows: “[A]lthough the instruction lowered the threshold for a guilty verdict, because belief in guilt was also lowered, there was no significant effect on the number of guilty verdicts.” (Wright & Hall, 2007: 96).

This study, like so many others, has a downside in that it relies upon students for its group of participants. It also has the limitation of occurring in settings remarkably different from an actually jury—these were participants in a university setting rather than a group of jurors who were assembled in a courtroom. Further still, this study assess how a juror would make decisions prior to group deliberation and the power of group dynamics for altering one’s perspectives can be strong. However, the limitations of this study can simultaneously be assets. Its crisp, simple design lets us have more certainty that the results are due to the variables manipulated rather than intervening or moderating variables. It also is worth remembering that every jury is a group of jurors who sat down, watched a trial, heard judicial instructions about the law, and formed an opinion before deliberations. Understanding how people form their individuals opinions that they ten take with them into the deliberations is quite important in and of itself.

2. Alternative Explanations for What Happened Tend to Decrease Likelihood of Conviction

That jury instructions may actually cause people to convict under lower certainty is alarming—at least to a practitioner and believer in a high burden of proof. Is there anything we can do to counteract this effect?

Thankfully, yes.

Researchers at University of Virginia and Georgetown University considered whether creating an alternate narrative could reduce the likelihood of conviction. You see, in the early ‘90s, Nancy Pennington and Reid Hastie suggested the “Story Model” of jury deliberations. I talked a little bit about it here. A quick recap is that jurors work to build a story of what happened rather than performing a surgical breakdown of proof by each legal element.

The theory makes sense and is intuitive enough; one of its implications is that, where jurors are not sure which of two stories is the right one, there should probably be reasonable doubt. For example, if prosecutors allege a defendant committed a crime and the defendant presents credible evidence that it had to be someone else because he wasn’t at the scene, a jury should acquit. In fact, unless the jury decides that the defendant’s “story” is lacking in believability, they should acquit. But does this work in practice?

To find out, the authors designed an experiment where participants read a trial transcript. The prosecution case was designed to make it appear virtually certain that the defendant was guilty and the defense transcript introduced evidence that suggested another individual was actually the culprit. The experimenters varied the specifics of the alternate story such that some participants received a greater number of potential alternatives. Participants read the trial transcript for their assigned condition, then read a description of “beyond a reasonable doubt,” and finally made determinations of guilt. Notably, participants were asked to indicate their level of confidence in the guilt of the defendant and, where applicable, of any other suspects that the defense suggested.

The results show that an “alternative story” does indeed increase the likelihood of acquittal. It’s worth pointing out that participants who chose to acquit didn’t merely feel “less certain” of guilt but actually felt more certain in innocence. This effect is not a minor one—the researchers found a sizeable change in confidence when an alternative story was provided to jurors.

Hold your horses, because there’s more. Although an alternate story where the defense provides additional suspects beyond the defendant decreased likelihood of conviction, jurors still found the defendant as “more likely” to be guilty than the other suggested perpetrators. This is quite an important point because it highlights that jurors actually were operating on the principles of “beyond a reasonable doubt.” They may have felt the defendant was most likely out of a group of suspects but they didn’t feel convinced enough to convict. That’s justice as it should be.

This research is seemingly common-sense. After all, providing juries with another story should, if believed, cause them to be less clear on the prosecution’s theory. Some might say this research just goes to confirm what we already know…and yet, I’m not sure I’d agree. First of all, research can be valuable merely for providing empirical support for what we assume to be true. Secondly and more importantly, this disconfirms a concern that I’ve had for some time: that jurors see all defense attorneys as just out to trick them or manufacture reasonable doubt. If jurors felt that way, they would pay no heed to an alternative theory or explanation. Accordingly, this gives me some encouragement that jurors take their jobs seriously and aren’t out just to convict.

3. Final Thoughts

These two pieces of research show us interesting things about juries. They show us that jury instructions on reasonable doubt can actually lower an individual’s threshold for the amount of certainty he or she needs to feel in order to convict. On the other hand, they also show us that jurors are willing to examine a defense’s contentions rather than flatly rejecting them or tending to presume guilt. If there are any take-home points, they would be: (1) push for a jury instruction that does not overly emphasize the nature of “beyond a reasonable doubt” as a lower standard than complete certainty, and (2) build your theory of the case to explain why the prosecution’s story is the incorrect one. Those two methods appear to have an impact on how juries process and evaluate their confidence in guilt.

The concept of proving something “beyond a reasonable doubt” has a long history of evading any real meaning. Judges who have tried to come up with concrete jury instructions have sometimes been reversed on appeal or otherwise criticized for meddling with the standard. Legally speaking, no one really knows exactly what it means. In truth, that’s probably no accident. For example, the law often uses a “reasonable standard” or a “Totality of the Circumstances” test to avoid bright line rules and make the determination of some legal issue rest on its facts. It is a way of turning fact-finders into legal decision makers. It is a way of building flexibility into the system. This is both a blessing and a curse because it allows for flexibility and equity while simultaneously removing any guidance. Jurors are almost always lay persons who don’t practice law for a living and would probably very much like more guidance and clarity they receive. Yet the law refuses to tell them what to do with a detailed definition or explanation of “beyond a reasonable doubt.” Essentially, the law tells them: “use your best judgment based on what you understand this legal concept to mean.” In the face of such an instruction, jurors have worked hard to comply. And as the research presented here tends to show, those jurors tend to be doing a decent job.

-Zachary Cloud


Tenney, E.R., Cleary, H. M. D., & Spellman, B. A. (2009). Unpacking the Doubt in “Beyond a Reasonable Doubt”: Plausible Alternative Stories Increase Not Guilty Verdicts. Basic and Applied Social Psychology, 31, 1-8.

Wright, D.B. & Hall, M. (2007). How a “Reasonable Doubt” Instruction Affects Decisions of Guilt. Basic and Applied Social Psychology, 29, 91-98


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