Jury Competence

Many observers praise the abilities of juries in making decisions in both criminal and civil cases. Others, however, criticize the competence of juries, arguing that juries are not effective legal decision makers. Psychologists have conducted a variety of studies to evaluate how juries make decisions, using simulation and field experiments, archival data, and interviews of jurors and judges. Overall, juries show a relatively high degree of competence—jurors take their decision-making tasks seriously, understand the nature of the adversary process, attempt to make decisions that achieve many (sometimes conflicting) goals simultaneously, and perform at a level that is similar to that of judges. However, there are clear areas in which jury performance could be improved, and a variety of procedural mechanisms have been developed to assist juries in their decision-making tasks.

Models of Juror Decision Making

Research has shown that jurors attend to the trial evidence presented and show relatively high levels of comprehension and recall of the facts at issue. Mathematical models of juror decision making—i.e., Bayesian models, algebraic models, and stochastic process models— posit that jurors begin with a preliminary judgment and update that judgment as evidence is introduced throughout the trial. In contrast, the “story model” of juror decision making is an explanation-based model of juror decision making that contends that jurors construct a narrative story that best accounts for the (often conflicting) trial evidence and then select the verdict option that best fits that narrative.

Jurors’ Use of Evidence

In reaching their verdicts, jurors endeavor to make decisions that account for the evidence and that follow the law. Research has shown that in doing so, jurors do tend to make use of the evidence that the law defines to be relevant to their decisions. For example, jurors use evidence about offenders’ conduct in determining criminal guilt and civil liability and evidence about the severity of plaintiffs’ injuries in setting compensation. Indeed, the substantive trial evidence turns out to be the best predictor of verdicts.

Extralegal Evidence

In some instances, however, jurors may use information that has been held to be legally irrelevant. First, research has demonstrated that jurors sometimes use information that is related to the case but that is not part of the legally admissible evidence. For instance, jurors have difficulty ignoring evidence to which they are exposed but that is ruled to be inadmissible by a judge—even when they are explicitly told to disregard such evidence. In addition, jurors are influenced by information about a case that that they obtain through the media—pretrial publicity—but that is not part of the evidence admitted in the court.

Second, jurors sometimes use evidence that is appropriately offered for one purpose to inform another decision for which that evidence is legally irrelevant. For example, evidence about a criminal defendant’s prior record is not admissible to establish guilt, but it may be admitted to discredit the defendant’s testimony. However, jurors have difficulty in limiting their use of such information. In the same way, jurors have been shown to conflate their liability and damages decisions, using evidence that is relevant to one in making decisions about the other. Similarly, there is evidence that jurors take into account their assessment of a plaintiff’s own fault in determining the level of damage that plaintiff has suffered. Because these decisions are supposed to be made separately and then combined by the judge, this can result in a “double discounting” of the plaintiff’s damages.

Third, jurors have also been found to rely on preexisting cognitive schemas about the law in ways that may be inconsistent with legal rules. For example, jurors may bring with them ideas about offenses or verdict categories that may conflict with the relevant law. Similarly, in the absence of guidance from the court about whether or how their decisions should take into account the possibility that a party is insured or will incur attorney fees, jurors must rely on their own notions about the likelihood and nature of any insurance or fees and about how their decisions should be informed by these possibilities.

Some, but not all, of the difficulties that jurors have in appropriately dealing with these evidentiary issues may be related to difficulties that they have in understanding jury instructions (see below). In addition, some studies have shown that jurors are better able to avoid misusing evidence in some of these ways when given instructions that explain the rationale underlying the legal rules. For example, jurors are better able to keep evidence about plaintiff or defendant culpability from influencing their damage awards when the reasons for doing so are explained.

Hearsay Evidence

There has also been research examining how jurors use particular types of evidence. For example, some studies have examined how jurors assess hearsay evidence—testimony by a witness relaying what was said by another person (the “declarant”) that is used to prove the truth of the matter asserted. Jurors appear to be able to competently assess hearsay evidence, distinguishing more reliable hearsay testimony as more accurate, more helpful to their decisions, and of higher quality than less reliable hearsay testimony.

Confession and Eyewitness Evidence

How jurors assess incriminating evidence, such as confession evidence or eyewitness testimony, has also been studied. With regard to confession evidence, there is some evidence that jurors are not influenced by factors—such as evidence of coercion—that are associated with the reliability of confessions. There is similar evidence with respect to eyewitness testimony. Psychological research has documented a number of factors that can influence the accuracy of an eyewitness, including the presence of a weapon, the use of a disguise, or other conditions under which the eyewitness observed the suspect. However, unaided, juror decisions—such as verdicts, culpability judgments, or assessments of the accuracy of the eyewitness—may not be influenced by these factors. In addition, juror decisions can be insensitive to differences in lineup procedures. In contrast, there is evidence that jurors are influenced by the confidence of the testifying eyewitness, even though confidence is only weakly correlated with witness accuracy. Expert testimony about the influences on eyewitness accuracy (e.g., witnessing conditions), however, has been shown to increase juror sensitivity to such factors and to decrease the effect of witness confidence. When experts testify for both sides, however, jurors appear to become skeptical of all eyewitness testimony, including identifications made under relatively good conditions.

Scientific and Statistical Evidence

Finally, it has been found that jurors can have particular difficulty with certain types of complex evidence, such as statistical, scientific, or other forms of expert testimony. On the one hand, expert evidence often proves helpful to juries—for example, helping them better evaluate other evidence, such as eyewitness testimony or confessions (see above). In addition, there is evidence that jurors carefully evaluate such expert testimony, approaching expert witnesses with some skepticism and working to evaluate both the experts (e.g., their credentials and motives) and the substantive testimony. On the other hand, it has been found that jurors sometimes have difficulty in understanding and using such evidence properly. For example, there is evidence that jurors are not skilled at identifying flaws in the design of scientific research studies and may not discount the value of such research appropriately. Studies with confounding variables, missing control groups, problems with the validity of measures, and the opportunity of experimenter demand are as influential as better-designed studies, and witnesses presenting such flawed research are not seen as any less credible than witnesses presenting studies without such flaws. Similarly, interviews with jurors as well as experimental studies have shown that jurors experience difficulty in reasoning about and making inferences from statistical evidence, such as probability estimates and information about population base rates. Recent research, however, suggests that improved judicial instructions may help jurors better understand scientific testimony.

Civil Decision Making

Civil juries in particular have been criticized as unpredictable, arbitrary, biased against plaintiffs, and prone to making large damage awards. Data from actual cases, however, provide little evidence of juries that are out of control; most jury awards are modest. However, while jury damage awards are appropriately influenced by legally relevant factors, they can also be highly variable, such that different amounts are awarded in cases involving what seem to be similar injuries. In addition, studies have found that juries tend to overcompensate small injuries but undercompensate large injuries. Moreover, there is evidence that for noneconomic damages, such as damages for pain and suffering, and for punitive damages, jurors have some difficulty in translating their judgments into dollar figures and can be influenced by cognitive biases such as anchoring.

Concern is often expressed that juries are biased in favor of plaintiffs and against defendants, particularly those defendants with “deep pockets.” However, there is evidence that jurors are relatively skeptical of plaintiffs and their claims. In addition, there is little evidence that the wealth of the defendant influences jurors’ liability determinations or their compensatory damage awards. In contrast, however, corporate defendants do seem to be held to a higher (“reasonable corporation”) standard of conduct than are defendants who are individuals.

Jury Instructions

The primary way in which juries are schooled in the requirements of the law is through the legal instructions that trial judges give them to guide their decisions. These instructions explain the jury’s role, inform the jury about the legal rules that govern the case that it is to decide, define the standard and burden of proof, and define the possible alternative verdicts available to the jury. Typical instructions focus on stating the law precisely rather than on comprehensibility; contain many legal terms of art; and consist of lengthy, complex sentences. Perhaps not surprisingly, therefore, a large body of research has demonstrated that while jurors perform relatively well at understanding and remembering the factual evidence presented at trial, they often have difficulty in understanding, remembering, and applying these legal instructions. For example, jurors have particular difficulty in understanding the “beyond a reasonable doubt” standard in criminal cases, the “negligence” standard in civil cases, and legal terms such as “aggravation” and “mitigation” in capital punishment instructions. Similarly, typical instructions limiting the use of particular evidence or indicating that particular evidence is to be ignored are poorly understood.

Significantly, there is evidence that the comprehension of and ability to follow instructions is improved when instructions are rewritten following the basic principles of psycholinguistics. Removing legal jargon, negatives, and words with multiple meanings; using more common words; replacing abstract concepts with concrete terms; using party names; simplifying sentence structure; and providing an analytical structure to guide decision making can all help make instructions more comprehensible. In addition, there is some evidence that providing jurors with written copies of instructions can lead to improved memory for and comprehension of the legal instructions. As noted above, research has also shown that jurors are better able to follow legal instructions when they include explanations and reasons for the underlying legal rules.


Much research examining jury competence has been conducted by examining the decisions and judgments of individual jurors. Indeed, the predeliberation preferences and judgments of individual jurors are predictive of ultimate jury verdicts. Juries, however, operate as group decision-making bodies. Thus, it is important to consider the ways in which the deliberative process might influence jury competence. Importantly, juries may be advantaged over an individual decision maker (such as a judge) in that they can draw on the memories and understandings of the collective. Research has demonstrated that jurors who deliberate are more likely to engage in complex reasoning about the law and the evidence and make more counterarguments, have better memory for the trial evidence, are less susceptible to some judgmental biases, and reach more consistent decisions. On the other hand, group deliberation has been found to worsen judgmental biases in some circumstances, and group decisions can become more extreme.

Comparing Jurors with Judges

Any discussion of jury competence must consider the standard by which competence is to be evaluated. One common standard by which to evaluate jury competence is to compare jurors with the most likely alternative—trial court judges. In general, research has shown that judges and jurors show substantial agreement in their decisions. High rates of agreement are found both in cases classified by judges as relatively easy to understand and in cases involving a high degree of legal and factual complexity. Moreover, in those cases in which there is disagreement between jury and judge, the judges do not attribute the disagreement to jury misunderstandings of the case. Rather, judges are more likely to disagree with jury decisions in cases that are “closer calls”—that is, in cases in which there are credible arguments to support either decision. Several studies have also shown relatively high rates of agreement between juries and other expert decision makers; for example, jury verdicts in medical malpractice cases tend to correlate with the judgments of physicians.

In addition, research has shown that judges and jurors engage in similar decision processes. For example, judges and jurors consider the same factors in assessing plaintiffs’ pain and suffering in personal injury cases, rank the severity of cases in similar ways, take into account similar factors in setting punitive damage awards, and are influenced by the same variables when assigning blame in criminal cases. Judges also suffer from many of the same difficulties that jurors do. For example, judges, like jurors, are typically not trained in math or science and have difficulty in evaluating statistical and scientific evidence. In addition, judges, like jurors, have been shown to be unable to ignore inadmissible evidence. Similarly, judges are subject to the same sorts of cognitive illusions as are jurors; their decisions are influenced by anchoring, outcome and hindsight bias, and other heuristics.

Judges—as the trial participants who are in a position to most closely observe the jury—are also well suited to comment on jury competence. When asked their opinions about juries, judges report high levels of satisfaction with juries and believe that juries understand the issues and attempt to follow the legal instructions.

Aids to Jury Decision Making

A variety of aids have been proposed to assist juries with their decision tasks. Many of these aids have been controversial, but over time more courts have become willing to incorporate some of these procedures into their processes. This increased openness has also facilitated empirical assessments of the effects of these reforms in field settings. These reforms attempt to improve jury decision making through a range of mechanisms. First, a number of reforms are aimed at assisting jurors in making sense of and remembering the evidence. For example, instructing juries about the law prior to the introduction of evidence provides jurors with a framework that helps them structure the trial evidence as they hear it. Similarly, allowing jurors to take notes and ask questions of witnesses helps jurors understand the testimony presented and remember it better. Providing access to trial transcripts, witness lists, or trial summaries helps jurors refresh their memories, allows them to clarify the evidence when memories differ, and makes it more likely that they will systematically process the evidence. Allowing jurors to discuss the case throughout the trial may help jurors comprehend the evidence and formulate useful questions.

Other reforms are targeted at helping jurors follow the legal rules. For example, bifurcating the trial—that is, separating trials into multiple parts at which different questions are considered may help jurors focus on the evidence that is most relevant to the separate decisions at hand. For example, decisions on punitive damages could be postponed until after the jury has determined liability and compensatory damages. Then, evidence relevant to punitive damages could be heard at a separate proceeding. Similarly, asking jurors to respond to a series of questions (“interrogatories” or a “special verdict”) can be used to focus jurors on the relevant legal questions. Reforms directed at judicial instructions are also likely to improve jurors’ ability to understand and follow the law. Rewriting judicial instructions to make them more understandable and easier to follow, drafting instructions to assist jurors with scientific and statistical evidence, and formulating reason-based instructions may all help jurors comprehend and apply the law better.


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Return to the overview of Trial Consulting in Forensic Psychology.