Juries and Joined Trials

Joinder is a legal term that refers to the combination of several counts, parties, or indictments in a single trial. Although there has been limited empirical research examining joinder trials, the research that has been conducted has focused almost entirely on the influence of the inclusion of additional indictments on juror decision making. In this context, a joinder trial refers to a trial in which one defendant is tried for multiple offenses that have similar characteristics or arise from the same incident. The court has the discretion to try a defendant for each offense individually in separate trials or combine the offenses into a single trial if the offenses are related. However, there has been a general consensus among researchers that trials with joined offenses lead to a proconviction bias; jurors are more likely to vote for conviction when offenses are joined than when the defendant is tried separately for each offense. Although the courts have addressed the potential prejudice inherent in joining offenses or defendants by specifying safeguards to protect defendants, the adequacy of these safeguards is still subject to debate.

The Law Concerning Joined Trials

Rule 8(a) of the Federal Rules of Criminal Procedure allows for defendants to be tried for two or more offenses in a single trial if the offenses are similar in character or are part of a single scheme or plan of action. Furthermore, Rule 8(b) allows for two or more defendants to be tried in a single trial if they are accused of jointly engaging in the same criminal transaction. Although these rules are for federal courts, many states have patterned their own rules of criminal procedure after these federal rules. The primary purpose of the combination of offenses is judicial expediency. Separate trials for each offense or defendant would result in many more trials, increasing court costs. In essence, the issue becomes balancing the need to conserve resources with providing defendants fair trials.

The law does recognize that joining offenses and/or defendants may result in a pro-prosecution bias. Rule 14 of the Federal Rules of Criminal Procedure states that the court must protect defendants from the prejudice that may arise from joined trials by separating the charges into separate trials, severing the trials of two or more defendants, or employing any other remedy necessary. If defendants wish to be tried for each of the charges separately or be tried separately from other defendants, they can make a motion for severance of the offenses or from the defendant. However, judges frequently will rule in favor of joining offenses and defendants, unless a case can be made that a joined trial will prejudice the jury. Prejudice may be inferred if the defendant would be likely to rely on contradictory defense strategies if the charges were severed, if the jury would be likely to infer from one of the charges that the defendant has a criminal disposition, or if the jury would be likely to accumulate evidence across charges when determining the defendant’s guilt.

Judges may be disposed to join offenses or defendants because case law argues that any prejudice that does arise from joinder can be easily remedied. Specifically, the U.S. Supreme Court has ruled that prejudice from joining offenses or defendants in a single trial can be prevented by instructing jurors to consider the evidence for each charge individually and decide independently the verdicts for each of the charges or defendants.

Empirical Evidence

Researchers have sought to determine whether the courts’ assumptions about the ways that jurors decide verdicts in joined trials are supported by empirical evidence. Although there has been limited research in the area, the studies that have been conducted produced relatively consistent findings. Some researchers have posited that joining offenses may lead to a deprivation of the defendant’s right to an unbiased trial. The empirical evidence suggests that when offenses are combined in a single trial, defendants are more likely to be convicted than if the offenses were tried separately. Several jury simulation studies show that trials with joined offenses result in higher conviction rates than if each offense was tried in separate trials. Moreover, although the courts have opined that the prejudice that results from joining offenses can be prevented by proper judicial instruction, most studies suggest that judicial instruction is insufficient to prevent this bias toward conviction. In addition, other factors can affect the nature and degree of the prejudicial nature of joined trials.

In one of the first studies to examine the effects of joining offenses in a single trial, researchers examined the differences in conviction rates when mock jurors made judgments in severed trials, where the defendant was charged with a single count of rape in each of two trials, or in a joined trial, in which the defendant was tried for two charges of rape in the same trial. When the offenses were joined, the judge also instructed the mock jurors that they were to consider each charge with its respective evidence separately. Despite the judicial instruction, the jurors were more likely to find the defendant guilty of the first offense when the offenses were joined than when the offenses were severed. Joinder did not influence rates of conviction on the second offense, but of course, in real cases, the same jury would never hear evidence for both of the severed offenses. This bias toward conviction has been demonstrated across a number of studies that vary on a number of dimensions, including participant type, the presence of deliberations, and the medium used to present the trial (written, audiotaped, or videotaped stimulus).

Studies have also examined the mediators of this pro-prosecution bias in joined trials. As noted previously, courts have acknowledged three possible sources of prejudice in a joined trial: (1) confusion in relating the evidence to the charges, (2) the accumulation of evidence across the charges, and (3) jurors inferring that the defendant has a “criminal disposition” because he or she is being tried for multiple crimes. As early studies merely demonstrated that joining offenses increased convictions and did not address how joining offenses altered juror evaluations that lead to verdicts, researchers such as Sarah Tanford and Steven Penrod began to examine empirically whether various attributes of the joined trial, such as charge similarity, evidence similarity, and judicial instruction, increase jurors’ confusion about which evidence relates to which charges, their accumulation of evidence across charges, and their tendency to draw inferences about the defendant’s criminal disposition. After watching a videotaped mock trial in which the similarity of the charges, the similarity of the evidence, and the presence of judicial instructions were manipulated, jurors were more likely to render a guilty verdict when the offenses were joined than when they were tried separately, which is consistent with earlier research. In contrast, evidence similarity had no effect on verdicts, and charge similarity was related to verdicts in some studies and not others. Across studies, joining offenses did result in confusion of the evidence, an accumulation of prosecution evidence, and negative inferences about the defendant’s criminality. Although the jurors’ perceptions of the defendant’s criminality mediated the effects of joinder on verdicts, confusion of the evidence did not. Early research by this research team using representative jurors suggested that elaborated judicial instruction designed to reduce the prejudicial effects of joinder had no effect on verdicts; however, follow-up research conducted with undergraduate students found the same instructions to have an ameliorative effect.

Edie Greene and her colleagues have explored other potential mediators of the prejudicial effect of joinder on verdicts. In two experiments, these researchers replicated previous findings that conviction rates are higher in joined trials. They also tested several previously examined mediators of these effects, including negative inferences about the defendant and evidence confusion. Their findings comported with previous research findings that jurors’ inferences that a defendant had a criminal disposition seem to drive the increased conviction rate in joined trials. They also examined previously untested mediators of the joinder effect. They hypothesized that jurors who possess the knowledge that the defendant is charged with multiple offenses may be more distressed than jurors who are aware of only a single offense. This increase in distress may lead jurors to lower their criterion required for a conviction; that is, jurors in joined trials may have a lower threshold for finding a defendant guilty than jurors who hear the severed charges. It is also possible that joining offenses creates a greater memory load for jurors in joined trials and this greater load leads to less accurate or detailed memories of the trial evidence. A third hypothesis is that the greater amount of information presented at joined trials may cause jurors to remember only the more salient information that confirms their verdict choice. There was no evidence to support the viability of any of these additional potential mediators of the prejudicial joinder effect.

Although there are some areas in which the empirical evidence is equivocal, there are certain findings about the effects of joinder on jurors that have been repeatedly demonstrated. There is consensus that the joinder of offenses in a single trial results in a pro-prosecution bias. There is also consensus that the effects of joining offenses on the verdict are mediated by jurors’ perceptions of the defendant’s criminality rather than by confusion over the evidence. Similarly, although the courts have stated that judicial instruction is an adequate safeguard against prejudicial joinder, there is consensus that the judicial instructions generally provided when offenses are joined in one trial are ineffective in protecting jurors’ decisions from the prejudicial effects of joinder. There is some evidence that alternative judicial instructions that are designed to reduce the prejudicial effects of joinder may work under certain circumstances; however, further research is needed to replicate these findings and to determine the limitations of these elaborated instructions in reducing the prejudicial effects of joinder on juror decisions.


  1. Greene, E., & Loftus, E. I. (1985). When crimes are joined at trial. Law and Human Behavior, 9, 193-207.
  2. Horowitz, I. A., & Bordens, K. S. (1985). Joinder of criminal offenses: A review of the legal and psychological literature. Law and Human Behavior, 9, 339-353.
  3. Tanford, S., & Penrod, S. (1984). Social inference processing in juror judgments of multiple-offence trials. Journal of Personality and Social Psychology, 47, 749-765.
  4. Tanford, S., Penrod, S., & Collins, R. (1985). Decision making in joined criminal trials: The influence of change similarity, evidence similarity, and limiting instructions. Law and Human Behavior, 9, 319-337.

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