Encyclopedia of Psychology and Law

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 dis-

cretion 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 tri-

als 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 defen-

dants 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 defen-

dants 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 determin-

ing the defendant’s guilt.

Judges may be disposed to join offenses or defen-

dants 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 sin-

gle trial can be prevented by instructing jurors to con-

sider 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 pro-

duced relatively consistent findings. Some researchers

have posited that joining offenses may lead to a depri-

vation 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 pre-

vent this bias toward conviction. In addition, other

factors can affect the nature and degree of the prejudi-

cial 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

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