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 video-
taped stimulus).
Studies have also examined the mediators of this
pro-prosecution bias in joined trials. As noted previ-
ously, courts have acknowledged three possible
sources of prejudice in a joined trial: (1) confusion in
relating the evidence to the charges, (2) the accumula-
tion of evidence across the charges, and (3) jurors
inferring that the defendant has a “criminal disposi-
tion” 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 empiri-
cally 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 accu-
mulation of evidence across charges, and their ten-
dency 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 accumula-
tion 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 evi-
dence 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 ame-
liorative 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 convic-
tion 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 dis-
position 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 defen-
dant 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 accu-
rate or detailed memories of the trial evidence. A
third hypothesis is that the greater amount of infor-
mation presented at joined trials may cause jurors to
remember only the more salient information that con-
firms their verdict choice. There was no evidence to
support the viability of any of these additional poten-
tial mediators of the prejudicial joinder effect.
Although there are some areas in which the empir-
ical 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 instruc-
tion is an adequate safeguard against prejudicial join-
der, 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, fur-
ther research is needed to replicate these findings and
to determine the limitations of these elaborated394 ———Juries and Joined TrialsJ-Cutler (Encyc)-45463.qxd 11/18/2007 12:43 PM Page 394