'insanity defense' and diminished capacity
Evidence: A brief guide to admissibility
The proceedings in federal court are governed in part by the Federal Rules
of Evidence. FRE Rule 402 provides that, with certain exceptions, "All
relevant evidence is admissible. . . ." Conversely, Rule 402 states, "Evidence
which is not relevant is not admissible."
Rule 401 defines "relevant evidence." It states:
"Relevant evidence" means evidence having any tendency to make the
existence of any fact that is of consequence to the determination of the
action more probable or less probable than it would be without the evidence.
Therefore, the first inquiry a judge must make when evidence is offered
is whether it is relevant under Rule 401. This is obviously a pretty broad
standard, and very little evidence seriously offered could be said to be
Prejudice, Confusion, and Waste of Time
A second hurdle is FRE Rule 403, which permits the district court to exclude
relevant evidence "if its probative value is substantially outweighed by
the danger of unfair prejudice, confusion of the issues, or misleading
the jury, or by considerations of undue delay, waste of time, or needless
presentation of cumulative evidence."
Rule 403 requires the judge to conduct a "balancing test," to weigh
the usefulness of the evidence in determining the issues against its likelihood
to obscure the issues. According to the Advisory Committee that wrote this
Rule, it is intended to permit the court to exclude evidence which may
have the effect of "inducing decision on a purely emotional basis, at one
extreme, to nothing more harmful than merely wasting time, at the other
extreme. . . ."
Another concern in a criminal trial is that the trial should focus on whether
this particular defendant performed this particular action of which (s)he
is accused on this particular occasion. This focus is emphasized in FRE
Rule 404, which states that character evidence - that is, evidence about
the person's tendencies or personality in general - is not generally admissible
for the purpose of proving action on a particular occasion. Thus the prosecution
could probably not present witnesses who testified to the defendant's violent