The '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:

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 "irrelevant."

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. . . ."

Character evidence

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 character generally.