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In criminal cases, the prosecution frequently offers evidence of uncharged conduct (usually prior criminal activity), claiming that it will tend to prove the mens rea element of the charged offense. When such an offer is made, Rule 404(b) of the Federal Rules of Evidence (FRE) creates a dilemma for the trial court. On the one hand, the rule forbids the introduction of uncharged conduct evidence (UCE) for one purpose: proof of a person's character in order to show that the person acted in conformity with that character trait on a particular occasion. On the other hand, it permits the introduction of the evidence for any other purpose - including proof of intent or of other mens rea elements of the offense.

The dilemma exists because admission of the uncharged conduct will frequently lead the jury to draw both the permitted and the forbidden inference. Rule 403 mandates admission of the UCE unless "its probative value is substantially outweighed by the danger of unfair prejudice," but this general mandate has provided little meaningful guidance in resolving the dilemma created when UCE is offered to prove mens rea.

This article uses visual and mathematical models to analyze that dilemma. It describes the basic model itself, isolates and identifies various effects of UCE and their probative or prejudicial nature, and, introduces a powerful new analytic tool, "windfall analysis," based on the concepts of evidentiary ideals and evidentiary windfalls.

It concludes that the Seventh Circuit is wrong to uniformly reject conditional concessions as an adequate substitute for the evidence. It also concludes that, to the extent that the other circuits suggest that conditional concessions should always be accepted, those circuits may also be wrong. It then sets forth a proposition not yet adopted by any of the circuits: that there are situations in which a trial court should require conditional concessions as a pre-requisite for excluding UCE even though that evidence would be excludable under 403 even without that concession.

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Pacific Law Journal