The balancing test that applies to this type of evidence is that of probative value versus prejudicial potential. On one side, if the conduct illustrates a pattern of behavior, or has some other value as evidence in support of the act alleged, it is appropriate under Rule 401(b) of the FRE for a trial judge to allow it into evidence. (Weeks, 2005) On the other hand, the jury may be tempted to assign probative value to prior acts as they relate to the charged act. (Weeks, 2005) (which is why prior bad acts are generally inadmissible to begin with).
The jury may err on two bases with respect to this type of evidence. (Weeks, 2005) They may decide the case at hand based sole, or disproportionately upon a perceived pattern of behavior, or they may overestimate the probative value of such evidence. (Weeks, 2005) In order to admit such evidence, a trial judge must determine that the true probative value of the evidence significantly outweighs these considerations. In terms of prejudicial potential, numerous studies have confirmed the notion that juries are very likely to weigh prior bad acts as significant elements of their consideration.
(Weeks, 2005) Juries are far more likely to convict or find judgment against defendants in cases where prior bad acts are introduced as evidence. (Weeks, 2005) On a legal bases, the rule (403(b) ) only prohibits such evidence being introduced solely on the basis of character. (Weeks, 2005) It is considered relevant as evidence of pattern of behavior, but the appellate law for this type of evidence is complicated, and often, decisions are based upon the particular facts of a given case, rather on an overall finding of admissibility. (Weeks, 2005)