The Rules of Evidence do not permit evidence of past conduct to prove conduct on a particular occasion. Federal Rule of Evidence 404(a) (evidence “of a person’s character or a trait of character is not admissible for the purpose of proving action in conformity therewith on a particular occasion”). The concern with such character evidence is twofold:
We fear that evidence of an individual’s bad character will prejudice the jury, causing them to find against him on the basis of past actions because he is “a bad guy,” or because he “deserved it.” We also reject the deterministic idea that an individual is bound to repeat past mistakes despite attempts to alter and reform one’s behavior as this would itself belie the deterrent effect of our penal system.
Robert P. Duffield II, “Distorting the Limits of FRE 406: A Tough Habit to Break,” 38 Rutgers L.J. 897, 910 n.64 (Spring 2007).