American courts have long excluded evidence of a person's prior bad acts. This tradition reflects a fear that the jury will place too much weight on past crimes and prior misdeeds. '[I]t is said to weigh too much with the jury and to so over persuade them as to prejudice one with a bad general record and deny [the accused] a fair opportunity to defend against a particular charge.' Michelson v. United States, 335 U.S. 469, 476, 69 S. Ct. 213, 93 L. Ed. 168 (1948); see also H. Richard Uviller, Evidence of Character to Prove Conduct: Illusion, Illogic, and Injustice in the Courtroom, 130 U. Pa. L. Rev. 845, 884 (1982) ('[A]s the special conditions of predictive value coalesce, the potential for prejudice also rises.'). The risk is that jurors will focus on evidence of prior acts, believing that someone with a criminal record cannot change and discounting any evidence to the contrary.
Over the past two hundred years, the prior-acts rule has changed much in form but little in function. In the early days of the common law, courts used an inclusionary approach: evidence of prior acts was presumptively admissible unless it was relevant only ...